Question

The following throws a compile error because foo.getClass() isn’t the same capture group as F extends Foo somehow:

public <F extends Foo> F create (final F foo){

     return foo.getClass().cast(foo);

   }

The following works fine.

return foo;

Thanks.

Answer 1

Object#getClass() returns a Class<? extends Foo>, not a Class<F extends Foo>.

The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called.

The erasure of <F extends Foo> is the upper bound, Foo. Therefore the Class#cast() call is roughly equivalent to

return (Foo) foo;

and not

return (F) foo;

As you’ve probably figured out by now, a Foo is not an F extends Foo (the declared method return type), so the compiler won’t let you return a Foo.