Method return type to fulfill multiple interfaces

Question

Is it possible to specify a method which returns a object that implements two or multiple interfaces?

Say we have the following interfaces:

interface Foo         


        

Answer 1

You can make T a class parameter:

class SomeImplementor implements FooBar {
    private T fSomeField;

    public T getFooBar() {
        return fSomeField;
    }

}

As to why your generics approach didn't work. Lets create the following two classes that implement Foo and Bar:

class A implements Bar, Foo{
   private int a;
   ...
}
class B implements Bar, Foo{
   private String b;
   ...
}
class SomeImplementor implements FooBar {
   private A someField;
   public  T getFooBar() {
      return someField;
   }
}

So we should now be able to execute the following:

SomeImplementor s = new SomeImplementor();
A a = s.getFooBar();
B b = s.getFooBar();

Although getFooBar() returns an object of type A, which has no valid cast to type B (where will the String member come from?), even though B fulfills the requirement of , i.e. is a valid T.

In short, the compiler (remember, generics is a compile-time mechanism) can't guarantee that every T of type can have an assignment to it of type A. Which is exactly the error you see - the compiler can't convert the given A to every valid T.