错误：方法未覆盖或从超类型实现方法

Question

Building queue implementation based on linked list. Cannot run application because of the two errors:

public class Queue<Integer> implements Iterable<Integer> {
    ...
    public Iterator<Integer> iterator()  {
            return new ListIterator(first);
        }
    private class ListIterator<Integer> implements Iterator<Integer> {// error #1
            private Node<Integer> current;

            public ListIterator(Node<Integer> first) {
                current = first;
            }

            public boolean hasNext(){ return current != null;                   }
            public void remove()    { throw new UnsupportedOperationException();}

            public int next() { // error #2
                if (!hasNext()) throw new NoSuchElementException();
                int item = current.item;
                current = current.next;
                return item;
            }
        }    
     }

error #1: error: Queue.ListIterator is not abstract and does not override abstract method next() in Iterator where Integer is a type-variable: Integer extends Object declared in class Queue.ListIterator

error #2: error: next() in Queue.ListIterator cannot implement next() in Iterator return type int is not compatible with Integer where E,Integer are type-variables: E extends Object declared in interface Iterator Integer extends Object declared in class Queue.ListIterator

How to get it working?

Answer 1

Boxing and unboxing in Java simplify code in many places, but method return types is not one of them. The next method must return an Integer , not an int . It must match the generic type parameter exactly.

public Integer next()

Second, you've declared a generic type parameter Integer in your Queue and ListIterator classes that has nothing to do with java.lang.Integer . Remove it:

//              here
public class Queue implements Iterable<Integer> {

and

//                      here
private class ListIterator implements Iterator<Integer> {