What is Composition in Java With Examples

Composition in Java is a powerful concept within object-oriented programming (OOP) that allows one class to contain an object of another class, creating a HAS-A relationship. Unlike inheritance (IS-A), which tightly couples classes, composition promotes loose coupling—leading to better maintainability, flexibility, and scalability in software design.

In this article, you'll explore the fundamentals of composition, how it differs from aggregation and inheritance, and how it enables code reusability with practical, real-world examples. We'll also cover its advantages, implementation strategies, and best practices for using composition in Java-based applications.

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A Brief Narration of Associations or Relationships in Java

In object-oriented programming, objects are related to each other and use the common functionality between them. This is where the topics of Inheritance, Association, Aggregation, and Composition in Java programs come.

Inheritance (IS-A) and Association (HAS-A) in Java

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1. Inheritance (IS-A)

An IS-A relationship signifies that one object is a type of another. It is implemented using ‘extends’ and ‘implements’ keywords.

Example: HP IS-A laptop

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2. Association (HAS-A)

A HAS-A relationship signifies that a class has a relationship with another class. For instance, Class A holds Class B’s reference and can access all properties of class B.

Example: Human body HAS-A Heart

3. Aggregation Vs Composition

Has-A relationship or Association can be divided into aggregation and composition. An aggregation container class and referenced class can have an independent existence. A composition reference class cannot exist if the container class is destroyed.

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Let’s take an example to understand aggregation and composition. A car has its parts e.g., engines, wheels, music player, etc. The car cannot function without an engine and wheels but can function without a music player. Here the engine and car have a composition relation, and the car and music player have an aggregation relationship. In the case of Aggregation, an object can exist without being part of the main object.

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Composition Vs. Inheritance

Understanding the difference between composition and inheritance is essential for mastering object-oriented programming. While inheritance establishes an IS-A relationship between classes, composition in Java is based on a HAS-A relationship, promoting more modular and maintainable code. 

This section breaks down when to use each approach, highlighting the advantages of choosing composition in Java for flexibility and better code reuse in complex applications.

Composition in Java

A composition in Java between two objects associated with each other exists when there is a strong relationship between one class and another. Other classes cannot exist without the owner or parent class. For example, A ‘Human’ class is a composition of Heart and lungs. When the human object dies, nobody parts exist.

The composition is a restricted form of Aggregation. In Composition, one class includes another class and is dependent on it so that it cannot functionally exist without another class.

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Implementation of Composition in Java

The engine and car relationship are implemented using Java classes as below. In Java, the ‘final’ keyword is used to represent Composition. This is because the ‘Owner’ object expects a part object to be available and function by making it ‘final’.

public class Car {

           private final Engine engine; 

    public Car(){

    engine  = new Engine();

}

}

 class Engine {

private String type;

}

Let us take another example that depicts both inheritance and composition.

In this program, the class Honda is a Car and extends from the class Car. The car engine Object is used in the Honda class.

class CarEngine {
    public void StartEngine(){

        System.out.println(“The car engine has Started.”);

    }

    public void stopEngine(){

        System.out.println(“The car engine has Stopped.”);

    }

} 

class Car {

    private String colour;

    private int maxi_Speed;

    public void carDetails(){

        System.out.println(“Car Colour= “+colour + “; Maximum Speed= ” + maxi_Speed);

    }

    //Setting colour of the car

    public void setColour(String colour) {

        this.colour = colour;

    }

    //Setting maximum car Speed

    public void setMaxiSpeed(int maxi_Speed) {

        this.maxi_Speed = maxi_Speed;

    }

} 

class Honda extends Car{

    public void HondaStart(){

        CarEngine Honda_Engine = new CarEngine(); //composition

        Honda_Engine.startEngine();

        }

} 

public class Main {

    public static void main(String[] args) {   

        Honda HondaJazz = new Honda();

        HondaJazz.setColour(“Black”);

        HondaJazz.setMaxSpeed(160);

        HondaJazz.carDetails();

        HondaJazz.HondaStart();

    }

}

Output:

Car Colour = Black; Maximum Speed = 160

The car engine has started. 

The output is derived using composition and shows the details of the Honda Jazz car.

UML Denotations of Association

The relationships of association, aggregation, and composition in Java between classes A and B are represented as follows in UML diagrams:

Association in Java: A—->B

Composition in Java: A—–<filled>B

Aggregation in Java: A—–<>B

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Features of Composition in Java

Composition is a fundamental concept in Java programming that enables the creation of complex objects by combining simpler ones along with description, what is composition in java with example.  It facilitates code reuse, maintainability, and flexibility in designing software applications. Let’s explore the key features of composition in Java:

• Object Composition

Object composition involves creating complex objects by combining simpler ones. In Java, this is achieved by defining classes that contain references to other classes as instance variables. This allows objects to be composed of other objects, forming a hierarchical structure.

• Has-a Relationship

Composition establishes a “has-a” relationship between classes, where one class contains objects of another class. This relationship signifies that a class has references to other classes to fulfill its functionality. For example, a Car class may have instances of Engine, Wheels, and Seats classes and understanding what is composition in java.

• Code Reusability

Composition promotes code reusability by allowing the reuse of existing classes within new classes. Instead of inheriting behavior through inheritance, classes can reuse functionality by containing instances of other classes. This enhances modularity and reduces code duplication, composition example in java.

