Java Generics compilation error: incompatible types: Item#1 cannot be converted to Item#2

Question

I am building my own collection class implementing the Iterable interface in Java. However, I am getting a compilation error saying:

Nikolass-MacBook-Pro:week2 nburk$ javac Deque.java 

Deque.java:115: error: incompatible types: Item#1 cannot be converted to Item#2
         Item currentItem = current.item;
                                   ^
  where Item#1,Item#2 are type-variables:
    Item#1 extends Object declared in class Deque
    Item#2 extends Object declared in class Deque.DequeIterator
1 error

Here are the relevant parts of my code:

public class Deque<Item> implements Iterable<Item>  {

   // return an iterator over items in order from front to end
   public Iterator<Item> iterator() {
      return new DequeIterator<Item>();
   }

   private class DequeIterator<Item> implements Iterator<Item> {

      public DequeIterator() {

      }

      public Item next () {
         Item currentItem = current.item; // this line causes the ERROR
         current = current.next;
         return currentItem;
      }
   }
}

I believe the issue somewhat is due to the fact that I am using the generics in a wrong way and my declaration of Item might not be correct. However, I was comparing my implementation with this quick tutorial and don't see any differences that might cause the issue on my side. Does anybody what I'm missing and how I can get rid of the error?

Answer 1

In

private class DequeIterator<Item> implements Iterator<Item> {
    public DequeIterator() {
    }
} 

you're declaring a new type variable called Item. You aren't reusing the one declared with Deque<Item>. These may be different types. Instead, get rid of the parameter declaration

private class DequeIterator implements Iterator<Item> {

Presumably, current.item is a reference to an object of type Item declared in Deque, but you're trying to assign it to a variable of type Item declared in DequeIterator.

---

You'll also need to remove parameterizations of DequeIterator since it isn't generic any more. So, for example

return new DequeIterator<Item>();

changes to

return new DequeIterator();