在尾部插入单链表

Question

Every time I run this, it returns a bunch of: null, null, null for the items when I test the function out.

//enqueue()
//adds newItem to the back of this Queue

public void insertItemLast(Object newItem){//make sure that it is not empty so we can do the cool stuff in here
    if(newItem == null)
        return;//user inputs nothing
    else {
        Node P = new Node(newItem);
        P.next = null;
        if(head == null){
            head = P;
            tail = P;
            //tail.next = null;
        }else{
            tail.next = new Node(newItem);
            tail = new Node(newItem);

            //tail.next = null;

        }

    }
    numItems++;
}//end enqueque

Answer 1

You create two different links instead of just one.

Your else should be :

 } else {
    tail.next = new Node(newItem);
    tail = tail.next;
}

Actually, you can make it even simpler. Just use P for the new link of the list in all cases :

public void insertItemLast(Object newItem){
    if(newItem == null)
        return;//user inputs nothing
    else {
        Node P = new Node(newItem);
        P.next = null;
        if(head == null) {
            head = P;
            tail = P;
        } else {
            tail.next = P;
            tail = P;
        } 
    }
    numItems++;
}//end enqueque

Answer 2

You correctly assigned a new Node to tail.next , but you didn't update tail ; you instead assigned another new Node to tail , effectively breaking the tail off of the list.

To advance the old tail to the new tail -- the newly inserted Node -- replace

tail = new Node(newItem);

with

tail = tail.next;