java design patterns problem solving approach

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Java Design Patterns: Problem solving Approach Paperback – March 2, 2018

• Print length 174 pages
• Language English
• Publication date March 2, 2018
• Dimensions 8.5 x 0.4 x 11.69 inches
• ISBN-10 198574340X
• ISBN-13 978-1985743403
• See all details

Product details

• Publisher ‏ : ‎ CreateSpace Independent Publishing Platform (March 2, 2018)
• Language ‏ : ‎ English
• Paperback ‏ : ‎ 174 pages
• ISBN-10 ‏ : ‎ 198574340X
• ISBN-13 ‏ : ‎ 978-1985743403
• Item Weight ‏ : ‎ 1.21 pounds
• Dimensions ‏ : ‎ 8.5 x 0.4 x 11.69 inches

About the author

Alban andahi.

My name is Alban....Alban Andahi. I have been a programmer for over a decade, written software professionally as a developer, software engineer and systems architect. I have also worked as a contributing factor to many professional books which have emerged best sellers across the globe.

Apart from being a significant figure in the software world, I'm also an entrepreneur and a mentor. contacted me on Email: [email protected]

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Why solve a problem twice? Design patterns let you apply existing solutions to your code

Software design patterns are like best practices employed by many experienced software developers. You can use design patterns to make your application scalable and flexible.

The most satisfying problems in software engineering are those that no one has solved before. Cracking a unique problem is something that you can use in job interviews and talk about in conferences. But the reality is that the majority of challenges you face will have already been solved. You can use those solutions to better your own software.

Software design patterns are typical solutions for the reoccurring design problems in software engineering. They're like the best practices employed by many experienced software developers. You can use design patterns to make your application scalable and flexible .

In this article, you'll discover what design patterns are and how you can apply them to develop better software applications, either from the start or through refactoring your existing code.

Note: Before learning design patterns, you should have a basic understanding of object-oriented programming.

What are design patterns?

Design patterns are solutions to commonly occurring design problems in developing flexible software using object-oriented programming . Design patterns typically use classes and objects, but you can also implement some of them using functional programming . They define how classes should be structured and how they should communicate with one another in order to solve specific problems.

Some beginners may mix up design patterns and algorithms . While an algorithm is a well-defined set of instructions, a design pattern is a higher-level description of a solution. You can implement a design pattern in various ways, whereas you must follow the specific instructions in an algorithm. They don’t solve the problem; they solve the design of the solution.

Design patterns are not blocks of code you can copy and paste to implement. They are like frameworks of solutions with which one can solve a specific problem.

Classification of design patterns

The book, Design Patterns- Elements of Reusable Object-Oriented Software written by the Gang of Four (Erich Gamma, John Vlissides, Ralph Johnson, and Richard Helm) introduced the idea of design patterns in software development. The book contains 23 design patterns to solve a variety of object-oriented design problems. These patterns are a toolbox of tried and tested solutions for various common problems that you may encounter while developing software applications.

Design patterns vary according to their complexity, level of detail, and scope of applicability for the whole system. They can be classified into three groups based on their purpose:

• Creational patterns describe various methods for creating objects to increase code flexibility and reuse.
• Structural patterns describe relations between objects and classes in making them into complex structures while keeping them flexible and efficient.
• Behavioral patterns define how objects should communicate and interact with one another.

Why should you use design patterns?

You can be a professional software developer even if you don't know a single design pattern. You may be using some design patterns without even knowing them. But knowing design patterns and how to use them will give you an idea of solving a particular problem using the best design principles of object-oriented programming. You can refactor complex objects into simpler code segments that are easy to implement, modify, test, and reuse. You don’t need to confine yourself to one specific programming language; you can implement design patterns in any programming language. They represent the idea, not the implementation.

Design patterns are all about the code. They make you follow the best design principles of software development, such as the open/closed principle ( objects should be open for extension but closed for modification ) and the single responsibility principle ( A class should have only one reason to change ). This article discusses design principles in greater detail.

You can make your application more flexible by using design patterns that break it into reusable code segments. You can add new features to your application without breaking the existing code at any time. Design patterns also enhance the readability of code; if someone wants to extend your application, they will understand the code with little difficulty.

What are useful design patterns?

Every design pattern solves a specific problem. You can use it in that particular situation. When you use design patterns in the wrong context, your code appears complex, with many classes and objects. The following are some examples of the most commonly used design patterns.

Singleton design pattern

Object oriented code has a bad reputation for being cluttered. How can you avoid creating large numbers of unnecessary objects? How can you limit the number of instances of a class? And how can a class control its instantiation?

Using a singleton pattern solves these problems. It’s a creational design pattern that describes how to define classes with only a single instance that will be accessed globally. To implement the singleton pattern, you should make the constructor of the main class private so that it is only accessible to members of the class and create a static method (getInstance) for object creation that acts as a constructor.

Here’s the implementation of the singleton pattern in Python.

The above code is the traditional way to implement the singleton pattern, but you can make it easier by using __new__ or creating a metaclass).

You should use this design pattern only when you are 100% certain that your application requires only a single instance of the main class. Singleton pattern has several drawbacks compared to other design patterns:

• You should not define something in the global scope but singleton pattern provides globally accessible instance.
• It violates the Single-responsibility principle.

Check out some more drawbacks of using a singleton pattern .

Decorator design pattern

If you’re following SOLID principles (and in general, you should), you’ll want to create objects or entities that are open for extension but closed for modification. How can you extend the functionality of an object at run-time? How can you extend an object’s behavior without affecting the other existing objects? You might consider using inheritance to extend the behavior of an existing object. However, inheritance is static. You can’t modify an object at runtime. Alternatively, you can use the decorator pattern to add additional functionality to objects (subclasses) at runtime without changing the parent class. The decorator pattern ( also known as a wrapper ) is a structural design pattern that lets you cover an existing class with multiple wrappers.

For wrappers, it employs abstract classes or interfaces through composition (instead of inheritance). In composition, one object contains an instance of other classes that implement the desired functionality rather than inheriting from the parent class. Many design patterns, including the decorator, are based on the principle of composition. Check out why you should use composition over inheritance .

The above code is the classic way of implementing the decorator pattern. You can also implement it using functions.

The decorator pattern implements the single-responsibility principle. You can split large classes into several small classes, each implementing a specific behavior and extend them afterward. Wrapping the decorators with other decorators increases the complexity of code with multiple layers. Also, it is difficult to remove a specific wrapper from the wrappers' stack.

Strategy design pattern

How can you change the algorithm at the run-time? You might tend to use conditional statements. But if you have many variants of algorithms, using conditionals makes our main class verbose. How can you refactor these algorithms to be less verbose?

The strategy pattern allows you to change algorithms at runtime. You can avoid using conditional statements inside the main class and refactor the code into separate strategy classes. In the strategy pattern, you should define a family of algorithms, encapsulate each one and make them interchangeable at runtime.

You can easily implement the strategy pattern by creating separate classes for algorithms. You can also implement different strategies as functions instead of using classes.

Here’s a typical implementation of the strategy pattern:

In the above code snippet, the client code is simple and straightforward. But in real-world application, the context changes depend on user actions, like when they click a button or change the level of the game. For example, in a chess application, the computer uses different strategy when you select the level of difficulty.

It follows the single-responsibility principle as the massive content main (context) class is divided into different strategy classes. You can add as many additional strategies as you want while keeping the main class unchanged (open/closed principle). It increases the flexibility of our application. It would be best to use this pattern when your main class has many conditional statements that switch between different variants of the same algorithm. However, if your code contains only a few algorithms, there is no need to use a strategy pattern. It just makes your code look complicated with all of the classes and objects.

State design pattern

Object oriented programming in particular has to deal with the state that the application is currently in. How can you change an object’s behavior based on its internal state? What is the best way to define state-specific behavior?

The state pattern is a behavioral design pattern. It provides an alternative approach to using massive conditional blocks for implementing state-dependent behavior in your main class. Your application behaves differently depending on its internal state, which a user can change at runtime. You can design finite state machines using the state pattern. In the state pattern, you should define separate classes for each state and add transitions between them.

State pattern follows both the single-responsibility principle as well as the open/closed principle. You can add as many states and transitions as you want without changing the main class. The state pattern is very similar to the strategy pattern, but a strategy is unaware of other strategies, whereas a state is aware of other states and can switch between them. If your class (or state machine) has a few states or rarely changes, you should avoid using the state pattern.

Command design pattern

The command pattern is a behavioral design pattern that encapsulates all the information about a request into a separate command object. Using the command pattern, you can store multiple commands in a class to use them over and over. It lets you parameterize methods with different requests, delay or queue a request’s execution, and support undoable operations. It increases the flexibility of your application.

A command pattern implements the single-responsibility principle, as you have divided the request into separate classes such as invokers, commands, and receivers. It also follows the open/closed principle. You can add new command objects without changing the previous commands.

Suppose you want to implement reversible operations (like undo/redo) using a command pattern. In that case, you should maintain a command history: a stack containing all executed command objects and the application’s state. It consumes a lot of RAM, and sometimes it is impossible to implement an efficient solution. You should use the command pattern if you have many commands to execute; otherwise, the code may become more complicated since you’re adding a separate layer of commands between senders and receivers.

According to most software design principles including the well-established SOLID principles, you should write reusable code and extendable applications. Design patterns allow you to develop flexible, scalable, and maintainable object-oriented software using best practices and design principles. All the design patterns are tried and tested solutions for various recurring problems. Even if you don't use them right away, knowing about them will give you a better understanding of how to solve different types of problems in object-oriented design. You can implement the design patterns in any programming language as they are just the description of the solution, not the implementation.

If you’re going to build large-scale applications, you should consider using design patterns because they provide a better way of developing software. If you’re interested in getting to know these patterns better, consider implementing each design pattern in your favorite programming language.

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Design patterns represent the best practices used by experienced object-oriented software developers. Design patterns are solutions to general problems that software developers faced during software development. These solutions were obtained by trial and error by numerous software developers over quite a substantial period of time. This book will take you through step by step approach and examples using Java while learning Design Pattern concepts.

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• Print length 174 pages
• Language English
• Publication date 2 March 2018
• Dimensions 21.59 x 1.02 x 29.69 cm
• ISBN-10 198574340X
• ISBN-13 978-1985743403
• See all details

Product details

• Publisher ‏ : ‎ Createspace Independent Pub (2 March 2018)
• Language ‏ : ‎ English
• Paperback ‏ : ‎ 174 pages
• ISBN-10 ‏ : ‎ 198574340X
• ISBN-13 ‏ : ‎ 978-1985743403
• Item Weight ‏ : ‎ 549 g
• Dimensions ‏ : ‎ 21.59 x 1.02 x 29.69 cm

About the author

Alban andahi.

My name is Alban....Alban Andahi. I have been a programmer for over a decade, written software professionally as a developer, software engineer and systems architect. I have also worked as a contributing factor to many professional books which have emerged best sellers across the globe.

Apart from being a significant figure in the software world, I'm also an entrepreneur and a mentor. contacted me on Email: [email protected]

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Mastering Design Patterns in Java: Your Ultimate Guide

Are you finding it challenging to master design patterns in Java? You’re not alone. Many developers grapple with this task, but there’s a tool that can make this process a breeze.

Think of Java’s design patterns as a blueprint – a blueprint that simplifies the software development process, making it more manageable and efficient.

This guide will walk you through the most common design patterns in Java, from basic to advanced levels. We’ll explore their core functionality, delve into their advanced features, and even discuss common issues and their solutions.

So, let’s dive in and start mastering design patterns in Java!

TL;DR: What are Design Patterns in Java?

Design patterns in Java provide solutions to common software design problems and enhance efficient communication among developers. There are three main types: Creational, Structural, and Behavioral . Key examples include: Singleton pattern, Factory pattern, Builder pattern, Adapter pattern, Decorator pattern, and Strategy pattern .

Here’s a simple example:

In this example, we’ve created a Singleton class. The Singleton class maintains a static reference to the lone singleton instance and returns that reference from the static getInstance() method.

This is just a basic example of a design pattern in Java. There’s a lot more to learn about design patterns and their implementation in Java. Continue reading for more detailed information and advanced usage scenarios.

Table of Contents

Understanding Singleton and Factory Design Patterns

Delving into observer and decorator patterns, exploring composite and state patterns, common issues with design patterns in java, understanding the fundamentals of design patterns, the relevance of design patterns in large-scale software development, design patterns in microservices architecture, exploring related concepts: solid principles and clean code practices, wrapping up: java design patterns, singleton pattern: the one and only.

