IS a vs HAS a in Java

1. IS-A Relationship in Java
2. HAS-A Relationship in Java
3. Key Differences Between IS-A Relationship and HAS-A Relationship

One of the key features of using an object-oriented programming language is that we can reuse a code multiple times. Generally, we can use two concepts, Inheritance and Composition, to achieve reusability.

Inheritance is an IS-A relationship, and Composition is a HAS-A relationship. Both the concepts differ in a certain way but have the same goal, reusability.

Let us see the difference between these two concepts below.

IS-A Relationship in Java

An Inheritance or the IS-A relationship in Java refers to the relationship of two or more classes. Inheritance can be achieved by utilizing the extends keyword in Java. We can inherit a parent class to use its methods and variables in a child class.

In the sample code below, we have two classes, Animal and Dog, the Animal class has a function whatAmI() that returns a String, while the Dog class extends the class Animal which is a concept of Inheritance.

This Dog is an Animal relationship. The Dog class can now access the whatAmI() method from its parent class.

class Animal {
  public String whatAmI() {
    return "I belong to Animal class";
  }
}

class Dog extends Animal {
  public void aboutMe() {
    System.out.println("I am a Dog class and " + whatAmI());
  }
}

Output:

I am a Dog class and I belong to Animal class

HAS-A Relationship in Java

Unlike Inheritance, the Composition or the HAS-A relationship does not use any keyword like extends. We create an object of the parent class or whichever class we want to use in this method.

In the following example, again, we have two classes, Legs and Dog.

Legs has a method howManyLegs(). In the Dog class, we create an object of the Legs class and use that object to call the howManyLegs() function.

Now, here we can see that the Composition concept is being applied, Dog HAS-A Legs or Dog class HAS-A property of Legs.

class Legs {
  public String howManyLegs() {
    return "I have four legs";
  }
}

class Dog {
  Legs legs = new Legs();

  public void aboutMe() {
    System.out.println("I am a Dog class and " + legs.howManyLegs());
  }
}

Output:

I am a Dog class and I have four legs

Key Differences Between IS-A Relationship and HAS-A Relationship