CODEWITHSUNDEEP
java
user979616
user979616

Reputation: 273

regarding this keyword (java)

I have a question regarding the output of the following program. The output is null. This is what I thought as well. Im thinking its because the methods called before display simply modify a copy of head and not head itself. Im assuming that I could get around this using a this.head= something right?

Heres the code:

public class List {
    private Node head;
    public List (){
        int max=3;
        int i;
        head=null;
        Node aNode=new Node(0);
        for (i=0; i<max; i++) {
            aNode.setNum(i);
            add (aNode);
            aNode.setNext(null);
            }
        }
    public void add(Node aNode) {
        Node temp;
        if(head==null)
            head=aNode;
        else {
            temp=head;
            while(temp.getNext()!=null)
                temp=temp.getNext();
            temp.setNext(aNode);
            }
        }
    public void display() {
        Node temp=head;
        while(temp!=null) {
            System.out.println(temp.getNext());
            temp=temp.getNext();
            }
        }
    }


public class Node {
    private int num;
    private Node next;
    public Node (int n) {num=n; next=null;}
    public int getNum() {return num;}
    public void setNum(int n) {num=n;}
    public void setNext(Node n) {next=n;}
    public Node getNext() {return next;}
    }

public class Driver {
    public static void main(String args[]) {
        List aList=new List();
        aList.display();
        }
}

Upvotes: 0

Views: 200

Answers (3)

Joop Eggen
Joop Eggen

Reputation: 109547

The add relies on receiving a new Node with next being null. So move Node aNode = new Node(); inside the for-loop.

Some sanitary remarks.

  • (Unimportant) Use current instead of temp, or anything else.
  • Fields in classes are by default null/0/0.0/false.

Upvotes: 2

Dan Hulme
Dan Hulme

Reputation: 15280

Look at this code from the constructor:

Node aNode=new Node(0);
for (i=0; i<max; i++) {
    aNode.setNum(i);
    add (aNode);
    aNode.setNext(null);
    }

You create one new node, and then keep trying to add that node to the list. You need the first line to be inside the for loop, so you create lots of nodes. After the constructor completes, your list only contains one node, with the value 3. Then later, in display():

System.out.println(temp.getNext());

You start by calling getNext() on the first node. Since there's only one node, getNext() returns null, which is what you print out. You should replace this line with

System.out.println(temp);

The error is nothing to do with the this keyword at all. You only need this.foo (when foo is some data member of your class) to disambiguate when you have a both a member and a local variable or parameter with the same name.

Upvotes: 0

Dave
Dave

Reputation: 6179

Before I answer your question, here is a side note...

Im thinking its because the methods called before display simply modify a copy of head and not head itself.

This is NOT correct.

Here is why...

    public void display() {

      // Basically, this says, make temp a REFERENCE of head...NOT A COPY!!!!
      Node temp=head;
      while(temp!=null) {
          System.out.println(temp.getNext());
          temp=temp.getNext();
      }
    }

Now, to answer your question, the reason temp is null is because head is null. And the reason head is null is because you never initialize it.

From you constructor...

    public List (){
      int max=3;
      int i;

      // Here you're saying "set head to null".
      // So when you call display, head is NULL.  You MUST initialize this.
      head=null;

      Node aNode=new Node(0);
      for (i=0; i<max; i++) {
        aNode.setNum(i);
        add (aNode);
        aNode.setNext(null);
      }
    }

Upvotes: 1

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