The Singleton pattern is a design pattern that restricts the instantiation of a class to a single instance. This is useful when exactly one object is needed to coordinate actions across the system.

Here’s a simple Singleton pattern implementation in Java:

In this code, we’ve created a Singleton class. The Singleton class maintains a static reference to the lone singleton instance and returns that reference from the static getInstance() method.

The Singleton pattern has several advantages, such as controlled access to the sole instance, reduced namespace, and the ability to permit a variable number of instances. However, it can be difficult to unit test due to its global access nature, and it may mask bad design, such as not defining proper objects for the job.

Factory Pattern: Creating Objects Made Easy

The Factory pattern is another common design pattern in Java. This pattern involves a single class (the factory) which is responsible for creating instances of other classes.

Here’s a simple Factory pattern implementation in Java:

In this code, we’ve created an AnimalFactory class. This factory class creates different types of Animal objects based on the input it receives.

The Factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes. However, the complexity of the code increases as the number of classes increases, and it can become difficult to manage and maintain.

Observer Pattern: Keeping Tabs

The Observer pattern is a design pattern where an object (known as a subject) maintains a list of objects depending on it (observers), automatically notifying them of any changes to state.

Here’s a simple Observer pattern implementation in Java:

In this code, we’ve created a JobPost class. When a new job is added, all registered job seekers will get notified.

The Observer pattern is a great way to achieve loose coupling between objects. It allows for a highly modular structure, where the subject and observers can be reused independently. However, observers are not aware of each other and can lead to unexpected updates.

Decorator Pattern: Adding Flair

The Decorator pattern is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class.

Here’s a simple Decorator pattern implementation in Java:

In this code, we’ve created a Coffee class and a SimpleCoffee class that extends Coffee. We then created a MilkCoffee class that also extends Coffee and adds a cost to it.

The Decorator pattern is useful for adding and removing responsibilities from objects at runtime. It provides a flexible alternative to subclassing for extending functionality. However, it can result in many small objects that can be hard to track and debug.

Composite Pattern: Building Blocks

The Composite pattern is a structural design pattern that allows you to compose objects into tree structures and then work with these structures as if they were individual objects.

Here’s a simple Composite pattern implementation in Java:

In this example, we’ve created a Component interface, a Leaf class that implements Component, and a Composite class that also implements Component. The Composite class can contain both Leaf and Composite objects, allowing for complex, tree-like structures.

The Composite pattern is great for representing part-whole hierarchies. It makes it easier to add new kinds of components. However, it can make design overly general and harder to restrict components.

State Pattern: Adapting to Change

The State pattern is a behavioral design pattern that lets an object alter its behavior when its internal state changes. The object will appear to change its class.

Here’s a simple State pattern implementation in Java:

In this code, we’ve created a State interface and two concrete states that implement the State interface. We also have a Context class that uses the State interface to change its behavior based on its current state.

The State pattern allows a class to change its behavior at runtime. It also encapsulates state-specific behavior and divides the behavior of a class. However, the State pattern can be overkill if a state machine has only a few states or rarely changes.

When implementing design patterns in Java, developers often run into a few common issues. Let’s explore these problems and provide some solutions and workarounds.

Misuse of Patterns

One of the most common problems is the misuse of patterns. Sometimes, developers force the use of design patterns where they’re not needed, leading to unnecessary complexity.

For example, using the Singleton pattern unnecessarily can lead to problems with unit testing and hiding dependencies.

Overuse of Patterns

Another common issue is the overuse of patterns. While design patterns can be powerful tools, using them excessively can lead to overly complex and hard-to-maintain code.

For instance, overusing the Factory pattern can lead to an explosion of factory classes, making the code harder to understand and manage.

Solutions and Workarounds

To avoid these issues, it’s important to understand the purpose and use case of each design pattern. Use design patterns only when they solve a specific problem and simplify the code, not just for the sake of using them.

Additionally, remember that design patterns are not concrete designs. They can be modified and adapted to fit the specific needs of your project. Don’t be afraid to adapt a pattern or create your own if it serves your project better.

In conclusion, while design patterns in Java are powerful tools, they must be used judiciously and appropriately. Understanding the potential issues and how to avoid them will help you use design patterns more effectively in your Java projects.

Design patterns are fundamental to software development, providing a standardized and optimized approach to solving common problems. They are essentially well-proven solutions to recurring issues in software design.

What are Design Patterns?

In the context of software development, a design pattern is a reusable solution that can be applied to common problems encountered in software design. They represent best practices evolved over a period of time and are named in a way that can convey their intent.

In the above example, we’ve implemented a Singleton pattern. This pattern ensures that a class only has one instance and provides a global point of access to it.

Why are Design Patterns Important?

Design patterns provide a standard terminology and are fundamental to efficient communication among developers. They allow developers to discuss possible solutions efficiently, as they provide a common language for certain complex structures. Moreover, design patterns encourage code reuse, reducing the overall development effort.

The Role of Design Patterns in Java Programming

In Java programming, design patterns play a crucial role in helping developers write flexible and reusable code. They provide solutions to common programming challenges and follow the ‘Gang of Four’ design patterns, which are divided into three categories: Creational, Structural, and Behavioral. Each pattern has a specific role and use-case in the Java programming language.

In summary, understanding design patterns and their role in Java programming is crucial for writing efficient, reusable, and understandable code.

Design patterns are not just for small projects or simple programs. They play a critical role in large-scale software development as well. When dealing with complex systems with numerous interacting parts, design patterns can help manage this complexity by providing a clear, standardized structure.

For instance, the Factory pattern can be used to manage and control the creation of objects in a large application, while the Observer pattern can help maintain consistency in a system with many interconnected parts.

In the world of microservices architecture, design patterns still hold a significant place. Microservices architecture is about developing a single application as a suite of small services, each running its own process and communicating with lightweight mechanisms. Here, patterns like the Aggregator, Proxy, and Chained Microservices patterns can be incredibly useful.

Design patterns are just one tool in a developer’s toolkit. Other concepts, like the SOLID principles and clean code practices, are also crucial for writing efficient, maintainable code.

The SOLID principles are a set of five principles for designing software that is easy to manage and maintain. They include principles like Single Responsibility, Open-Closed, and Liskov Substitution, which all aim to make software more understandable, flexible, and maintainable.

Clean code practices, on the other hand, are about making the code as clear, simple, and readable as possible. This includes practices like choosing descriptive names, keeping functions and classes small, and using comments sparingly and effectively.

Further Resources for Mastering Design Patterns in Java

Java Design patterns are an inherent part of Java Object Oriented Programming. To delve further into the benefits of design in Java OOP, and other concepts, Click Here!

To further your understanding of design patterns in Java, here are some additional resources:

• Java Abstract Class Basics – Learn how to declare abstract methods and classes, and extend abstract classes.

Lambda Expressions in Java – Explore lambda expressions in Java for writing concise and expressive code.

Java Design Patterns – A complete guide to all the classic design patterns in Java.

Design Patterns – A great resource for learning about design patterns in various programming languages.

Design Patterns Series – A series of articles explaining various design patterns and their implementation in Java.

In this comprehensive guide, we’ve delved into the world of design patterns in Java, a powerful tool for efficient and reusable code. We’ve explored the most common design patterns, from basic to advanced, and discussed their implementation, advantages, and potential pitfalls.

We began with the basics, introducing the Singleton and Factory patterns, which are fundamental to many Java applications. We then moved on to more complex patterns, such as the Observer and Decorator patterns, providing code examples and discussing their use cases and benefits.

Next, we ventured into expert-level territory, introducing alternative approaches such as the Composite and State patterns. We provided code examples for these patterns, discussed their advantages and disadvantages, and provided recommendations for their use.

We also touched on common issues that developers may encounter when implementing design patterns, such as misuse and overuse, and provided solutions and workarounds for these issues.

Here’s a quick comparison of the design patterns we’ve discussed:

Whether you’re a beginner just starting out with Java, or an experienced developer looking to deepen your understanding of design patterns, we hope this guide has been a valuable resource.

Mastering design patterns in Java can significantly improve your coding efficiency and the quality of your software. So keep practicing, keep learning, and happy coding!

About Author

Gabriel Ramuglia

Gabriel is the owner and founder of IOFLOOD.com , an unmanaged dedicated server hosting company operating since 2010.Gabriel loves all things servers, bandwidth, and computer programming and enjoys sharing his experience on these topics with readers of the IOFLOOD blog.

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Design Patterns for Beginners With Java Examples

In this article, learn more about design patterns and look at some java examples as well as a video..

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In this guide, we provide an introduction to the world of design patterns. For each pattern, we further understand the pattern and the context in which it is applicable, with real-world examples.

In this post, you will learn:

• What are design patterns
• Why we use design patterns
• Different types of design patterns
• When we use design patterns
• How do you implement different design patterns in Java
• Real-world examples of design patterns.

What Are Design Patterns?

We have been building object-oriented software for over 40 years now, starting with Smalltalk, which was the first object-oriented language.

The programming world has encountered a large number of problems, and a variety of solution have been proposed to tackle them.

An attempt was made by a group of four people, famously called the “Gang-Of-Four” or GoF, to come up with a set of common problems and solutions for them, in the given context.

This catalog of common problems and their solutions is labeled as GOF (Gang of Four) Design Patterns.

Why Design Patterns?

The advantages of design patterns are:

• To provide standard terminology that everybody understands
• Not to repeat the same mistakes over and over again

What Are the Types of Design Patterns?

The design patterns we talk about here, are from the perspective of an object-oriented world. There are mainly three different kinds of design patterns:

• Creational Patterns
• Structural Patterns
• Behavioral Patterns

Creational patterns deal with the creation of objects.

Structural patterns deal with the composition of objects.

It deals with questions such as:

• What does a class contain?
• What are the relationships of a class with other classes? Is it inheritance or composition?

Behavioral patterns focus more on the behavior of objects, or more precisely, interactions between objects.

How does an object communicate with another object?

Exploring Creational Design Patterns

We explore the following creational design patterns:

The Prototype Pattern

A prototype represents a fully initialized instance, to be copied or cloned.

Let’s take an example:

Let’s consider the design of a chess game. Every game of chess has the same initial setup — the king, queen, rook, bishop, knight, and the pawns all have their specific places. Let’s say we want to build software to model a chess game.

Every time a new Chess game is played, we need to create the initial board layout.

Instead of repeating the creation of chessboard each time,

• We can create an object that contains the initial setup
• Clone from it — each time a new Chess game is played.

The object with the initial setup of the chessboard is the prototype. And, we are using the prototype pattern.

Isn’t it simple?

In the prototype pattern, you have a fully initialized instance. Here, the initial board layout is readily available.

Whenever a new chess game is started, for example, in any of the numerous online Chess portals, this initialized instance is merely copied or cloned.

The Builder Pattern

The Builder Pattern separates object construction from its representation. What does that mean?

Assume that you go out for a multi-course dinner to a restaurant. Such a dinner would have many options, such as starters, main course, and desserts. You would probably choose two or three out of the presented options. A particular client may want to have dinner with the first two options only, leaving out the desserts option. Yet, another would prefer the main course and desserts, skipping the starters entirely.

Similar situations might arise in designing software. You may need to build an object using a subset of the options that are available. Or, you may need to create the object in multiple ways. This is where the builder pattern comes in handy.

To understand it further, let’s look at a small piece of code.

Let’s say you’re writing software for a machine that prepares coffee. The main ingredients of coffee are coffee, milk, and sugar.

Depending on which part of the world you are from, you choose whether or not you have sugar and milk.

The builder pattern steps in to provide these coffee options for you.

Have a look at the code inside main() .

What we have inside the Coffee is a Builder , to which we pass the mandatory type of the coffee. Chained to that call, we make other calls adding in our preferences of the other ingredients.

Someone else who wants a different coffee can easily build it. This leads to a huge amount of flexibility in building objects.

Other approaches to solving this problem, such as the use of setters, have many inherent problems. These solution lead to code that is difficult to read, and also behave erratically in multithreaded programs. The builder pattern solves all those problems.

The advantages of using the builder pattern are:

• It simplifies object creation
• Leads to more readable code
• Does not allow the values to be modified

The Singleton Pattern

The singleton pattern is the most famous among all the design patterns. What this pattern does is very clear from its name — it allows only one instance of a class per JVM at any point in time.

