Changing references to Objects java

Question

Code from CCI book:

public class ListNode {

  int val;
  ListNode next = null;

  ListNode(int x) {
    this.val = x;
  }

And this:

  public void appendToTail(int d) {
    ListNode end = new ListNode(d);
    ListNode n = this;
    while (n.next != null) {
      n = n.next;
    }
    n.next = end;
  }

Original: 5 -> 10 -> 15

As I understand, in this case "n" and "this" refer to same object yet, after debugging and stepping through:

"n" becomes 15 -> 20

"this" becomes 5 -> 10 -> 15 -> 20

How can this be? When I change "n", "this" should change as well?

So everything I did on "n" ,BESIDES appending 20, did not reflect on "this".

UPDATE:

Anyone who has same question should read this.

Can you please post your entire code block? The behavior depends on where the `appendToTail` method lives. — Barett, Oct 13 '16 at 21:38
@Barett it lives in ListNode class. Above is the whole code. — Anton Kim, Oct 14 '16 at 00:09

score 1 · Accepted Answer · answered Oct 13 '16 at 21:31

Inside your while loop you say:

n = n.next

When that line of code executes you change the value of n from "this" to n.next. This points to the current link object in the list and next points to a completely different object. As you walk your way through the linked list the value of n keeps updating as you pass through each link in the linked list.

When you get to the end of the linked list there is no next, i.e. value of n is null. So you exit the while loop and add a new link to the end of the linked list.

beatngu13 · Answer 2 · 2016-10-13T22:04:09.647

0

From the Java Language Specification:

[…] the keyword this denotes a value that is a reference to the object for which the instance method or default method was invoked […]

You only invoke appendToTail(int) on one particular node of your list, so this always refers to the same object. If you want to "step into" each node during debugging, you could change the implementation as follows:

public void appendToTail(int d) {
    if (next != null) {
        next.appendToTail(d);
    } else {
        next = new ListNode(d);
    }
}

edited Oct 13 '16 at 22:04

answered Oct 13 '16 at 21:53

beatngu13

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Thanks, that makes sense. – Anton Kim Oct 14 '16 at 01:10

Changing references to Objects java

2 Answers2