How do I use paramertized generic types in an inner class?

Question

Hello, I am trying to implement an inner class that has generic parameterized type.

Here is my code (short version):

public class AVLTree<T extends Comparable<? super T>> implements Iterable<T> {
 ...

 private class BinaryNode<T extends Comparable<? super T>> {
  ...
 }

 private class TreePreOrderIterator<E extends Comparable<? super E>> implements Iterator<E> {
  ...
 }

}

It does not work, and Eclispe/Java is giving me a warning that the type T paramterizing the inner class is 'hiding'. Any thoughts on how I can fix this?

EDIT: I added the other inner class I'm having problems with - "TreePreOrderIterator". The generic type T will be the same for AVLTree, BinaryNode, and TreePreOrderIterator, but only the inner classes need to access fields in AVLTree.

EDIT2: Also, the Iterator accesses a BinaryNode, which is becoming a conflict...

(Note: This is part of bigger project I'm doing for a class. If any other information is needed, please ask.)

Answer 1

Just rename T to something else so it wouldn't conflict with the outer class T.

The type parameter names are visible in all the class scope, including the inner classes. So inner classes or instance methods should declare yet another type parameter whose name conflict with the class type parameter. However, static nested classes and static methods don't have that problem.

Answer 2

It's because the parameter T (as the parameter for the BinaryNode<T> class) is the same as that used for AVLTree<T> -- the parameter used for AVLTree is normally in scope for the definition of the inner class BinaryNode, but because you're giving a new parameter the same name, it will be hidden. You could instead use:

private class BinaryNode<U extends Comparable<? super T>> { ... }

Answer 3

If you want T in BinaryNode to be the same type as the enclosing T associated with AVLTree, remove the declaration of T in BinaryNode.

If you want the T in BinaryNode to be different than the enclosing T associated with AVLTree, but you want to be able to access properties of the parent AVLTree, rename T to something else.

If you don't need to access properties of the enclosing AVLTree, make BinaryNode static.