A good real-world comparison would probably be the president of a nation.

However, there is a disclaimer here — there can only be one instance of that class, per JVM . If you have a Java application that runs as part of a cluster of application servers, each server runs a separate JVM instance. Therefore, you are allowed to have one instance of the singleton created on each application server, at any given point of time.

There are a few things to remember whenever you create a singleton class.

• The constructor needs to be private , to prevent the possibility of other objects creating instances of your class.
• In Java, build a Singleton using an Enum .
• JEE 7 has a built-in annotation named @Singleton , along with other related annotations.
• The main disadvantage of using the singleton pattern is that the resulting code is difficult to unit test. Make a clear decision as to where you absolutely need to use a singleton, and where you don’t.
• In frameworks such as Spring, the objects that are managed are called beans, and beans are singletons by default. What Spring does well is ensure that all of this is in the background.

The Factory Method Pattern

The intent of the factory method pattern is to create a family of object types. Let’s look at a code example.

This code implements a PersonFactory . This class has a static method named getPerson() that accepts a person’s name and gender as parameters. Depending on the gender String passed in, it either returns a Male or a Female object.

If somebody wants to create a male person, they invoke the getPerson() method on the PersonFactory with a gender argument of "M" . Similarly, you can create a female person by invoking the getPerson() method on the PersonFactory with a gender argument of "F" .

We are passing in an identifier of the type of object we need, at the time of creation, while still referring to the generic type, Person .

The Male and Female classes are hidden behind the PersonFactory implementation.

The advantage of using the abstract method pattern is that you can add additional types to the factory, without much change in the other classes using this class. In our example, you can add more types of gender, without affecting the existing code that deals with other genders, which all use Person .

What about the complexity involved in creating an object?

It greatly simplifies the task of object creation. The PersonFactory makes the decision of what object to create, and delivers it to us.

Structural Design Patterns

Let us now have a look at the structural design patterns we want to explore.

The Proxy Pattern

A proxy is an object that represents another object.

Let’s look at a real-world example.

Your debit card is a proxy for your bank account. Whenever you make a transaction using a debit card, the corresponding money is deducted from the bank account.

The debit card is a proxy for your bank account, which is the actual object.

Similar to that, in programming, you might have to program interactions with remote objects. In such situations, you create a proxy object that takes care of all external communications. You would communicate with the proxy as if it were residing on your local machine.

A good example is the EJB Home and Remote interfaces.

A proxy hides the complexity involved in communicating with the real object.

The Decorator Pattern

The decorator pattern allows us to add responsibilities to objects, dynamically.

In object-oriented programming, we typically use a lot of inheritance.

Let’s say a particular pizza place has 10 types of pizza. Our implementation has 10 classes for these types of pizza.

Now, there is a requirement to make these pizzas available with three types of toppings. If we would want to create individual classes for each pizza and topping combination, we have a total of 30 classes to manage.

Instead of doing this, can we make the pizza-topping relationship dynamic? Can we add a topping on top of an existing pizza?

We need to use a topping as a decorator on top of any pizza.

Another example would be adding a discount on a pizza order.

Let’s say that you have an order, and based on some criteria, you want to offer a discount to the customer. There might be a variety of discounts that might be applicable at different times. If you add a different type of a discount to each type of order, then in a static relationship, you need to maintain hundreds of classes.

Treating a discount as a decorator on an order makes the relationship dynamic.

A very good example where the decorator pattern is implemented in Java is the Java I/O packages. This is reflected in the way we create an input stream in an I/O program:

You have a FileInputStream . If you want to make it buffered, then add a decorator to it in the form of a BufferedInputStream . If you want the buffered FileInputStream to have line numbers in addition, then add a decorator for a LineNumberInputStream .

Decorator pattern enables you to add behavior to existing objects at run time. This allows the user of the interface to decide how they want to create objects.

The drawback of this approach is the complexity involved in creating objects. The user needs to understand a lot of classes and their relationships before being able to use the power of the decorator.

The Facade Pattern

A facade is a single class that represents an entire subsystem.

Let’s take the example of an event manager. An event manager is a go-to person when you want to organize an event. They handle several aspects of an event such as the decorations, food, sending out invitations to guests, music arrangements, and other similar things. The event manager acts as the facade of the event organization subsystem.

Consider the case of a distributed system. You typically have the need for multiple calls across layers.

Take, for instance, a system that offers the service for online book orders. Whenever an order comes in, several things need to be taken care of, such as checking for the stock, reserving the order, accepting the payment, updating the stock, and generating the invoice.

We can create a single facade, such as the order interface, which would manage all incoming orders and provide an interface to the customer.

The advantage of using the facade pattern is that it reduces the number of network calls, as well as reduces coupling among classes.

It succeeds in establishing a transaction boundary between communicating objects. Facades, like services, are good hubs to implement transactions.

As long as the interface of the facade remains the same, the implementation details of the subsystem can change.

The Adapter Pattern

An adapter is used to match interfaces of different classes.

Let’s take a real-world example of power adapters.

Problem : If you buy a mobile phone in India, it comes with a charger that only works with power sockets used in India. If you take the same charger to the US, for example, it will not work, as it will not fit into sockets there.

Solution : The solution is to use a travel adapter that you can use with your charger when you travel. You can plug in your charger into the travel adapter, and the travel adapter is used to connect to the socket in a particular country.

Similarly, when you try to talk to a system that uses a different message format or a language, you need an adapter to translate messages.

An interesting example is the communication between a Java program and a web service. Before sending out the data to the service, we need to convert the object into XML or JSON format. We are implementing the adapter pattern!

The Flyweight Pattern

Let’s consider a few scenarios

• Creation of an object takes a lot of time and involves multiple instances
• Each instance of an object occupies a lot of memory
• Some objects might be used several times across the same application with the same values

In these scenarios, you might not want to create a new instance every time it is needed.

How about caching an instance and reusing it when needed?

A flyweight represents creating a fine-grained instance that is being used for efficient sharing.

A really good real-world example is the public switched telephone network (PSTN).

In the PSTN, there are always a limited number of lines, and for simplicity, let’s assume this number is 10. However, there are thousands of customers that use these lines. Since all 1000 customers would not make calls at about the same time, it is possible to efficiently switch calls coming in, among the existing 10 lines.

In the software world, a good example of a Flyweight pattern is JDBC connections.

A connection pool is a set of connections to the database. The application may be firing a lot of queries, but we don’t create a new connection whenever a new query comes in. As soon as a query comes in, we match it to an available connection, and the query gets fired. Once query execution is done, the connection is released back into the pool.

Using such a tool allows us to avoid the cost involved in creating and closing a connection.

Behavioral Design Patterns

Let us now have a look at the behavioral design patterns.

The Chain of Responsibility Pattern

The Chain of Responsibility Pattern represents a way of passing a request between a chain of objects.

The best example of this pattern can be seen in the exception handling mechanism of most programming languages.

Suppose you have a method1() calling method2() , and method2() , in turn, calls method3() . Assume that method3() throws an exception.

If method3() has no exception handling, then the exception is passed on to method2() to handle it. If again method2() has no exception handling inside it, then the exception is passed on to method1() . If even method1() cannot handle it, it gets thrown out of method1() as well.

Consider a real-world example of a loan approval process.

A bank clerk has permissions to approve loans within a certain amount. If the amount goes above that, then it goes to the supervisor. The supervisor has a similar, albeit larger loan approval limit set for him. If the loan amount exceeds that limit, then it goes to his supervisor, and so on.

With the Chain Of Responsibility pattern, we have a chain of objects that are ready and wait to process requests. When a new request enters the system, it goes to the first object in the chain to attempt processing. Depending on the processing condition, the request travels up the chain and gets fully processed at some level, or maybe not processed at all.

The Iterator Pattern

The Iterator pattern is one of the most simple design patterns. You have a set of elements arranged in a collection, and you want to sequentially access those elements. A good example of an Iterator is a TV remote, which has the “next” and “previous” buttons to surf TV channels. Pressing the “next” button takes me one channel in the forward direction, and pressing the “previous” button takes me one channel in the backward direction.

In the programming works, examples of the Iterator class and the enhanced for loop in Java are examples of the Iterator pattern.

The State Pattern

The state pattern is used to alter an object’s behavior when its state changes.

Take a look at this Java example:

Let’s take the example of a fan wall control. The fan wall control controls the speed with a fan rotates. It has speed levels ranging from 0 to 5. When it is at level 0, the fan does not rotate, and it rotates the fastest at level 5.

When you rotate the knob of the fan control, the level changes, and this causes the speed of the fan to change as well. This is a classic case of a change in state (level) causing a change in behavior (speed).

A FanwallControl object is composed of a SpeedLevel object. SpeedLevel is an interface that has four different implementations. Initially, the level is at Off , and when you click rotate at that time, the new speed is at SpeedLevel1 . The happens successively, and if you rotate at SpeedLevel3 , the level returns to Off .

In case you need to define an additional speed level, just add in a new class that implements the SpeedLevel interface and implement its rotate method.

This is an excellent example that highlights the advantages of an extensible class.

The Strategy Pattern

The strategy has the task of encapsulating an algorithm inside a class. Let’s look at a Java code example:

The class ComplexClass intends to perform a lot of complex logic within it. One part of that logic is to sort a set of values. One direct way would be to implement the entire sorting logic within ComplexClass . This would make it very inflexible since, if you wanted to change the sorting logic tomorrow, that entire code needs to change.

When we use the strategy pattern, we separate the algorithm of how the sorting is done from ComplexClass .

We define an interface named Sortable , which has a method named sort() . Any actual sort algorithm is an implementation of Sortable , and needs to override sort() method.

Now, ComplexClass is given a particular Sortable implementation as a constructor argument. ComplexAlgorithm does not care what exact sorting algorithm is being used; it is happy that that object implements the sort() method of Sortable .

A lot of flexibility results due to the use of the strategy pattern. You can dynamically change the strategy and pass in the right one according to the context.

The Observer Pattern

The observer pattern is a way of notifying a change to a number of classes.

If you are a fan of cricket, you may want to know whenever Sachin Tendulkar scores a century so that you can celebrate.

All such similar people would register themselves to the event of Sachin scoring a century. Each of these people is now an observer for that event. Whenever Sachin does score a century, a centralized program will notify each observer.

Another example is that of online bidding. A group of bidders at an auction register themselves to receive notifications when a higher bid is placed. As soon as a bid higher than the current one is placed, all the registered bidders get to know about it.

There are two main parts to implementing the Observer design pattern.

• Registration — where the interested objects register themselves with the centralized program to receive notifications
• Notification — where the registered observers receive notifications from the centralized program

Here is a simple implementation of the Observer pattern:

We have created an instance of SachinCenturyNotifier  and registered three fans with it.

Whenever Sachin scores a century, the call notifier.sachinScoredACentury() would be made and all three fans would be notified.

The Visitor Pattern

The visitor pattern allows us to add a new operation to a class without changing the class.

There are a lot of scenarios when designing frameworks, where we don’t want other people to modify the code in the framework. We want others to extend the functionality without touching the framework code. They are allowed to add new operations, but not to change the existing operations.

The visitor pattern allows you to do this.

A good real-world example of the visitor pattern is the operation of a taxi company.

As soon as a person calls a taxi company, and a cab is dispatched, the company accepts a visitor. Once the visitor, or customer enters the taxi, he is no longer in control of where he is going. The cab driver is now in control.

If we look at it as object-oriented code, the driver class is in control of the customer class. The driver class can add new operations on top of the customer/visitor.

The Template Method Pattern

The template method pattern is used to defer the exact steps of an algorithm to a subclass.

A good real-world example of this pattern is how we go about creating a house plan. Any good house plan consists of a floor plan, the foundation, plumbing, framing, and wiring. Such a plan is almost identical for each house.

If you were to model this in software, you could create a template class with this standard behavior defined. A subclass could extend this and give actual implementations. Such details could include the wooden flooring type, the wall paint colors, and any added wings as required.

A good example of the template method pattern is within the Spring framework, in the form of AbstractController :

handleRequest() merely takes care of the basic things. However, it leaves the lions' share for the implementation of the method handleRequestInternal() . This method is defined by subclasses, where more specific logic can be implemented.

The template method pattern is all about doing the high-level steps and leaving the low-level details to the subclasses. The subclasses can override the low steps and provide their own implementation.

