Java中的单链表 - get（）方法

Question

I am new to Java and trying to implement a singly linked list in Java. I have included the use of generics. The code is shown below:

public class LinkedListX<E>{

   Node <E> head;
   int size;

   LinkedListX(){
       head = null;
       size = 0;
   }

   LinkedListX (E e){
       head = new Node(e);
       size = 1;
   }

   void add(E e){
       if (head == null){
           head = new Node (e);
       } else {
           Node current = this.head;
           while (current.next != null){
               current = current.next;
           }
           current.next = new Node(e);
       }
       size++;
   }

   E get (int n){
       if (n > size){
           return null;
       }
       Node current = head;
       for (int i=1;i<n;i++){
           current = current.next;
       }
       return current.e;
   }

   private class Node<E> {
       private E e;
       private Node next;

       Node (E e, Node n){
           this.e = e;
           this.next = n;
       }
       Node (E e) {
           this.e = e;
       }
   }

}

the get() method gives error message

Incompatible types, Require:E, Found: java.lang.Object" at "return current.e

I think I am using generics in a wrong way. Could someone let me know the right way to code this method?

Thanks.

Answer 1

As Node is an inner class it can also access the outer classes generic parameter. And you never assign a different value to E than the one from the outer class. So just remove the <E> from Node class declaration:

private class Node{
    // the rest
}

Answer 2

You're storing a Node as next without using its generic attributes - there are various places where you should be using Node<E> instead of just Node .

The problem basically is that you try to return E , but you save Node which is mapped by the compiler to Node<Object> - and Object isn't E

Answer 3

To use correctly generic, you may change :

• in get() : Node current = head; to Node<E> current = head;
• all your new Node() to new Node<>() (shortcut for new Node<E>()
• in Node class use also <E>

---

You can also just remove the <E> from all your Node (to all the Node class), you will still be able to use E for the data)

Answer 4

You mix two problems:

1. This line causes the problem (4 lines above the type mismatch):

    Node current = head; 

   You have to include generics as well, otherwise, its raw type is recognized as Object .

    Node<E> current = head; 

   Once you use a generic object, always include its type with <> .

2. The second thing is that you already use generics on the outer class LinkedListX <E> thus all its inner work with the E parameter. For this, the generics within the Node class can be omitted.

    private class Node { // the implementation } 

   Otherwise, the warning complaint appears:

   The type parameter E is hiding the type E

Answer 5

Node current = head;

The above line is a raw type declaration, and in such case 'java.lang.Object' is the default Type Variable.

Using a parameterised type Node Node<E> current = head; will solve the problem.

Answer 6

Your code has a lot of raw references of Node which is generating a lot of warnings . In your get(int n) snippet Node is raw and not generic so Java type inference algorithm failed to identify it as E and considering its as an Object. Please use the below code and check the difference in Eclipse

public class LinkedListX<E> {

    Node<E> head;
    int size;

    LinkedListX() {
        head = null;
        size = 0;
    }

    LinkedListX(E e) {
        head = new Node<E>(e);
        size = 1;
    }

    void add(E e) {
        if (head == null) {
            head = new Node<E>(e);
        } else {
            Node<E> current = this.head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = new Node<E>(e);
        }
        size++;
    }

    E get(int n) {
        if (n > size) {
            return null;
        }
        Node<E> current = head;
        for (int i = 1; i < n; i++) {
            current = current.next;
        }
        return current.e;
    }

    static private class Node<E> {
        private E e;
        private Node<E> next;

        @SuppressWarnings("unused")
        Node(E e, Node<E> n) {
            this.e = e;
            this.next = n;
        }

        Node(E e) {
            this.e = e;
        }
    }
}