• Encapsulation

Encapsulation is maintained through composition as the internal details of the composed objects are hidden from the outside world. Each class manages its own state and behavior, providing a clear separation of concerns. This enhances code maintainability and reduces the risk of unintended side effects and composition writing examples.

• Flexibility and Modifiability

Composition offers greater flexibility compared to inheritance, as it allows classes to change behavior dynamically by replacing or modifying the objects they contain. This promotes a modular design approach, where individual components can be modified or extended without affecting the entire system.

• Dynamic Behavior

With composition, the behavior of an object can be dynamically changed at runtime by replacing its constituent objects. This dynamic composition enables the creation of highly adaptable and customizable systems, where different configurations of objects can be used to achieve varying functionality, with an understanding of aggregation and composition in java.

• Loose Coupling

Composition helps in achieving loose coupling between classes, as objects are accessed through interfaces rather than concrete implementations. This reduces dependencies between classes and promotes better code maintainability and testability.

• Granular Control

With composition, developers have granular control over the behavior and state of objects. They can selectively expose certain functionalities of composed objects while encapsulating others, providing a clear interface for interaction with the object, like composition in java.

Benefits of Composition in Java

Composition in Java offers a more flexible and modular approach to building software compared to inheritance.In this section, we’ll explore the key benefits of using composition in Java and why it’s often the preferred choice for scalable and maintainable application design.

• Encapsulation and Modularity

Composition promotes encapsulation, which is the practice of bundling data and methods that operate on that data within a single unit, with an understanding of object composition in java. By encapsulating related functionalities into separate classes, you can create modular and reusable components. This modularity enhances code organization and makes it easier to understand, maintain, and extend, with examples of composition.

• Code Reusability

One of the primary benefits of composition is code reusability. Instead of inheriting behaviors from a single parent class, you can compose objects by combining multiple classes to achieve the desired functionality. This approach allows you to reuse existing classes in different contexts, reducing code duplication and promoting a more efficient development process.

• Flexibility and Loose Coupling

Composition promotes loose coupling between classes, which means that the components of a system are independent and can be modified or replaced without affecting other parts of the codebase. This flexibility is essential for building scalable and maintainable applications, as it enables developers to make changes to one part of the system without impacting the entire application.

• Better Control over Behavior

With composition, you have finer control over the behavior of your objects compared to inheritance. Instead of inheriting all the characteristics of a parent class, you can selectively choose which functionalities to include in a class by composing it with the appropriate components. This granularity allows you to design classes that are tailored to specific requirements, leading to cleaner and more efficient code.

• Avoiding the Diamond Problem

Inheritance can result in the “diamond problem,” where ambiguity rises when a class inherits from any two or more classes that have an ancestor. Composition helps avoid this issue by allowing you to combine functionalities from multiple classes without creating complex inheritance hierarchies. This simplifies the design and prevents potential conflicts that may arise from multiple inheritance.

• Facilitates Testing and Debugging

Composition facilitates unit testing and debugging by enabling you to isolate and test individual components independently. Since each class encapsulates a specific set of functionalities, you can easily mock dependencies and simulate different scenarios during testing. Additionally, debugging becomes more manageable as the codebase is divided into smaller, more focused units.

When to Use Composition in Java?

In Java programming, composition is a powerful concept used to build complex objects by combining simpler ones. It involves creating objects by incorporating other objects within them. Knowing when to utilize composition is essential for writing clean, maintainable, and efficient code, with various composition writing sample.

Understanding Composition

Composition establishes a “has-a” relationship between classes, where one class contains another as a part of its state. This is different from inheritance, which establishes an “is-a” relationship. In composition, the contained object does not inherit behavior from the containing class but rather is used to provide functionality or data, with english composition examples.

Encapsulation and Code Reusability

One of the primary use cases for composition is encapsulating functionality. By breaking down complex systems into smaller, more manageable components, you can encapsulate related functionality within separate classes. This promotes code reusability and modular design, making your codebase easier to understand and maintain.

Flexibility and Modifiability

Composition allows for greater flexibility in software design. Since objects are composed of other objects, you can easily modify or replace components without affecting the entire system. This modular approach simplifies testing and debugging and enables you to adapt your code to changing requirements more efficiently when association aggregation and composition in java.

Preventing Tight Coupling

Using composition helps avoid tight coupling between classes, which can make code brittle and difficult to maintain. By relying on interfaces or abstract classes, you can decouple components, making them interchangeable and reducing dependency on specific implementations. This enhances the scalability and extensibility of your codebase.

Promoting Single Responsibility Principle

Composition encourages adhering to the Single Responsibility Principle (SRP), which states that a class should have only one reason to change. By breaking down functionality into smaller, focused classes, each responsible for a specific task, you can create more cohesive and understandable code. This improves code maintainability and reduces the risk of introducing bugs when making modifications.

Example: GUI Components

Consider a graphical user interface (GUI) framework where various components such as buttons, text fields, and panels are composed to create complex interfaces. Each component encapsulates its behavior and appearance, allowing developers to mix and match them to design diverse user interfaces efficiently.

Conclusion

Composition in Java offers many advantages while programming and is one of the favoured design methods. In this article, we have tried to make you understand this important concept with real-life examples and practical code. Composition offers flexibility and robust code. Its code reusability feature helps in avoiding code duplication and achieving cost-effectiveness. This makes it one of the widely used methods in various programs. 

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