The Command Pattern

The command pattern encapsulates a command request as an object.

Let’s take a real-world example.

Consider the scenario when a customer goes to a restaurant and wants to place an order for a meal. The writer merely writes the order he gets on a piece of paper and passes it on to the chef. The chef executes the order and then prepares the meal. He passes the piece of paper to the manager.

The verbal order from the customer has now become a paper object. This piece of paper is the command object. The command object contains all the details needed to execute the request.

Similarly, in object-oriented programming, we can encapsulate all the details of a request into an object and pass that object to execute it.

In web applications, when a user types in the details on a form, these details are captured in a single request object, which is then passed across.

The interface java.lang.Runnable is also a good example of how this pattern is implemented. We create threads in Java by extending the Runnable interface, which has all the logic for execution in its start() method. When we want to create and start a thread, we pass this class to the start() method.

The Memento Method

The memento pattern captures and later restores an object’s internal state.

A lot of games that we play offer the option of performing an intermediate save. At a certain point in the game, you can save it and later come back to it.

To implement this, we need to save the internal states of the game objects and restore them at a certain point in time.

This save-revert functionality can be implemented by using serialization in a language such as Java.

The memento pattern is very useful in implementing undo/redo operations.

For example, if you are working on a text document in a word processor. If at a certain point, you decide to undo changes, you can see each undo until you reach a point where you are satisfied. You have now reverted to an earlier saved state of the document.

The Mediator Pattern

The mediator pattern is used to define simplified communication between classes.

Take the example of an Air Traffic Controller (ATC). Let’s say that at any point of time in India, we have about 500 flights in air. We need to decide the routes that each of these flights needs to take. This also includes deciding the times at which each of these flights takes off and lands. It would be a highly complex situation if each of these 500 flights needs to talk with each other and arrive at an acceptable schedule of routes.

That’s why we have the concept of an ATC. The flights communicate with the ATC, and having assimilated the information from all the flights, the ATC makes the decisions and communicates them back the flights.

In the software world, a good example of the Mediator pattern is the ESB (Enterprise Service Bus). In a distributed system, instead of letting the applications talk to each other, an application drops in a message to the ESB. The ESB routes the request to the application that needs to handle the request. It acts as the Mediator.

Do check out our video on the same topic:

In this article, we had a quick look over a variety of design patterns.

A design pattern is an approach to solve a problem in a given context. We focused on understanding the context in which a particular pattern may be applicable to real-world examples.

Published at DZone with permission of Ranga Karanam , DZone MVB . See the original article here.

Opinions expressed by DZone contributors are their own.

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Implement Design Patterns in Java [In-Depth Tutorial]

April 25, 2023

Design patterns are commonly used solutions to recurring software design problems. They provide a standard approach to solving complex programming challenges and allow developers to build robust, reusable, and maintainable code. In Java, there are several design patterns available that help developers to write more efficient and effective code.

Implementing design patterns in Java requires an understanding of the patterns themselves, as well as a familiarity with Java syntax and programming concepts. In this guide, we will provide an overview of some commonly used design patterns in Java and show you how to implement them in your code. We will cover the creational, structural, and behavioral design patterns, and provide practical examples of each pattern in action. By the end of this guide, you will have a solid understanding of how to implement design patterns in Java and how they can help you to write better code.

What are Design Patterns in Java?

Design patterns are reusable solutions to common software design problems. They are best practices or templates that developers can use to solve problems that occur repeatedly in software development. They provide a framework for solving problems that occur in software design by offering a set of guidelines to follow when approaching a particular problem.

Design patterns emerged from the field of architecture, where architects faced similar problems in building structures that required the use of common solutions. In software engineering, design patterns can help in creating software that is flexible, maintainable, and scalable.

In object-oriented programming, code is organized into objects that interact with each other. A design pattern provides a blueprint for creating these objects and their interactions. It can help to simplify and clarify the design of a system, making it easier to understand, maintain, and modify.

Design patterns have several benefits, including:

• Reusability : Design patterns can be reused in multiple projects, saving development time and effort.
• Scalability : Design patterns help to create scalable code, making it easier to maintain and modify as the project grows.
• Maintainability : Design patterns make it easier to maintain code, as they provide a clear structure and organization for the code.
• Consistency : Design patterns provide a consistent approach to solving problems, making it easier for developers to understand and work with each other's code.

There are many design patterns, each with its own purpose and usage. Creational patterns are used to create objects and manage their creation. Structural patterns are used to organize code into larger structures, making it easier to manage and modify. Behavioral patterns are used to manage communication between objects and control the flow of data.

Types of Design Patterns

Design patterns can be broadly categorized into three main types: creational patterns, structural patterns, and behavioral patterns. Each type serves a different purpose and provides a different set of guidelines for developers to follow.

Creational Patterns

Creational patterns are used to create objects and manage their creation. They provide a set of guidelines for creating objects in a way that is flexible, scalable, and reusable. Some examples of creational patterns include:

• Singleton : This pattern ensures that only one instance of a class is created and provides a global point of access to that instance.
• Factory Method : This pattern provides an interface for creating objects, allowing subclasses to decide which class to instantiate.
• Abstract Factory : This pattern provides an interface for creating families of related objects without specifying their concrete classes.
• Builder : Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
• Prototype : Specifies the kind of object to create using a prototypical instance and creates new objects by cloning this prototype

Structural Patterns

Structural patterns are used to organize code into larger structures, making it easier to manage and modify. They provide a set of guidelines for creating objects that work together to form larger, more complex structures. Some examples of structural patterns include:

• Adapter : Converts the interface of a class into another interface that clients expect, allowing classes with incompatible interfaces to work together.
• Bridge : Decouples an abstraction from its implementation, allowing the two to vary independently.
• Composite : Composes objects into tree structures to represent part-whole hierarchies, allowing clients to treat individual objects and compositions of objects uniformly.
• Decorator : Attaches additional responsibilities to an object dynamically, providing a flexible alternative to subclassing for extending functionality.
• Facade : Provides a unified interface to a set of interfaces in a subsystem, simplifying client interactions with the subsystem.
• Flyweight : Reduces memory usage and object creation overhead by sharing and reusing objects with a shared state, supporting large numbers of fine-grained objects efficiently.

Behavioral Patterns

Behavioral patterns are used to manage communication between objects and control the flow of data. They provide a set of guidelines for creating objects that interact with each other in a specific way. Some examples of behavioral patterns include:

• Observer : This pattern allows an object to notify other objects when its state changes, maintaining consistency across the system.
• Command: This pattern encapsulates a request as an object, allowing it to be passed as a parameter and executed at a later time.
• Template : This pattern defines the skeleton of an algorithm in an operation, deferring some steps to subclasses, allowing subclasses to redefine certain steps without changing the algorithm's structure.

Creational Design Pattern

1. singleton pattern.

The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. This is useful when a single object is needed to coordinate actions across the system. Here's an example of how to implement Singleton in Java:

In the Singleton class, we have a private static instance variable instance . The constructor is marked as private to prevent other classes from creating instances. The getInstance() method checks whether the instance is null, and if it is, it creates a new instance. If an instance already exists, it returns the existing instance. This ensures that only one instance of the Singleton class is created.

2. Factory Method Pattern

The Factory Method pattern defines an interface for creating an object but allows subclasses to decide which class to instantiate. It lets a class defer instantiation to subclasses. Here's an example of how to implement the Factory Method in Java:

In the example, we define an Animal interface with a speak() method. Two classes, Dog and Cat , implement the Animal interface. The AnimalFactory is an abstract class with an abstract method createAnimal() and a static method getFactory() that takes a string and returns a specific factory instance based on the input. The DogFactory and CatFactory classes extend AnimalFactory and override the createAnimal() method to return instances of Dog and Cat , respectively.

3. Abstract Factory

The Abstract Factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.

We define a Button interface with a render() method. Two classes, WindowsButton and MacButton , implement the Button interface. The GUIFactory interface has a createButton() method. The WindowsFactory and MacFactory classes implement the GUIFactory interface and override the createButton() method to return instances of WindowsButton and MacButton , respectively.

4. Builder Pattern

The Builder pattern separates the construction of a complex object from its representation, allowing the same construction process to create different representations.

5. Prototype Pattern

The Prototype pattern specifies the kind of object to create using a prototypical instance and creates new objects by cloning this prototype.

In the Shape interface, we define a clone() method. Two classes, Rectangle and Circle , implement the Shape interface and override the clone() method. The clone() method creates a new instance of the class and copies the properties of the original object. When using the prototype pattern, you can create a new object by simply calling the clone() method on an existing object, avoiding the need for complex object initialization logic.

Structural Design Patterns

1. adapter pattern.

The Adapter pattern converts the interface of a class into another interface that clients expect. It allows classes with incompatible interfaces to work together.

Use Case: Integrating a third-party library with a different interface or making a legacy code work with a new system.

We define a MediaPlayer interface (target) with a play() method. The AdvancedMediaPlayer class (adaptee) has two methods, playVlc() and playMp4() . We create an adapter class, MediaPlayerAdapter , that implements the MediaPlayer interface and has an instance of AdvancedMediaPlayer as a private member. In the play() method, we delegate the calls to the appropriate methods of the AdvancedMediaPlayer class based on the audio type.

2. Bridge Pattern

The Bridge pattern decouples an abstraction from its implementation, allowing the two to vary independently.

Use Case: Developing cross-platform applications with different UI controls or connecting remote services using different communication protocols.

We define an abstract Shape class with a Renderer member. The Shape class has an abstract draw() method. The Circle class (refined abstraction) extends Shape and overrides the draw() method, delegating the call to the Renderer member. The Renderer interface (implementor) has a renderCircle() method. The VectorRenderer and RasterRenderer classes (concrete implementors) implement the Renderer interface and override the renderCircle() method.

3. Composite Pattern

The Composite pattern allows treating a group of objects as a single instance of an object. It composes objects into tree structures to represent part-whole hierarchies.

Use Case: Implementing a file system with files and directories, or creating a GUI with complex nested UI components.

We define an abstract Graphic class with a draw() method and add() and remove() methods that throw UnsupportedOperationException by default. The Circle class (leaf) extends Graphic and overrides the draw() method. The GraphicGroup class (composite) extends Graphic , overrides the draw() method to draw all its child graphics, and also overrides the add() and remove() methods to manage child components.

4. Decorator Pattern

The Decorator pattern attaches additional responsibilities to an object dynamically. It provides a flexible alternative to subclassing for extending functionality.

Use Case: Adding features to an existing class without modifying its code, or extending a class when subclassing is not practical.

We define a Coffee interface (component) with cost() and getDescription() methods. The Espresso class (concrete component) implements the Coffee interface and overrides the cost() and getDescription() methods. The CoffeeDecorator abstract class (decorator) implements the Coffee interface and has a Coffee member. The Milk class (concrete decorator) extends CoffeeDecorator and overrides the cost() and getDescription() methods to add the cost and description of milk to the base coffee.

5. Facade Pattern

The Facade pattern provides a unified interface to a set of interfaces in a subsystem. It defines a higher-level interface that makes the subsystem easier to use.

Use Case: Simplifying interactions with a complex system, or providing a simpler API to a third-party library.

We define three subsystem classes, SubsystemA , SubsystemB , and SubsystemC , each with their respective operation methods. The Facade class has instances of all three subsystems and provides a performOperation() method, which calls the operation methods of all subsystems. Clients interact with the Facade class, which simplifies the interaction with the subsystems.

6. Flyweight Pattern

The Flyweight pattern uses sharing to support large numbers of fine-grained objects efficiently, reducing memory usage and object creation overhead.

Use Case: Creating many similar objects with a shared state, such as rendering characters in a text editor or game objects in a video game.

We define a Shape interface (flyweight) with a draw() method. The Circle class (concrete flyweight) implements the Shape interface and has a color property. The ShapeFactory class provides a static getCircle() method that returns a Circle instance with the specified color. If a circle with the given color doesn't exist, it creates a new one, stores it in the shapeMap , and returns it. This way, Circle instances with the same color are shared and reused.

