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> {...