incompatible types : java.lang.Object cannot be converted to T

Question

Here is my code:

package datastructures;

import java.util.Iterator;

public class Stack{
    private class Node{
        T data;
        N         


        

Answer 1

Node's data field doesn't know it's type.

Try giving the type when you initialize head.

private Node<T> head;
private Node<T> newNode(T data){
    Node new_node = new Node<T>();
    new_node.data = data;
    new_node.next = null;
    return new_node;
}

Now, new_node knows that the data field is of type T.

Answer 2

When you declare Node as

private class Node<T>

you're declaring a generic type with another T as the T in the enclosing type. You're hiding T. So, in short, it's as if you were declaring it as

private class Node<E>

Just use

private class Node

Answer 3

(For Java 8) You can use a typed variable with the diamond operator:

private Node newNode(T data) {
    Node<T> new_node = new Node<>();
    new_node.data = data;
    new_node.next = null;
    return new_node;
}

Otherwise the generic is initalized as Object and therefore the error is shown.

Answer 4

You are declaring Node inner type with its own type parameter, so T of Node shadows T of Stack, and they are essentially different types.

Remove declaration of type parameter <T> in your Node type (that's probably what you mean for it)