Behavioral Design Patterns

Behavioral design patterns define the ways in which objects communicate and interact with one another. They improve the flexibility and maintainability of code by decoupling components. Here are three commonly used behavioral design patterns, with code examples and explanations:

1. Observer Pattern

The Observer pattern allows an object to notify other objects when its state changes, maintaining consistency across the system. Here's an example of how to implement Observer in Java:

We define a Subject class that maintains a list of Observer instances. The Subject class provides methods to attach observers, set state, and notify observers. When the state is updated, the Subject notifies all attached observers. We define an abstract Observer class with a reference to the Subject and an abstract update() method. The BinaryObserver (concrete observer) extends the Observer class and implements the update() method to display the subject's state in binary format.

2. Strategy Pattern

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable, allowing the algorithm to vary independently from clients that use it.

We define a SortingStrategy interface (strategy) with a sort() method. The BubbleSortStrategy and QuickSortStrategy classes (concrete strategies) implement the SortingStrategy interface and provide their respective sorting algorithms. The Sorter class (context) contains a reference to a SortingStrategy instance and provides a setStrategy() method to change the strategy at runtime. The sort() method in the Sorter class delegates the sorting task to the currently set strategy.

3. Template Method Pattern

The Template Method pattern defines the skeleton of an algorithm in an operation, deferring some steps to subclasses, allowing subclasses to redefine certain steps without changing the algorithm's structure.

We define an abstract Game class with three abstract methods: initialize() , startPlay() , and endPlay() . The play() method (template method) in the Game class calls these methods in a specific order. Subclasses like Football and Basketball extend the Game class and provide their own implementations for the abstract methods. The play() method in the Game class ensures that the steps are executed in the same order for all concrete classes, while allowing subclasses to provide their own implementations for each step.

In summary, implementing design patterns in Java is a powerful tool for solving recurring software design problems, and it helps developers to build maintainable, efficient, and reusable code. The three types of design patterns are creational, structural, and behavioral. Each pattern has a specific set of benefits and drawbacks, and selecting the appropriate design pattern for a given situation is important. Implementing design patterns in Java requires an understanding of the pattern itself and familiarity with Java syntax and programming concepts. By following best practices and guidelines for using design patterns, developers can avoid common mistakes and apply patterns effectively in their projects. Overall, incorporating design patterns into Java programming can help developers write better code and achieve their software development goals.

Further Reading

Desing Pattern in Java

Bashir Alam

He is a Computer Science graduate from the University of Central Asia, currently employed as a full-time Machine Learning Engineer at uExel. His expertise lies in Python, Java, Machine Learning, OCR, text extraction, data preprocessing, and predictive models. You can connect with him on his LinkedIn profile.

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Book description

Design patterns have become a staple of object-oriented design and programming by providing elegant, easy-to-reuse, and maintainable solutions to commonly encountered programming challenges. However, many busy Java programmers have yet to learn about design patterns and incorporate this powerful technology into their work.

Java™ Design Patterns is exactly the tutorial resource you need. Accessible and clearly written, it helps you understand the nature and purpose of design patterns. It also serves as a practical guide to using design patterns to create sophisticated, robust Java programs.

This book presents the 23 patterns cataloged in the flagship book Design Patterns by Gamma, Helm, Johnson, and Vlissides. In Java™ Design Patterns, each of these patterns is illustrated by at least one complete visual Java program. This practical approach makes design pattern concepts more concrete and easier to grasp, brings Java programmers up to speed quickly, and enables you to take practical advantage of the power of design patterns.

Key features include:

Introductory overviews of design patterns, the Java Foundation Classes (JFC), and the Unified Modeling Language (UML)

Screen shots of each of the programs

UML diagrams illustrating interactions between the classes, along with the original JVISION diagram files

An explanation of the Java Foundation Classes that illustrates numerous design patterns

Case studies demonstrating the usefulness of design patterns in solving Java programming problems

After reading this tutorial, you will be comfortable with the basics of design patterns and will be able to start using them effectively in your day-to-day Java programming work.

0201485397B04062001

Table of contents

• About the Author
• Acknowledgments
• Defining Design Patterns
• The Learning Process
• Studying Design Patterns
• Notes on Object-Oriented Approaches
• The Java Foundation Classes
• Java Design Patterns
• Inheritance
• Composition
• JVISION UML Diagrams
• Visual SlickEdit Project Files
• How a Factory Works
• Sample Code
• The Two Subclasses
• Using the Factory
• Thought Questions
• Programs on the CD-ROM
• The Swimmer Class
• The Event Classes
• Circle Seeding
• Our Seeding Program
• Other Factories
• Thought Question
• A GardenMaker Factory
• How the User Interface Works
• Adding More Classes
• Creating a Singleton Using a Static Method
• Exceptions and Instances
• Throwing an Exception
• Creating an Instance of the Class
• Providing a Global Point of Access to a Singleton Pattern
• Building a CommPortManager
• An Investment Tracker
• Calling the Builders
• The List Box Builder
• The Check Box Builder
• Cloning in Java
• Using the Prototype
• Using the Prototype Pattern
• Prototype Managers
• Cloning Using Serialization
• Summary of Creational Patterns
• Moving Data between Lists
• The Object Adapter
• The Class Adapter
• Two-Way Adapters
• Pluggable Adapters
• The Class Diagram
• Extending the Bridge
• Java Beans as Bridges
• An Implementation of a Composite
• Computing Salaries
• The Employee Classes
• The Boss Class
• Building the Employee Tree
• Self-Promotion
• Doubly Linked List
• Consequences of the Composite Pattern
• A Simple Composite
• Composites in Java
• Decorating a CoolButton
• Using a Decorator
• Decorating Borders in Java
• Nonvisual Decorators
• Decorators, Adapters, and Composites
• Building the Façade Classes
• Consequences of the Façade Pattern
• Selecting a Folder
• Flyweight Uses in Java
• Sharable Objects
• Copy-on-Write
• Enterprise Java Beans
• Summary of Structural Patterns
• Applicability
• The List Boxes
• Receiving the Help Command
• A Chain or a Tree?
• Kinds of Requests
• Examples in Java
• Command Objects
• Building Command Objects
• The Command Pattern
• The Command Pattern in the Java Language
• Anonymous Inner Classes
• Simple Report Example
• Interpreting the Language
• Objects Used in Parsing
• Reducing the Parsed Stack
• The Syntax Tree
• Enumerations in Java
• The Filtered Enumeration
• Consequences of the Iterator Pattern
• Composites and Iterators
• An Example System
• Interactions between Controls
• Initialization of the System
• Mediators and Command Objects
• Consequences of the Mediator Pattern
• Single Interface Mediators
• Implementation Issues
• Implementation
• Watching Colors Change
• The Message to the Media
• The JList as an Observer
• The MVC Architecture as an Observer
• The Observer Interface and Observable Class
• Switching between States
• How the Mediator Interacts with the StateManager
• State Transitions
• Mediators and the God Class
• The Context Class
• The Program Commands
• The Line and Bar Graph Strategies
• Drawing Plots in Java
• Kinds of Methods in a Template Class
• AbstractTableModel
• AbstractBorder
• AbstractListModel
• Drawing a Standard Triangle
• Drawing an Isoceles Triangle
• The Triangle Drawing Program
• Templates and Callbacks
• When to Use the Visitor Pattern
• Visiting the Classes
• Visiting Several Classes
• Bosses Are Employees, Too
• Catch-All Operations Using Visitors
• Double Dispatching
• Traversing a Series of Classes
• Installing and Using Swing
• Ideas behind Swing
• The Swing Class Hierarchy
• Setting the Look and Feel
• Setting the Window Close Box
• Making a JxFrame Class
• A Simple Two-Button Program
• More on JButton
• Radio Buttons
• The JToolBar
• JToggleButton
• A Sample Button Program
• Action Objects
• Design Patterns in the Action Object
• List Selections and Events
• Changing a List Display Dynamically
• A Sorted JList with a ListModel
• Sorting More-Complicated Objects
• Getting Database Keys
• Adding Pictures in List Boxes
• A Simple JTable Program
• Cell Renderers
• Rendering Other Kinds of Classes
• Selecting Cells in a Table
• Patterns Used in This Image Table
• The TreeModel Interface
• 34. Sandy and the Mediator
• 35. Herb's Text Processing Tangle
• 36. Mary's Dilemma
• Bibliography

Product information

• Title: Java™ Design Patterns: A Tutorial
• Author(s): James W. Cooper
• Release date: February 2000
• Publisher(s): Addison-Wesley Professional
• ISBN: 9780201485394

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Posted on Mar 26, 2023

From Problems to Solutions: Understanding Design Patterns

An interesting fact in software development is that you are almost always not the first person to face a specific problem. The problem may have been occurring since the dawn of software development. In this blog post, we are going to take a look at design patterns, why we need them, and an overview of some of the design patterns.

Design patterns are a systematic approach for addressing recurring problems that programmers face. They describe a universal standard to solve those problems. The provided solution is in such a way that it can be reused despite the specific domain of the problem at hand. Therefore, next time you face a programming problem, beware that you don’t have to rediscover the solution from scratch.

Why Design Patterns?

• Experience makes one perfect: You, as a developer, have to first understand existing good designs, their application area, and trade-offs so that you can reuse them or develop your own.
• Shared language of design: Design patterns provide a common vocabulary for programmers. This helps to effectively communicate with one another, reducing misunderstandings and increasing productivity.
• Better scalability: Design patterns help to make systems more scalable, allowing them to grow and adapt as needs change.
• Improved code structure: Design patterns help to structure code in a consistent and organized manner, making it easier to understand and maintain. This can lead to improved code quality and reduced development time.

Elements of a Design Pattern

• Name - is a handle used to describe the design issue. It is necessary because it gives a common ground of understanding among programmers.
• Problem - is used to describe when the pattern can be used.
• Solution - is a template that describes elements and their relationships without providing implementation detail.
• Consequence - is used to describe the trade-offs of the pattern. Examples: time and space trade-offs, flexibility, extensibility, etc.

Types of Design Patterns

Design Patterns: Elements of Reusable Object-Oriented Software, commonly known as the Gang of Four , is a cornerstone in software engineering that brought design patterns into the mainstream. The book breaks down 23 different design patterns that fall into the following 3 categories:

• Creational - They are concerned with object-creation techniques.
• Structural - They are concerned with the combination of objects and classes in a flexible and effective way.
• Behavioral - They are concerned with the interaction between classes and objects and their responsibility.

Introduction to Six Design Patterns

The following section is going to provide an overview of 6 software design patterns (2 from each category) and how they can be used to solve common programming challenges.

1. Singleton

The Singleton pattern is a type of creational pattern that ensures the creation of a single instance of a class and provides a global point of access to that instance.

Suppose you are building a Database connection class:

• You don’t want to create a separate DB instance for every object that needs to access it. If you do so, it’s going to be costly. This concept goes for other shared resources like files as well.
• You want to provide global and easy access to that instance. Similar to global variables, the Singleton pattern gives you access to an object from anywhere in your program.

There are various ways of implementing a Singleton pattern. However, all of the implementations share the following basic concepts:

• Unlike a regular constructor that creates a new object whenever you call it, a Singleton class has a private constructor. A private constructor prevents other objects from instantiating an object of the Singleton class.
• A private instance variable.
• A static method that returns a reference to that instance. This static method basically acts as a constructor and calls the private constructor under the hood to either create an object (when requested for the first time) or return the cached one (for subsequent requests).

2. Factory Method

The factory method is also another type of creational pattern that provides an interface for creating objects but lets the subclasses decide which class to instantiate. In other words, the subclasses can change the type of objects that will be created.

Suppose you are creating a logging framework by using the factory method design pattern to log different kinds of messages including, but not limited to, error messages, warning messages, and informational messages. In this use case:

• The logger can provide a factory method for creating different types of messages.
• The client code that uses this logger doesn’t need to know the exact implementation of the messages, it only needs to know the interface for creating them.
• You can provide flexibility. If the object creation process changes, for example, a new class is added, only the factory method needs to be updated. No need to change the client code that uses those objects.
• Replace direct object construction calls with calls to a specific factory method.
• Create an object by calling the factory method, rather than invoking a constructor.

Adapter is a type of structural design pattern that allows objects with incompatible interfaces to work together. It lies at an intermediary position between the client and the interface that the client wants to communicate with.

Suppose you are building a video format conversion tool by following the Adapter design pattern:

• Your software needs to adapt between different video formats, such as MP4, AVI, and FLV. It should be able to take a video in any format as input and convert it to the desired format. Communication between the target and the client happens via the adapter.
• If in the future you want to support new formats, the pattern makes it easier because the video format conversion is decoupled from the specific video formats.
• Define an adapter class that converts the incompatible interface of a class (adaptee) into another interface (target) clients expect.
• Use this adapter to work with (reuse) other classes that do not have the required interface.

