Java中的通用类型-对象类型不匹配

Question

I tried to create a LinkedList class. But I had 2 problem:

1)The Node last=null declaration is giving me a raw-type error but in above of that declaration there is no error like it. 4 same declarations but only the last one gives an error.

2) In the get() method I want to return V type and as you can see the value variable is already in V type. But it gives me "cannot convert Object to V" error. But temp.value is already V.

public class Linkedlist<V> {

    public class Node <V> {

        private Node next=null;

        private String key;

        private int size;

        private V value=null;

        public Node(V value, String key){
            this.key=key;
            this.value=value;
        }
    }

    Node root=null;
    Node temp=null;
    Node temp1=null;
    Node last=null;

    last=root;

    public void add(V value, String key){
        last.next = new Node(value,key);
        last=last.next;
    }


    public void remove(String key){
        temp=root;
        if(isEmpty())
        System.out.println("list is empty!");

        else{
            if(temp.next!=null){

                if(!temp.next.key.equals(key)){
                    remove(temp.next.key);
                }

                else if(temp.next.key.equals(key)){
                    if(temp.next==last)
                    last=temp;
                    temp.next=temp.next.next;
                }
            }
            else
            System.out.println("there is no such element");
        }
    }


    public V get(String key){
        temp=root;

        if(temp.key.equals(key)){
            if(temp.next!=null)
            get(temp.next.key);

            else
            return null;
        }
        else if(temp.key.equals(key))
        return temp.value;

    }

Answer 1

The two problems you cite are really one and the same. Given a parameterized class Node<V> such as you declared, these ...

Node root=null;
Node temp=null;
Node temp1=null;
Node last=null;

... all declare objects of the raw type Node . Other code will interpret treat them as if their type parameters had been specified as Object . You should instead declare them like so:

Node<V> root=null;
Node<V> temp=null;
Node<V> temp1=null;
Node<V> last=null;

where the <V> is literal -- a reference to the class's type parameter, not any concrete type. If you do that then both your errors will go away.

Answer 2

Your inner class Node is generic, but you're using the raw form of the class. That means that a method that returns V gets type-erased, and it is now returning Object . But the Node class doesn't need to be generic. A non- static nested class (ie a nested class) can use its enclosing class's generic type parameter. So, remove the <V> on the Node class definition.

public class Node {

Other problems I see:

• last=root; appears to be outside any constructor, method, or initialization block.
• I don't see an isEmpty() method, but you may have not posted it for brevity.
• The get() method needs a return statement in the case that neither the if nor the else conditions are met.