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.

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Why Are Design Patterns in Java needed?

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;

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

When Should We Use Design Patterns in Java?

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.

Categorization of Design Patterns in Java

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. 

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How to Create Design Patterns in Java?

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.

Use of Design Patterns in Real Life Cases.

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.

Bottom Line

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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