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What is Composition in Java with Examples

The versatility of the Java programming language allows code reuse while reducing duplicity between classes. Java facilitates code reusability by building relationships between classes, and these relationships are of two types: inheritance and association. While inheritance lets us implement an is-a relationship and create a new class from an existing class, association or has-a relationship ensures code reusability. 

The has-a relationship is further divided into two categories: aggregation and composition in Java. This article will explore the concept of composition in Java and help you understand its fundamentals with examples.

An Overview of Relationships in Java

Relationships in Java refer to the different ways classes are related. Broadly, there are two types of relationships in Java:

  • Is-a relationship
  • Has-a relationship

Composition in Java

Source

Is-a relationship (Inheritance)

The inheritance feature in Java lets us implement is-a relationships. In this case, one class inherits another class. For instance, if a class ‘Flower’ inherits another class, ‘Lily,’ we can say that class ‘Lily’ has an is-a relationship with ‘Flower.’ It also implies that Lily is a flower.

Example: Lily ‘is a’ flower, computer ‘is a’ device, and so on.  

Has-a relationship (Association)

A has-a relationship signifies that a class uses the instance of another class, and such a relationship in Java is called association. For example, if a class P holds another class Q’s reference, it implies that P can access all properties of class Q. 

Example: Computer ‘has a’ processor, bee ‘has a’ sting, etc. 

An association in Java is a relationship between two discrete classes using their objects. The relationship can be of the following types:

  • One-to-many: One object associates with multiple objects.
  • One-to-one: One object associates with a single object.
  • Many-to-many: Many objects associate with other multiple objects.
  • Many-to-one: Many objects associate with one object.

Association is of two types: aggregation and composition.

Composition in Java

Composition in Java is a design technique to implement has-a relationships between classes. In this association, one class contains another class. Moreover, the contained class relies on the containing class and cannot exist independently. In other words, one class includes another class with interdependent functionality. 

For instance, a class ‘Car’ contains classes ‘Engine’ and ‘Battery.’ Thus, ‘Car’ is in a has-a relationship with ‘Engine’ and ‘Battery’, and the latter two cannot exist independently without ‘Car.’

Let’s consider another real-life example. A human body has kidneys, and the organs cannot exist without a human body. Thus, if we think ‘Human’ to be a class with a has-a relationship with the class ‘Kidneys,’ it is composition in Java example since the kidneys cannot exist independently without a human body. 

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Implementing Composition in Java Example

Now consider the following program demonstrating the concept of composition in Java. We will break it down into steps for better understanding.

Step 1: In the first step, we will create a class ‘Car’ and declare and define methods and data members.

class Car

{

// declaring data members and methods

private String color;

private int speed;

public void carFeatures()

{

System.out.println(“Car Color= “+color + ” Maximum Speed= ” + speed);

}

public void setColor(String color)

{

this.color = color;

}

public void setspeed(int speed)

{

this.speed = speed;

}

}

 

Step 2: Next, we will create a class ‘Fiat’ that extends the above class ‘Car.’ The class ‘Fiat’ uses the FiatEngine class object start() method through composition, implying that ‘Fiat’ class ‘has-a’ FiatEngine. Fiat is a car, so it extends from the class ‘Car.’ 

 

class Fiat extends Car

{

//inherits all properties of Car class

public void setStart()

{

FiatEngine e = new FiatEngine();

e.start();

}

}

 

Step 3: In the next step of the Java composition program, we will create a class FiatEngine through which we use the class object in the above class ‘Fiat.’

 

class FiatEngine

{

public void start()

{

System.out.println(“Engine has started.”);

}

public void stop()

{

System.out.println(“Engine has stopped.”);

}

}

 

Step 4: In the final step of the program, we will create a class CompositionExample, make an object of the ‘Fiat’ class, and initialize it.

 

class CompositionExample

{

public static void main(String[] args)

{

Fiat f = new Fiat();

f.setColor(“Black”);

f.setspeed(175);

f.carFeatures();

h.setStart();

}

}

 

Output:

Car Color= Black Maximum Speed= 175

Engine has started.

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Aggregation and Composition in Java

Aggregation and composition in Java are the two kinds of has-a relationships, with both being different in terms of dependency between classes. In aggregation, the container and reference classes have an independent existence, while a composition reference class ceases to exist without a container class. Thus, a reference class is dependent on its container class in composition.

Here’s a real-life example to understand the difference between aggregation and composition in Java. A car has various parts like an engine, brakes, battery, headlights, music player, etc. While the car can function without a music player, it cannot run without essential parts like an engine or battery. Hence, we can say that the car and the music player have an aggregation relationship while the engine and car have a composition relationship. 

What are the benefits of using composition in Java?

Now that you have a fundamental understanding of the composition concept in Java, here’s an overview of the benefits of this technique:

  • Composition in Java facilitates code reusability.
  • Composition in Java lets us control the visibility of other objects to client classes and reuse what we need.
  • Although Java does not support multiple inheritances, we can work around it using composition.
  • Composition in Java is flexible and lets us change the member objects at runtime to alter the program’s behavior.
  • Java composition offers better class testability, a feature particularly useful in test-driven development.
  • A composition-based design has fewer classes.

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Conclusion

Composition in Java is an excellent technique if the relationship between classes is has-a and you are looking to reuse code. Summing up, here are some key points you need to remember about composition in Java:

  • Composition in Java represents a has-a relationship.
  • In composition, one class contains another class, and the contained class relies on the containing class and cannot exist independently. 
  • Both entities in a composition association are dependent on each other.
  • We can achieve composition in Java through an instance variable that refers to other objects.

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What is composition and inheritance in Java?

Composition and inheritance in Java are two programming techniques to establish relationships between objects and classes. The composition represents a has-a relationship with one class containing another class. On the other hand, inheritance exhibits an is-a relationship with one class acquiring the properties and behavior of another class.

What is the purpose of composition in Java?

Composition in Java allows code reusability and provides better testability of a class. Although Java does not support multiple inheritances, the composition provides a way to do so.

Is composition an OOP?

Composition is a fundamental concept in object-oriented programming. It describes a has-a relationship where one class references one or more objects of other classes through instance variables.

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