CODEWITHSUNDEEP
javalinked-liststack
Christian Ross
Christian Ross

Reputation: 31

What's wrong with this Java Linked List representation of a stack of integers?

Okay, so with the following code I get a nullpointer exception from everything in my pop method. Therefore, I know that 'head' must be null when that method runs. Thing is I have no idea why and I've looked over my code a good bit now. Please help!

Here it is:

NODE CLASS:

public class StackNode{

  private StackNode link; //link to next node
  private int value;

  public StackNode(int value, StackNode linkValue){
    this.link = link;
    this.value = value;
  }
  public StackNode(){
   this.link = null;
  }
  public void setNodeData(int value){
   this.value = value; 
  }
  public void setLink(StackNode newLink){
   this.link = newLink; 
  }
  public int getValue(){
   return this.value; 
  }
  public StackNode getLink(){
   return link; 
  }
}

LINKED LIST CLASS:

public class IntStackList{

 private StackNode head;

 public IntStackList(){ this.head = null; }
 public void push(int value){
   this.head = new StackNode(value, head);
 }
 public int pop(){
   int value = this.head.getValue(); //get the int value stored in the head node
   this.head = head.getLink(); //sets the head to the next node in line
   return value;
 }
}

I am implementing this in a program that converts a decimal number to binary(for a class). I can print data from the first node AKA the head of the linked list but then I have the null issue when popping again.

Upvotes: 0

Views: 69

Answers (1)

MadProgrammer
MadProgrammer

Reputation: 347204

You're assigning link to itself in the constructor if the StackNode...

public class StackNode {

    private StackNode link; //link to next node
    private int value;

    public StackNode(int value, StackNode linkValue) {
        this.link = link;
        this.value = value;
    }

It should be 

this.link = linkValue;

Upvotes: 2

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