4. Decorator

Decorator is another type of structural design pattern that lets you attach new behaviors to objects without affecting the behavior of other objects from the same class. Even though inheritance can be used to alter the behavior of an object statically, a decorator helps you alter it dynamically as well.

Suppose you are building a text editor:

• The text editor can be your concrete component, and a spell checker can be your decorator.
• The spell checker class should have the same interface as the text editor class so that it can be used as a decorator.
• The spell-checker class can wrap the text editor and add spell-checking functionality to the editor. This makes it flexible to add new functionalities to the editor in the future.
• Define an interface for the components and decorators.
• Implement concrete components and concrete decorators that conform to the interface.
• The decorators should have a reference to the components they decorate and delegate to the components to perform basic behaviors. The decorators will have methods that add new behavior.
• Use the decorators to wrap the components in the desired order to attach the additional behavior.

Note - Adapter provides a different interface to its subject. Decorator provides an enhanced interface.

5. Observer

Observer is a behavioral design pattern that defines a one-to-many dependency between objects so that when one object, named the subject , changes its state, all of its dependents, named observers , are notified and updated automatically.

Suppose you are building a stock market website:

• You can use the Observer pattern to update your users with the latest stock prices.
• The website is the subject, and its users are the observers. Whenever the stock prices change, the website notifies its users.
• Define a Subject object.
• Define Observer objects. An observer can subscribe or unsubscribe to a stream of events.
• The relationship between the subject and observers shouldn’t be tightly coupled.
• When the subject’s state changes, all registered observers are notified and updated.

6. Strategy

Strategy is another type of behavioral design pattern that allows you to define a family of algorithms, encapsulate each one, and make them interchangeable. It enables the client to select the algorithm’s behavior at run time.

Suppose you are creating software that has sorting functionality:

• A sorting algorithm, such as quick sort, insertion sort, or bubble sort, can be selected at runtime and used to sort a collection of items.
• The sorting strategies can be easily replaced or modified, which makes it possible to add or remove functionality as needed
• Create a Strategy interface that defines common behavior.
• Create concrete strategy classes that conform to the Strategy interface. Each concrete strategy is a variation of the algorithm.
• Create a context class that defines a reference to a sorting strategy object.
• The context class should have a method that accepts a strategy object and invokes the selected strategy.

In conclusion, having a solid grasp of design patterns is crucial, especially as you advance your career in software engineering.

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Top comments (5)

Templates let you quickly answer FAQs or store snippets for re-use.

• Location Berlin
• Work Developer
• Joined Feb 27, 2023

Very good article, thank you. Today I work for 8 years as a developer in industry, but most time self-taught (no university or else). I know many patterns, how to create and use them, but I couldn't name them, except a few of them. In a job interview I were ask, what design patterns I know, and the only two I could name without thinking was Singletons and Factories, lol.

Your article helps me to make better research, because I can put the right words in my search engine of desire to find, what I want to archieve :) (easier as search for "technique to let react one object on state changing of another"...)

• Location Ethiopia
• Work Django Developer and Technical Writer
• Joined Jun 10, 2020

Thanks a lot. I am glad you found it useful :)

It is indeed important to understand and name at least the most common ones.

• Joined Dec 3, 2019

No one should ever go without. Thank you for spreading this around the world.

• Joined Mar 31, 2023

hello I'm a front-end engineer. I really like your article and would like to translate it, is it possible?

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Java: Learning Design Patterns using a Problem Solution approach

Are there any existing books/blogs out there that teaches design patterns using a problem solution approach?An example I have in mind is in your application you have tons of code that are repetitive. One way to solve is using Template Method design pattern or something like that. Basically you are presented with a problematic code and its going to get refactored with a design pattern.

What Is Design Patterns In Java And How To Use It

Design patterns in Java offer structured solutions to common coding challenges. In this article, we'll explore various patterns, their practical applications, and best practices. Ideal for developers looking to enhance their Java coding skills and build efficient software.

KEY INSIGHTS

• Design patterns in Java are tailored to specific project needs, not one-size-fits-all.
• Three main categories: Creational, Structural, and Behavioral , each addressing unique software aspects.
• Utilizing design patterns enhances code maintainability and reduces complexity .
• Practical application demonstrates their versatility in real-world scenarios .

Design patterns in Java offer solutions to common coding challenges. They provide a shared language for developers, making code more efficient and maintainable. By understanding these patterns, you can enhance your programming skills and create more robust applications.

Understanding The Basics Of Design Patterns

Types of design patterns in java, benefits of using design patterns, common challenges and solutions, practical examples of design patterns, best practices when implementing patterns, frequently asked questions.

Design patterns are reusable solutions to common problems encountered in software design. They aren't templates, but rather general guidelines that can be adapted to various situations. By using design patterns, developers can avoid reinventing the wheel, leading to more efficient and maintainable code.

Why Use Design Patterns?

Categories of design patterns, common misconceptions.

Design patterns provide a standard terminology and are a specific way to solve recurring design problems. This makes the software design more modular, facilitating communication between developers. When a developer says, "Let's use the Singleton pattern here," everyone on the team knows what they're talking about.

There are three main categories of design patterns:

• Creational Patterns : Deal with object creation mechanisms, trying to create objects in a manner suitable to the situation.
• Structural Patterns : Concerned with how classes and objects can be composed, to form larger structures.
• Behavioral Patterns : Deal with object collaboration and the delegation of responsibilities among objects.

For instance, the Singleton pattern is a creational pattern that ensures a class has only one instance and provides a global point to access it.

It's essential to understand that design patterns are not a one-size-fits-all solution. They should be applied only when they genuinely fit the problem at hand. Overusing or misusing patterns can lead to unnecessarily complex and confusing code. Always consider the specific needs of your project before deciding to implement a particular pattern.

In Java, design patterns are broadly categorized into three main types. Each type addresses a specific set of challenges and has its own set of patterns that fall under it.

Creational Patterns

Structural patterns, behavioral patterns.

Creational patterns deal with the process of object creation. They abstract the instantiation process, making the system independent of how its objects are created, composed, and represented. Some common creational patterns include:

• Singleton : Ensures a class has only one instance and provides a global point to access it.
• Factory Method : Provides an interface for creating instances of a class, with its subclasses deciding which class to instantiate.

Example of the Factory Method pattern:

Structural patterns focus on how classes and objects are composed to form larger structures. They ensure that changes in one part of a system don't affect other parts. Common structural patterns include:

• Adapter : Allows classes with incompatible interfaces to work together.
• Composite : Composes objects into tree structures to represent part-whole hierarchies.

Example of the Adapter pattern:

Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. They describe patterns of communication between objects. Some notable behavioral patterns are:

• Observer : Defines a dependency between objects so that when one object changes state, all its dependents are notified.
• Strategy : Defines a family of algorithms, encapsulates each one, and makes them interchangeable.

Example of the Observer pattern:

In conclusion, understanding these patterns and their appropriate use cases can significantly enhance the quality and maintainability of Java applications.

Design patterns offer a multitude of advantages when developing software. Their primary purpose is to provide solutions to recurring problems, but the benefits extend beyond just problem-solving.

Improved Code Reusability

Enhanced code maintainability, efficient problem-solving, clearer communication, scalability and performance.

One of the most significant advantages of using design patterns is code reusability . Patterns provide a blueprint for solving common problems, which means you can use the same solution in multiple places. This reduces redundancy and ensures consistency across the application.

Design patterns promote modularity and flexibility . When patterns are implemented correctly, making changes to one part of the system doesn't adversely affect other parts. This modular approach simplifies debugging and makes the codebase easier to maintain.

Patterns represent proven solutions to common challenges. Instead of spending time trying to come up with a solution from scratch, developers can leverage existing patterns. This speeds up the development process and ensures that the solution is both efficient and reliable.

Design patterns provide a common vocabulary for developers. When discussing solutions, developers can refer to patterns by name, ensuring that everyone has a clear understanding of the proposed approach. For instance, suggesting the use of the "Observer pattern" immediately conveys a specific design approach to the team.

Many design patterns, especially structural and behavioral ones, are geared towards creating scalable systems. By following these patterns, developers can ensure that the system can handle growth without significant performance degradation.

For example, the Proxy pattern can be used to control access to an object, allowing for lazy initialization and optimization:

Incorporating design patterns into software development practices can lead to more robust, efficient, and maintainable applications. They provide a foundation upon which to build, ensuring that the resulting software is of the highest quality.

In the realm of software development, especially when working with design patterns, developers often encounter specific challenges. Recognizing these challenges and knowing the solutions can streamline the development process.

Challenge: Code Duplication

Challenge: tight coupling, challenge: complex object creation.

One of the most frequent issues developers face is code duplication . Repeated code can make the system bloated, harder to maintain, and prone to errors.

Solution : Implementing the Prototype pattern can help in cloning objects, reducing the need for duplicating code.

Example using the Prototype pattern :

Tight coupling between classes can make the system rigid, less modular, and harder to refactor.

Solution : The Adapter pattern can be used to bridge the gap between two incompatible interfaces, promoting loose coupling.

Creating complex objects step-by-step can be cumbersome and error-prone.

Solution : The Builder pattern provides a solution by allowing the creation of different types of objects using the same construction process.

Example of the Builder pattern :

Understanding these common challenges and their respective solutions can help developers navigate the complexities of software design more efficiently. By leveraging design patterns, these challenges can be addressed head-on, leading to cleaner and more effective code.

Design patterns are best understood when seen in action. Let's delve into some practical examples that demonstrate the application of various design patterns in real-world scenarios.

Singleton: Database Connection

Observer: notification system, strategy: payment methods, factory method: ui elements.

A common use case for the Singleton pattern is when creating a database connection. It's crucial to ensure that there's only one active connection to the database to avoid unnecessary overhead.

The Observer pattern is often used in notification systems where multiple subscribers need updates when a particular event occurs.

Online shopping platforms often provide multiple payment methods. The Strategy pattern can be employed to switch between different payment strategies seamlessly.

Different operating systems have distinct UI elements. The Factory Method pattern can be used to create platform-specific UI components.

These examples showcase the versatility and practicality of design patterns. By understanding and implementing these patterns, developers can address common challenges efficiently and produce high-quality software.

Design patterns provide a foundation for building robust and maintainable software. However, their effectiveness largely depends on how they're implemented. Here are some best practices to consider when working with design patterns.

Understand The Problem First

Avoid overengineering, stay consistent, favor composition over inheritance, keep it simple, continuously refactor, document your decisions.

Before jumping into a pattern, it's essential to understand the problem you're trying to solve. Patterns are tools, not goals. Ensure that the pattern you choose aligns with the specific challenge at hand.

While patterns offer structured solutions, it's crucial to avoid overengineering . Not every problem requires a pattern. Sometimes, a simple, straightforward solution is more appropriate and efficient.

When implementing a pattern, consistency is key . Ensure that the pattern is applied uniformly across the codebase. This makes the code easier to read, understand, and maintain.

In many scenarios, it's beneficial to favor composition over inheritance . This approach provides greater flexibility and reduces the risk of tight coupling. The Decorator pattern is an excellent example of this principle.

While design patterns can address complex challenges, the implementation should remain as simple and intuitive as possible. Avoid adding unnecessary layers or components that don't directly contribute to the solution.

As software evolves, the initial patterns might no longer be the best fit. It's essential to continuously refactor and reassess the patterns in use. This ensures that the software remains adaptable and maintainable.

When implementing a pattern, always document your decisions . This includes the reason for choosing a particular pattern, its benefits, and any trade-offs considered. Proper documentation aids future developers in understanding and maintaining the system.

What are design patterns?

Design patterns are standardized solutions to recurring problems encountered in software design. They serve as blueprints that can be adapted to various situations, promoting best practices in coding.

Why are design patterns important?

Design patterns provide a shared language for developers, making code more efficient, maintainable, and scalable. They also offer proven solutions, reducing the need to "reinvent the wheel" for common challenges.

Are design patterns language-specific?

No, design patterns are language-agnostic. While the implementation might vary, the underlying concept remains consistent across different programming languages.

Can I use multiple design patterns in a single project?

Absolutely! In fact, it's common for multiple patterns to be used in a single project. The key is to ensure that each pattern is applied where it's most appropriate and beneficial.

