Covariant Return Type in Java

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Covariant Return Type in Java

Posted by Unknown Saturday 9 November 2013 0 comments

Object oriented programming (OOP) has a principle named substitutability. In this tutorial, let us learn about substitutability and support for covariant return type in Java. Covariant return type uses the substitutability principle.

Liskov Substitution Principle

Substitutability was introduced by eminent Barbara Liskov and Jeannette Wing. It is also called as Liskov substitution principle.
Let T be a super type and S be its subtype (parent and child class). Then, instances (objects) of T can be substituted with instances of S. Parent’s instances can be replaced with the child’s instances without change in behavior of the program.

Let WildAnimal be a supertype and Elephant be a subtype, then an instance obj1 of WildAnimal can be replaced by an insance obj2 of Elephant.

Covariant, Contravariant and Invariant

The subtyping principle which we discussed above as Liskov principle is called covariant. The reverse of it (instead of child replacing the parent, the reverse of it as parent replacing the child) is called contravariant. If no subtyping is allowed then, it is called invariant.

Covariant Type in Java

From the release of JDK 1.5, covariant types were introduced in Java. Following example source code illustrates the covariant types in java. In the below example, method overriding is used to demonstrate the covariant type.
In class Zoo, the method getWildAnimal returns ‘WildAnimal’ which is a super type. AfricaZoo extends Zoo and overrides the method getWildAnimal. While overriding, the return type of this method is changed from WildAnimal to Elephant. This demonstrates covariant type / Liskov substitution principle. We are replacing the supertype’s (WildAnimal) instance with subtype’s (Elephant) instance. This was not possible before JDK 1.5 IndiaZoo is just another example which demonstrates the same covariant type.

class WildAnimal {

  public String willYouBite(){

    return "Yes";

  }

}

class Elephant extends WildAnimal {

  public String whoAreYou() {

    return "Elephant";

  }

}

class BengalTiger extends WildAnimal {

  public String whoAreYou() {

    return "Bengal Tiger";

  }

}

class Zoo {

     WildAnimal getWildAnimal() {

         return new WildAnimal();

     }

 }

class AfricaZoo extends Zoo {

     @Override

     Elephant getWildAnimal() {

         return new Elephant();

     }

}

class IndiaZoo extends Zoo {

     @Override

     BengalTiger getWildAnimal() {

         return new BengalTiger();

     }

}

public class Covariant {

  public static void main(String args[]){

    AfricaZoo afZoo = new AfricaZoo();

    System.out.println(afZoo.getWildAnimal().whoAreYou());

    IndiaZoo inZoo = new IndiaZoo();

    System.out.println(inZoo.getWildAnimal().whoAreYou());

  }

}

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