Cannot find type parameter if it is an inner class of the subclass in Java

Question

Why does the following code have a compile error:

Foo.java:

public abstract class Foo<T> {
    public abstract T getInner();
}

MyFoo.java:

public class MyFoo extends Foo<MyFooInner> {
    public static class MyFooInner {
    }
    public MyFooInner getInner() {
        return new MyFooInner();
    }
}

Compiling the second class results in:

MyFoo.java:1: cannot find symbol
symbol: class MyFooInner
public class MyFoo extends Foo<MyFooInner> {
                               ^
1 error

Is there a way around this problem besides putting the inner class in its own file?

Answer 1

Use the following notation:

public class MyFoo extends Foo<MyFoo.MyFooInner> {...

UPDATE: Static nested classes are effectively a top level class as specified here:

A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.

So, the only way you can refer to a static nested class is mentioning its parent class somewhere. Otherwise it is a reference to an imported class or to a class within the same package.

UPDATE: To explain it even more, another way to reference the class is to import it like this:

import my.package.MyFoo.MyFooinner;

public class MyFoo extends Foo<MyFooInner> {...

Answer 2

To build on @Andrey Adamovich's answer, the reason for this is that types identified in a class declaration must make sense to members outside that class. This is the same as if I had written the following in another class file:

MyFooInner mfi = new MyFooInner()

This would not compile because that class wouldn't know what MyFooInner was - I would either need to qualify it by writing MyFoo.MyFooInner or else use import MyPackage.MyFoo.MyFooInner;. The same logic applies to class declarations.

For the same reason, nested classes that are declared private cannot be used in the parent class's declaration at all.