Do design patterns guarantee optimal solutions?

While design patterns provide proven solutions to common problems, they don't always guarantee the most optimal solution for every scenario. It's essential to assess the specific needs of your project before implementing a pattern.

Let’s test your knowledge!

Which design pattern provides a way to access the elements of a collection object in sequential manner without any need to know its underlying representation?

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Table of Contents

Why are design patterns in java needed, when should we use design patterns in java, categorization of design patterns in java, how to create design patterns in java, use of design patterns in real life cases., bottom line, everything you need to know about the design patterns in java.

Broadly speaking, Design Patterns are a set of general solutions to commonly-faced problems in Object-Oriented Software Development . These are well-proven solutions for specific problems that have been derived over years of trial-and-error experiments by experienced software developers. 

It’s important to understand that these are not mere solutions to the problems but template-solutions that can be customized according to your problem statements. Each and every design pattern acts as a blueprint that can be used to solve a design problem and can be embedded in any software module at any stage of development. They are language-independent strategies that will allow you to make your code flexible, maintainable, and reusable. 

In this guide, you will explore every aspect of Design Patterns in Java that you would need to understand and dive deep into the world of Object-Oriented Software Design and Development. Beginning with a basic introduction including the Whys and Hows of Design Patterns in Java, you will gradually move on to understanding the patterns with real-life examples.

Here's How to Land a Top Software Developer Job

For experienced Software Developers, Design Patterns are a common vocabulary that they use to devise general solutions to design problems. But why are we talking about Design Patterns specifically for Java? 

The reason is that Java internally follows design patterns. You can find Observer Patterns throughout Java Swing. The input and output stream readers use Adapter patterns. Hence, in order to get your hands dirty in Object-Oriented Java, it’s very essential that you become well-versed with Design Patterns in Java.

Design Patterns represent a general systematic solution to recurring design problems. Let’s discuss a few advantages of Design Patterns in Java;

• Since they are template solutions, they can be reused
• They are testified, well-proven solutions devised by experienced software developers over the years
• They inculcate transparency in the application design
• Communication of solutions among developers becomes easy with the use of design patterns as they are a general solution

Design Patterns helps developers devise a well-defined solution with the use of well-packed information that they bring to the table. It helps them to find a connection between the requirements of the project and an existing solution to similar problems. Hence, using design patterns during the analysis and design phase during the software development life cycle process would be a wise decision.

In broad terms, we can categorize design patterns into the following parts:

• Core Java Design Patterns.
• Creational Design Patterns
• Structural Design Patterns
• Behavioural Design Patterns
• J-EE Design Patterns

Creational Design Patterns are used preferably during the class initiation process and are mainly concerned with the process of creating objects. They can be further classified into - 

• Factory Pattern - Here, we define an interface or an abstract class but we let the subclasses decide which class can instantiate it. 
• Abstract Factory Pattern - It just defines an abstract class or an interface without specifying concrete sub-classes. In other words, it allows a class to return a factory of classes and thus is one level higher than Factory Pattern.
• Singleton Pattern - It allows you to define a class with a single instance accessible globally by all classes. For example, you can use a static keyword to create variables and methods accessible by all classes.
• Prototype Pattern - It says that if the cost of creating new objects is quite expensive, we can clone objects and customize them as per our requirements.
• Builder Pattern - It allows you to use a step-by-step approach for building complex objects using simple ones. It allows you to clearly define the construction of your objects.
• Object Pool Pattern - Object pool is a type of container that contains several objects. If we take an object from the pool, we can’t use it again before it is put back. It allows you to reuse expensive objects.

Structural Design Patterns typically deal with the relationship between the classes and objects by identifying them to simplify complex structures. They can be further classified into - 

• Facade Pattern - It helps to hide the complex functionalities of the subsystem by bringing all the complex interfaces under one umbrella of a simple and higher-level interface.
• Bridge Pattern - It allows you to separate the functional abstraction by decoupling it from the implementation so that both of them can vary independently.
• Composite Pattern - It helps you to create a hierarchy of classes that contains complex and primitive objects. It allows flexibility to the structure and makes it easier to add more functionalities.
• Decorator Pattern - It uses composition as an alternative to inheritance to add or extend the functionalities during the runtime.
• Adapter Pattern - It says that we can just mold or convert the interface of an existing class to create a new interface according to the client’s requirements.
• Flyweight Pattern - It's a simple approach that tells us to use existing objects until no matching object is found, in which case, we can create a new one.
• Proxy Pattern - It means that we can create another object to carry out a similar function which will help us to hide important information from original objects.

Behavioral Patterns are usually concerned with the interaction between the objects. It makes sure that the objects can easily communicate with each other while being loosely coupled. They can be further classified into - 

• Chain Of Responsibility Pattern - It says that each request should be passed to a chain of objects so that if one fails to act, it is automatically passed to the other.
• Strategy Pattern - It encapsulates the functionality of each class within itself making it easier to extend new behavior.
• Interpreter Pattern - It can be used for parsing expressions defined with a set of rules called grammar.
• Iterator Pattern - It is used to access elements of an aggregate object, sequentially.
• Mediator Pattern - It provides a mediator class to handle complex communications between classes.
• Memento Pattern - It stresses on saving the current state as it is changed, to revert back in case of failure.
• Command Pattern - It is used to separate an object which invokes an operation from the one which actually performs it.
• State Pattern - It stresses on creating objects for each state.
• Observer Pattern - It defines a one-to-one dependency between objects so that they can get updated easily in case of a change.
• Template Pattern - It avoids duplication by creating a separate class for common functionalities.

The J-EE Design Patterns specifically focuses on providing solutions to the enterprise edition-based frameworks such as Spring, etc. They can be further classified into types such as MVC Design Pattern, Dependency Injection Patterns, etc. 

Java provides you with a ton of functionalities and implementations to create your desired Design Pattern. Based on your requirement, you can leverage the use of interfaces, abstract methods or classes and structure them accordingly. It even provides you with functional interfaces to declare and define your classes.

Consider the following scenario. To implement a network interface, you can create an abstract class called the cellular network with three abstract methods for different types of pay per minutes plans. You can also create different subclasses describing the various different network plans. Depending upon the requirements, each network plan can define any of the abstract methods of the cellular network class. This is an example of a Factory Design Pattern.

Let’s take another example. Suppose there are several users who have registered on a website that displays cricket scores. Now, several of them might want to know when Kohli scores the next century. So, they register for such an event which notifies them as soon as he creates a century. In this case, we can see an observer pattern where the users are observers for an event.

In this Design Patterns in Java article, we discussed how we can leverage Design Patterns in Java to simplify the solutions by reusing standard template-solutions to already existing problems. We discussed the need for design patterns in Java, the different categories, and how and when to use them with some real-life scenarios.

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Complete Guide to Design Patterns

• Types of Software Design Patterns

1. Creational Design Patterns

• Creational Design Patterns

Types of Creational Patterns

• Factory method Design Pattern
• Abstract Factory Pattern
• Singleton Method Design Pattern in JavaScript
• Singleton Method Design Pattern
• Prototype Design Pattern
• Builder Design Pattern

2. Structural Design Patterns

• Structural Design Patterns

Types of Structural Patterns

• Adapter Design Pattern
• Bridge Design Pattern
• Composite Method | Software Design Pattern
• Decorator Design Pattern
• Facade Method Design Pattern
• Flyweight Design Pattern
• Proxy Design Pattern

3. Behvioural Design Patterns

• Behavioral Design Patterns

3. Types of Behvioural Patterns

• Chain of Responsibility Design Pattern
• Command Design Pattern
• Interpreter Design Pattern
• Mediator design pattern
• Memento Design Pattern
• Observer Design Pattern
• State Design Pattern
• Strategy Design Pattern
• Template Method Design Pattern
• Visitor design pattern
• Top Design Patterns Interview Questions

Software Design Pattern in Different Programming Languages

• Design Patterns in Java | Java Design Patterns Tutorial
• Python Design Patterns
• Modern C++ Design Patterns Tutorial
• JavaScript Design Patterns

Software Design Pattern Books

• Gang of Four (GOF) Design Patterns
• Design Patterns Gamma

Software Design Pattern in Development

• Design Patterns: Understand The Importance With Real Life Examples
• Latest Design Patterns for Web Development
• Design Patterns for Enhancing User Experience
• Design Patterns for Mobile Development
• Design Patterns for Relational Databases

Some other Popular Design Patterns

• Introduction to Pattern Designing
• Catalog of Design Patterns
• Front Controller Design Pattern
• Java Dependency Injection (DI) Design Pattern
• Caching Design Pattern
• Repository Design Pattern

Design patterns help in addressing the recurring issues in software design and provide a shared vocabulary for developers to communicate and collaborate effectively. They have been documented and refined over time by experienced developers and software architects.

Important Topics for Guide to Design Patterns

• What are Design Patterns?

Types of Design Patterns

• Use cases of Design Patterns
• Applications of Design Patterns
• When to avoid the use of Design Patterns ?
• How to learn Design Patterns?

1. What are Design Patterns?

Design patterns are basically defined as reusable solutions to the common problems that arise during software design and development. They are general templates or best practices that guide developers in creating well-structured, maintainable, and efficient code.

2. Types of Design Patterns

Basically, there are several types of design patterns that are commonly used in software development. These patterns can be categorized into three main groups:

• The Singleton method or Singleton Design pattern is one of the simplest design patterns. It ensures a class only has one instance, and provides a global point of access to it.
• The Factory Method pattern is used to create objects without specifying the exact class of object that will be created. This pattern is useful when you need to decouple the creation of an object from its implementation.
• Abstract Factory pattern is almost similar to Factory Pattern and is considered as another layer of abstraction over factory pattern. Abstract Factory patterns work around a super-factory which creates other factories.
• Builder pattern aims to “Separate the construction of a complex object from its representation so that the same construction process can create different representations.” It is used to construct a complex object step by step and the final step will return the object.
• Prototype allows us to hide the complexity of making new instances from the client.
• The concept is to copy an existing object rather than creating a new instance from scratch, something that may include costly operations. The existing object acts as a prototype and contains the state of the object.

• The adapter pattern convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces
• The bridge pattern allows the Abstraction and the Implementation to be developed independently and the client code can access only the Abstraction part without being concerned about the Implementation part
• Composite pattern is a partitioning design pattern and describes a group of objects that is treated the same way as a single instance of the same type of object. The intent of a composite is to “compose” objects into tree structures to represent part-whole hierarchies.
• It allows us to dynamically add functionality and behavior to an object without affecting the behavior of other existing objects within the same class.
• We use inheritance to extend the behavior of the class. This takes place at compile-time, and all the instances of that class get the extended behavior.
• Facade Method Design Pattern provides a unified interface to a set of interfaces in a subsystem. Facade defines a high-level interface that makes the subsystem easier to use.
• Proxy means ‘in place of’, representing’ or ‘in place of’ or ‘on behalf of’ are literal meanings of proxy and that directly explains Proxy Design Pattern.
• Proxies are also called surrogates, handles, and wrappers. They are closely related in structure, but not purpose, to Adapters and Decorators.
• This pattern provides ways to decrease object count thus improving application required objects structure. Flyweight pattern is used when we need to create a large number of similar objects

3. Behavioral Design Patterns

• It defines a one-to-many dependency between objects, so that when one object (the subject) changes its state, all its dependents (observers) are notified and updated automatically.
• that allows the behavior of an object to be selected at runtime. It is one of the Gang of Four (GoF) design patterns, which are widely used in object-oriented programming.
• The Strategy pattern is based on the idea of encapsulating a family of algorithms into separate classes that implement a common interface.
• The Command Pattern is a behavioral design pattern that turns a request into a stand-alone object, containing all the information about the request. This object can be passed around, stored, and executed at a later time
• Chain of responsibility pattern is used to achieve loose coupling in software design where a request from the client is passed to a chain of objects to process them.
• Later, the object in the chain will decide themselves who will be processing the request and whether the request is required to be sent to the next object in the chain or not.
• A state design pattern is used when an Object changes its behavior based on its internal state. If we have to change the behavior of an object based on its state, we can have a state variable in the Object and use the if-else condition block to perform different actions based on the state.
• Template method design pattern is to define an algorithm as a skeleton of operations and leave the details to be implemented by the child classes. The overall structure and sequence of the algorithm are preserved by the parent class.
• It is used when we have to perform an operation on a group of similar kind of Objects. With the help of visitor pattern, we can move the operational logic from the objects to another class.
• Interpreter pattern is used to defines a grammatical representation for a language and provides an interpreter to deal with this grammar.
• It enables decoupling of objects by introducing a layer in between so that the interaction between objects happen via the layer.
• It is used to restore state of an object to a previous state. As your application is progressing, you may want to save checkpoints in your application and restore back to those checkpoints later.
• Intent of Memento Design pattern is without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.

