Getting error during compiling java program

Question

I have below two classes

import java.util.*;
public interface Stack<Item> extends Iterable<Item>{
    void push(Item item);
    Item pop();
    boolean isEmpty();
    int size();
    Iterator<Item> iterator();
}

Second class is the :

import java.util.*;
public class LinkedStack<Item> implements Stack<Item>{

private Node head;
private int size;


private class Node{
    Item item;
    Node next;

    public Node(Item item){
        this.item = item;

    }

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

    }
}


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

public int size(){
    return size;
}

public void push(Item item){
    head = new Node(item,head);
    size++;
}   

public Item pop(){
    Item item = head.item;
    head = head.next;
    size--;
    return item;
}

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

class LinkedStackIterator<Item> implements Iterator<Item>{
    private Node current = head;

    public boolean hasNext(){
        return current != null;
    }

    public Item next(){
        Item return_item = current.item;
        current = current.next;
        return return_item;
    }

    public void remove(){};
}
}

I am getting typecast error in method public Item next():

Item return_item = current.item;

If I write above line as

Item return_item = (Item) current.item;

It works fine. Can anybody suggest the reason?

I am getting below compilation error:

LinkedStack.java:57: error: incompatible types Item return_item = current.item; ^ required: Item#2 found: Item#1 where Item#1,Item#2 are type-variables: Item#1 extends Object declared in class LinkedStack Item#2 extends Object declared in class LinkedStack.LinkedStackIterator 1 error

Answer 1

Because your LinkedStackIterator class is generic, when it refers to Item it means a different type to the Item used in your outer classes. One solution is to change LinkedStackIterator so it is not generic, i.e. change:

class LinkedStackIterator<Item> implements Iterator<Item>{

to

class LinkedStackIterator implements Iterator<Item>{

Side note: it can be quite confusing to Java programmers to see generic type variables given meaningful names like Item. You may wish to consider renaming this to something more expected like T.