What is a covariant return type?

java oop covariance
Pops picture Pops · Dec 10, 2009 · Viewed 72.2k times · Source

What is a covariant return type in Java? In object-oriented programming in general?

Answer

Andrzej Doyle picture Andrzej Doyle · Dec 10, 2009

Covariant return, means that when one overrides a method, the return type of the overriding method is allowed to be a subtype of the overridden method's return type.

To clarify this with an example, a common case is Object.clone() - which is declared to return a type of Object. You could override this in your own class as follows:

public class MyFoo
{

   ...

   // Note covariant return here, method does not just return Object
   public MyFoo clone()
   {
       // Implementation
   }
}

The benefit here is that any method which holds an explicit reference to a MyFoo object will be able to invoke clone() and know (without casting) that the return value is an instance of MyFoo. Without covariant return types, the overridden method in MyFoo would have to be declared to return Object - and so calling code would have to explicitly downcast the result of the method call (even thought both sides "know" it can only ever be an instance of MyFoo).

Note that there's nothing special about clone() and that any overridden method can have a covariant return - I used it as an example here as it's a standard method where this is often useful.

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