Question

I have written a class which is a base class of Class A and implements an interface of Class B.

Now my compiler is giving a wierd kind of error saying that “The return types of functiona from Class A is not compatible with return type of functiona in class B.”

My Code is as below,

public class X extends A implements B
{
}

public class A
{
  public Enumeration<String> test(){}
}

public interface B
{
  public Enumeration<Object> test();
}

Now I can’t understand why the compiler is giving such an error since already String is a type of an object, so what i understood is that automatic type conversion should happen in runtime because of that. Am i right? or my conceptual understanding has gone wierd on me?

Answer 1

What you’re trying to do is possible in Java. As Ernest stated, an Enumeration is not a subclass of Enumeration, since Java genercis lacks the concept of variance.

Anyway, you can express you intention using type wildcard. You have to change you interface this way:

public interface B
{
  public Enumeration<?> test();
}

Now your code compile fine. Just to let you know, you can also restrict your interface to some other type than Object. For example, if you have to build an interface that return Enumerations
of Number:

    class X extends A implements B
    {
    }

    class A
    {
        public Enumeration<Long> test(){return null;}
    }

    class C
    {
        public Enumeration<String> test(){return null;}
    }

    //This doesn't compile! String does not extend Number
    /*class Y extends C implements B
    {
    }*/

    interface B
    {
        public Enumeration<? extends Number> test();
    }