奇怪的错误，试图在Java中创建一个通用的链表类

Question

public class GenericLinkedList<T extends Comparable<T>> implements Cloneable {

GenericListNode<T> head;

/**
 * inserts a new node containing the data toAdd at the given index.
 * @param index
 * @param toAdd
 */
public <T> void add (int index, T toAdd) {
    GenericListNode<T> node = new GenericListNode<T>((T) toAdd);
    if (isEmpty()) {
        head = node;
    } else {

    }

}

This is the code I have and for some reason it has a problem with me doing

head = node;

It says:

Type mismatch: cannot convert from GenericListNode<T> to GenericListNode <T extends Comparable<T>>

It suggests Casting node to be

head = (GenericListNode<T>) node;

But it still gives me the error.

Answer 1

In this declaration

public <T> void add

you are defining a new type called T that is completely independent from the T defined at the class level. That's the notation for declaring a generic method.

Since the two types don't have the same bounds, they are not compatible and one cannot be converted to the other.

Get rid of that generic declaration.

Answer 2

Don't redefine T in your method:

public void add (int index, T toAdd) {
    GenericListNode<T> node = new GenericListNode<T>((T) toAdd);
    if (isEmpty()) {
        head = node;
    } else {

    }
}

T is already defined at "class-level", if you add it again on the method you are hiding the class-level one, thus you have two different types called T .

Answer 3

You are re-defining (read: shadowing) the generic definition of T . Just drop it from the method definition and you should be fine:

public class GenericLinkedList<T extends Comparable<T>> implements Cloneable {

    GenericListNode<T> head;

    /**
     * inserts a new node containing the data toAdd at the given index.
     * @param index
     * @param toAdd
     */
    public void add (int index, T toAdd) {
        GenericListNode<T> node = new GenericListNode<T>(toAdd);
        if (isEmpty()) {
            head = node;
        } else {

        }
    }
}