Java compilation failure when using generics parameter in inner class

Question

Please take a look at the code snippet below:

interface IFoo<E>{
    void doFoo(E env);
}

class A<E>{
    public void doA(E env){}
}

public class Foo<E> implements IFoo<E>{
    public A<E> a;

    @Override
    public void doFoo(E env) {
        a.doA(env);
    }

    private class FooInner<E> implements IFoo<E>{

        @Override
        public void doFoo(E env) {
            a.doA(env);
        }
    }
}

Eclipse complains inside of private inner class a.doA(env) with the following message.

The method doA(E) in the type A<E> is not applicable for the arguments (E)

It doesn't seem like accessibility issue because non-static inner class have an access to all instance variables of the outter class. It looks like I defined my generics wrong somewhere. Can anyone explain me what I am doing wrong here?

Answer 1

You've used the same generic parameter name for the inner class, so the type E of the inner class is shadowing the E of the outer class.

Remove generic parameter from the inner class, like this:

public class Foo<E> implements IFoo<E>{

    ...

    private class FooInner implements IFoo<E>{ // "E" here is the same "E" from Foo

        @Override
        public void doFoo(E env) {
            a.doA(env);
        }
    }
}

Answer 2

The type of the enclosing class is part of the type of the inner class. FooInner is already parameterized by E , because it's part of the outer class; the explicit parameterization is redundant and incorrect, because it's actually trying to introduce a new type parameter using the same name as the existing one. Just remove the <E> in private class FooInner<E> , and you're golden.