错误参数类型E隐藏类型E

Question

I am creating a generic linked stack. This error shows up when creating the new node shown next:

private class Node<E> {

What is wrong with my code that is causing this?

public class LinkedStack<E> implements StackBehavior<E> {

    private class Node<E> {
        private E element;
        private Node<E> next;
        private Node(E element) {
            this.element = element;
            this.next = null;

        }
        private Node(E element, Node<E> next) {
            this.element = element;
            this.next = next;
        }
    }

    private Node<E> top = null;

    public void push(E item) {
        top = new Node<E>(item, top);
    }

    public E pop() {
        if (top == null) {
            throw new EmptyStackException("Pop error: Stack is empty.");
        }
        E item = top.element;
        top = top.next;
        return item;
    }

    public E peek() {
        if (top == null) {
            throw new EmptyStackException("Peek error: Stack is empty.");
        }
        return top.element;
    }

    public boolean isEmpty() {
        return (top == null);
    }

    public String toString() {
        Node<E> curr = top;
        String stringStack = "top";
        while (curr != null) {
            stringStack += " --> " + curr.element;
            curr = curr.next;
        }
        return stringStack;
    }
}

Answer 1

In this declaration

public class LinkedStack<E> implements StackBehavior<E> {

you are declaring a new type variable named E .

In this inner class declaration

private class Node<E> {

you are declaring a new type variable also called E . Any use of Node.E inside Node hides the accessible type variable E declared in LinkedStack .

This is a warning, not an error, but consider changing names if you really need the type variable (but it doesn't seem like you do).

Answer 2

The problem is that <E> in Node<E> is hiding the <E> from top class LinkedStack<E> . There are two solutions for this:

• Remove the <E> from the inner class. Usage of E inside it will automatically refer to the generic E in top class.

• Make it an inner static class. This will make its generic E different from the E in top class.