3. Use cases of Design Patterns

Use Cases of Design Pattern

Design patterns are a valuable tool in software development, and they offer various benefits and uses, some of them are explained below :

• Design patterns help organize code in a structured and consistent way. This makes it easier to maintain, update, and extend the codebase. Developers familiar with the patterns can quickly understand and work on the code.
• Design patterns encapsulate solutions to recurring design problems. By using these patterns, we can create reusable templates for solving specific problems in different parts of your application.
• Complex software problems can be broken down into smaller, more manageable components using design patterns. This simplifies development by addressing one problem at a time and, in turn, makes the code more maintainable.
• Design patterns, particularly structural patterns, allow us to create a flexible and extensible architecture, making it easier to add new features or components.
• Code designed with patterns in mind is often more modular and easier to test. we can write unit tests for individual components or classes, leading to more reliable and robust software.
• Design patterns are not tied to a specific programming language or platform. They are general guidelines that can be applied across different technologies, making it easier to adapt your code to different environments.
• Design patterns provide a common language and a shared understanding among team members. They enable developers to communicate effectively and collaborate on software projects by referring to well-known design solutions.

4. Applications of Design Patterns

Basically, design patterns should be used when they provide a clear and effective solution to a recurring problem in our software design. Here are some situations where we can use the design patterns.

Application of Design Patterns

• Collaboration: Suppose i f we are working in a team or on a project with multiple developers, so there design patterns can facilitate collaboration by providing a common language and shared understanding of how to address specific design challenges.
• Recurring Problem: Suppose if we encounter a design problem that we have seen in different forms in multiple projects or if it’s a well-known and documented problem in software development, it’s a good indicator that a design pattern might be useful.
• Maintainability and Extensibility: When we wants to create code that is easy to maintain, extend, and modify over time, design patterns can help by providing a structured approach to problem-solving.
• Cross-Platform Development: When we need to create code that works on different platforms or with various technologies, design patterns provide a platform-agnostic way to solve common problems.
• Testing and Debugging: Design patterns can make our code more modular and testable, leading to improved testing and debugging processes.
• Design Review and Planning: During the design and planning phase of a project, we can proactively consider the use of design patterns to solve anticipated design challenges.

It’s important to note that design patterns are not a one-size-fits-all solution. They should be used judiciously, and not all problems require the application of a design pattern. Always consider the specific context and requirements of your project when deciding whether to use a design pattern.

5. When to avoid the use of Design Patterns ?

There are situations where it’s best to avoid or be cautious about the use of design patterns . Here are some cases when we have to avoid the use of design patterns:

• Over-engineering: Don’t use design patterns just for the sake of using them. If a simpler and more straightforward solution can solve the problem effectively, choosing a design pattern can introduce unnecessary complexity.
• Lack of Relevance: If a design pattern doesn’t directly address the problem there we have try to solve, it’s better not to force-fit it into our code. Instead, consider other, more suitable solutions.
• Tight Deadlines: When we have tight project deadlines, spending extra time implementing design patterns might not be practical. Prioritize meeting project milestones over applying design patterns that might not be immediately necessary.
• Misapplication: Applying design patterns incorrectly can lead to code that is harder to understand and maintain. It’s crucial to understand the patterns thoroughly and apply them in the right context.
• Resource Constraints: In resource-constrained environments, such as embedded systems or performance-critical applications, design patterns might introduce overhead that affects system efficiency. In these cases, consider more lightweight solutions.

In general, the decision to use or avoid design patterns should be based on a careful consideration of the specific context, the problem we are trying to solve, and the skill level and familiarity of our team with design patterns.

6. How to learn Design Patterns?

Learning design patterns is a valuable skill for software developers and architects. Here are some key points where we can focus to learn Design Pattern.

How to learn Design Patterns

6.1. Understand Object-Oriented Programming (OOP) Principles

Before diving into design patterns, ensure we have a solid understanding of OOP principles like encapsulation, inheritance, polymorphism, and abstraction, as design patterns often build upon these concepts.

6.2. Choose a Good Resource

Select a reputable book, online course, or tutorial that focuses on design patterns. Some well-regarded resources include “Design Patterns: Elements of Reusable Object-Oriented Software” by the Gang of Four (GoF), and online courses on platforms like Coursera, edX, or Pluralsight.

6.3. Start with the Basics

Begin with the fundamental design patterns. The Gang of Four book, for instance, covers 23 classic design patterns categorized into creational, structural, and behavioral patterns. Start with creational patterns like Singleton, Factory Method, and Abstract Factory.

6.4. Implement and Practice

Don’t just read about design patterns; implement them in our own code. Create small projects or code examples to apply the patterns that we learn. Practical experience is crucial for understanding how and when to use them.

6.5. Understand Context and Problem Solving

Focus on understanding the problem context that each design pattern addresses. Why was the pattern created, and in what situations is it most useful? Recognize the problems that each pattern aims to solve.

6.6. Collaborate and Discuss

Join programming forums, communities, or meetups where you can discuss design patterns with fellow developers. Peer discussions can provide different perspectives and insights.

6.7. Practice Patterns in Real Projects

Apply design patterns in real-world projects whenever we have the opportunity. This hands-on experience will help you gain a deeper understanding of their practical benefits and challenges.

6.8. Stay Updated

Keep up with the latest developments and trends in design patterns, as new patterns may emerge over time. The software development field is constantly evolving.

7. Conclusion

Always remember that the learning design patterns is an ongoing process, and it’s important to continuously practice and refine our knowledge. Over time, we will definately develop the ability to recognize when and how to apply design patterns effectively in our software design and development efforts.

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• Design Patterns

Last Updated on February 2nd, 2024

In this article, we will discuss about all GoF ( Gang of Four ) design patterns. The 23 design patterns by GOF are well known, and more are to be discovered on the way. We will start with some basic concepts that are also expected from a developer. Let’s start with our topic ‘Java Design Patterns’ and other concepts related to it.

Table of Contents

What is a Design Pattern?

In a software industry, Design Pattern is a description or guideline to solve a problem that occurs repeatedly while developing a software Application. Some people in the industry also call it a template as it solves the problems while developing the application. When we talk about these types of solutions to recurring problems in Java language, they become ‘Design Patterns in Java’ or ‘Java Design Patterns’.

Why these Design Patterns named as GoF?

It was  21 October 1994 , when four authors Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, with a foreword by Grady Booch, published a book titled as  Design Patterns  –  Elements of Reusable Object-Oriented Software  which launched the concept of  Design Pattern in Software design. These four authors are altogether known as the Gang of Four  ( GoF ). Since then, these are called GoF Design Patterns.

Why do we need to learn Design Patterns?

Design Patterns help us to solve a programming problem during project development. Moreover, Design Patterns will help us in fixing performance and memory related issues. If you have strong concepts in Java Design Patterns, your confidence level will also be high because design patterns itself provide you solutions for many common use cases. Also, you will never feel low while developing a project. Even you will be able to suggest a better solution to a problem instead of the existing one.

If you are strong in design patterns, you may complete your development earlier than the predicted time and number of bugs will also be very small. In contrast, if you are weak in design patterns, you will face a lot of problems while developing a project, even sometimes you may need to modify your code from the beginning, that can result in discomfort & irritation.

Additionally, if you are planning to apply for a job as an experienced developer, keep in mind that your interview may not be completed without having questions on Design Patterns.

Please note that Java Design Patterns are not closely related to Java Application Design. However, they help to design a better software. Particularly, they are useful to us while we think about the solution to a programming problem during project development.

What are the advantages/benefits of using Design Patterns?

1) Design Patterns help in finding the solution of a complex problem. 2) Using design patterns, we can make our code loosely-coupled. 3) Furthermore, it will have the option of reusable codes, which reduce the total development cost of the application. 4) Additionally, future developers feel the code more user-friendly. 5) It has all the standard approaches to find out solutions to the common problem of a software. 6) We can use the same pattern repeatedly in multiple projects. 7) Moreover, It helps in refactoring our code in a better way.

How are Java Design Patterns useful in your Professional Career?

If you know Design Patterns, it will lead to better code structure, better communication, and, best of all, it will save you a lot of time that you can spend on cooler things. Additionally, Design Patterns will also be useful in your Professional Career in multiple places.

♦ In Code Comments and Naming Conventions: When you’re writing code, clearly identify the patterns you’re using in the comments. Also, choose class and method names that follow any patterns. As a result, other developers who have to read your code will thank you for allowing them to quickly understand your implementation.

♥ In Architecture Documentation : When you write architectural documentation, using patterns will reduce the amount of documentation you need to write and gives the reader a clearer picture of the design.

♦ In Groups of Interested Developers: Share your knowledge as much as possible. Generally, many developers have heard about patterns, but don’t have a good understanding of what they are. Volunteer to give a brown-bag lunch on patterns or a talk at your local user group.

♥ In Project Meetings: When you meet with your team to discuss a software design, use design patterns to help stay “in the design” longer.

When to use Java Design Patterns?

We should use Design patterns in our project when we face recurring design problems or situations that can benefit from established, proven solutions. Here are some specific situations in which we should consider using java design patterns:

♦ Recurring Problems: When you find yourself facing the same design or architectural problem again and again across different projects, it’s an indication that a design pattern might be advantageous. Patterns provide a reusable blueprint for solving these problems efficiently.

♥ Complexity Management: Use design patterns to manage complexity in your software. As a project grows, it can become more and more challenging to maintain and extend. Design patterns help organize and structure code in a way that makes it easier to understand, modify, and extend.

♦ Maintainability: When you want to enhance the maintainability of your codebase, design patterns can help by encouraging separation of concerns, modularity, and clean code practices. This makes it simpler to fix bugs, add new features, and update existing functionality without causing unintended side effects.

♥ Scalability: In scenarios where your application needs to scale appropriately to handle increasing loads, design patterns can guide you in building scalable architectures. Patterns like the Observer and Strategy can be useful for handling different requirements and loads.

♦ Code Reusability: Design patterns encourage code reusability by encapsulating solutions to common problems in a reusable format. This reduces redundancy and saves development time.

♥ Legacy Code Integration: When working with legacy code that lacks a clear structure or is challenging to maintain, design patterns can help refactor and improve the codebase gradually, making it more modern and maintainable.

♦ Pattern Recognition: As you gain experience in software development, you will develop an awareness for recognizing situations where certain patterns can be applied effectively. Learning and using design patterns help improve this skill.

Click on the links below to go through the topics: Step by Step Tutorials

Links to Each Design Patterns in Detail With Examples

Creational design patterns.

• Singleton Pattern
• Factory Pattern
• Abstract Factory Pattern
• Builder Pattern
• Prototype Pattern

Structural Design Patterns

• Adapter Pattern
• Composite Pattern
• Proxy Pattern
• Flyweight Pattern
• Façade Pattern
• Bridge Pattern
• Decorator Pattern

Behavioral Design Patterns

• Template Method Pattern
• Mediator Pattern
• Chain of Responsibility Pattern
• Observer Pattern
• Strategy Pattern
• Command Pattern
• State Pattern
• Visitor Pattern
• Iterator Pattern
• Interpreter Pattern
• Memento Pattern

Each pattern has its specific use cases and may introduce complexity if applied inappropriately. Therefore, it’s crucial to understand the problem you are trying to sort out and the context in which you are working before selecting and applying a design pattern. Additionally, you should be open to modify or create custom patterns when the standard patterns don’t fit your specific needs accurately.

Design Pattern’s Cheat Sheet

In fact, our target is to make you remember the Design Patterns in Java as a vocabulary. So, now it’s time to provide the short descriptions of each design pattern so that you can memorize it easily. Below is the cheat sheet of Design patterns in Java as a one liner definition of each patterns, just to memorize while revising the concepts. However, keep in mind that patterns are not kept in sequence to make you exercise in a better way…

Links to OOPs Design Principles & SOLID Principles

OOPs Design Principles

SOLID Design Principles