Covariant return types, we like you but were doing okay without you

Until J2SE 1.4, it was illegal to have two methods in a class with similar signatures, but differing only in return types. Java 5 changes that so something like this is perfectly legal as long as the return type of the overriding method is a sub-class of the overridden method:

  class Foo {}

  class Bar extends Foo {}



  public class Bat {



    public Foo getObject() {

      return new Foo();

    }



  }



  public class Qux extends Bat {



    // Bar is a sub-class of Foo, so this is legal

    public Bar getObject() {

      return new Bar();

    }



  }

So what does that tell us?

With covariant return types, the language spec concedes that creators of classes truly understand the polymorphic relationship of types. It thus takes the burden off the shoulders of API users making their life a tad easier (though not much).

Now, I have ranted before about useless language feature in Java 5 (here and here) and feel this falls right in that category. Surely, no one can claim this is absolutely required (simple workarounds exist). Besides, I have apprehensions about readability issues it introduces.

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