Incompatible types using generics

Question

I'm implementing a LinkedList Stack implementation using Generics in Java as practice. I'm getting an error, and want to know why I am getting it as it is unclear to me.

The error:

Error: /Path/To/Code/Java/MyLinkedList.java:64: incompatible types
found: Item
required: Item

The code (it occurs in the Next() method of ListIterator towards the end. There are comments by it.):

import java.util.Iterator;
import java.util.NoSuchElementException;

public class MyLinkedList<Item> implements Iterable<Item> {
    private Node first;
    private int N; //size

    private class Node {
        private Node next;
        private Item item;

        private Node(Item item, Node next) {
            this.item = item;
            this.next = next;
        }

    }

    public int size() {
        return N;
    }

    public boolean isEmpty() {
        return this.first == null;
    }

    public void push(Item data) {
        Node oldfirst = this.first;
        this.first = new Node(data, first);
        this.N++;
    }

    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Underflow");
        Item item = this.first.item;
        this.first = this.first.next;
        this.N--;
        return item;
    }

    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Underflow");
        return first.item;
    }

    public String toString() {
       StringBuilder list = new StringBuilder();
        for ( Item item : this) {
            list.append(item + " ");    
        }

        return list.toString();
    }

    public Iterator<Item> iterator() { return new ListIterator(); }

    private class ListIterator<Item> implements Iterator<Item> {
       private Node current = first;
       public boolean hasNext() { return current != null; }
       public void remove() { System.out.println("Can't do dis, nigga"); }

       public Item next() {
            if (!hasNext()) throw new NoSuchElementException();

            //The line in question:
            Item item = current.item;
            //I managed to fix it if I do: Item item = (Item) current.item;
            //Why is that necessary?


            current = current.next; 
            return item;
       }
   }    
}

Answer 1

You have declared Item as type parameters in both your top-level class and the inner class. So, the Item in MyLinkedList<Item> is different from that in ListIterator<Item> , and hence are incompatible. You can just make your ListIterator class non-generic:

private class ListIterator implements Iterator<Item>

...and you should be fine.

Also, I would suggest to change the type parameter Item to some single letter like E , to avoid getting it confused to some actual class. As per convention, type parameters should be single upper-case letters.