inheritance(is-a) v/s composition (has-a) relationship in Java

IS-A Relationship

IS-A is a way of saying − This object is a type of that object. Let us see how the extends keyword is used to achieve inheritance. 

public class Animal {
}
public class Mammal extends Animal {
}
public class Reptile extends Animal {
}
public class Dog extends Mammal {
}

Now, if we consider the IS-A relationship, we can say −

• Mammal IS-A Animal
• Reptile IS-A Animal
• Dog IS-A Mammal
• Hence: Dog IS-A Animal as well

With the use of the extends keyword, the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass.

We can assure that Mammal is actually an Animal with the use of the instance operator.

Example

Live Demo

class Animal {
} class Mammal extends Animal {
} class Reptile extends Animal {
} public class Dog extends Mammal {
   public static void main(String args[]) {
      Animal a = new Animal();
      Mammal m = new Mammal();
      Dog d = new Dog();
      System.out.println(m instanceof Animal);
      System.out.println(d instanceof Mammal);
      System.out.println(d instanceof Animal);
    }
}

This will produce the following result −

Output

true
true
true

HAS-A relationship

These relationships are mainly based on the usage. This determines whether a certain class HAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.

Let's  look into an example −

Example

public class Vehicle{ }
public class Speed{ }
public class Van extends Vehicle {
   private Speed sp;  
}

This shows that class Van HAS-A Speed. By having a separate class for Speed, we do not have to put the entire code that belongs to speed inside the Van class, which makes it possible to reuse the Speed class in multiple applications.

In an Object-Oriented feature, the users do not need to bother about which object is doing the real work. To achieve this, the Van class hides the implementation details from the users of the Van class. So, basically what happens is the users would ask the Van class to do a certain action and the Van class will either do the work by itself or ask another class to perform the action.