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Thread: Problem with Binary Search Tree

  1. #1
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    Default Problem with Binary Search Tree

    Hi, I'm new to the forum and am not sure if this is the right place to post this, but I'm getting a problem with the following Binary Search Tree:

    import java.awt.*;
    import java.awt.event.*;
     
    import javax.swing.*;
     
    import java.util.Comparator;
    class BST{
    private BSTNode t=null;
    private BSTNode l = null;
    private BSTNode r = null;
    private BSTNode node;
    private StringComparator comp;// how to compare nodes
    public BST(Comparator c){
     
     
    }
     
    // constructor
    public BSTNode insert(Object newData){
    BSTNode temp=t;
    if (t == null)
    {
        t=new BSTNode(newData);
        t.left= new BSTNode(null);
        t.right= new BSTNode(null);
        return t;
    }
    else
    {
     
        while (temp.data != null)
        {
            if (comp.compare(temp.data, newData)<0)
                {
                    System.out.println("going left");
                    temp=temp.left;
                }
                else
                {
                    System.out.println("going right");
                    temp=temp.right;
                }
     
        }
        temp.data=newData;
        temp.left= new BSTNode(null);
        temp.right= new BSTNode(null);
        return temp;
    }
    //  if (comp.compare(t.data, newData)<0)
    //  return t.left = insert(newData);
    //else return t.right = insert(newData);
    }
     
     
    // can check if comp is null before insert
    public void setComparator(StringComparator c){
    if (t == null)
        c.compare(t.data, node.data);
    }
    // only allow if tree empty
    public void clear(){
     
        t = null;
    }// empty tree
    public BSTNode find(String key)
    {
     if (t == null)
     return null;
    if (key == t.data)
     return t;
    if (comp.compare(key, t.data)<0)
    return find(key);
    else
        return find(key);
    }
    public void getText(JTextField t, String key)
    {
    t.setText(key);
    }
    public String toString()
    {
    String fail= "The object was not found.";  
    return fail;}
    }

    In particular I'm getting a null pointer exception on line 33 (if (comp.compare(temp.data, newData)<0)).

    Any idea why this is happening? Help would be much appreciated!


  2. #2
    Super Moderator copeg's Avatar
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    Default Re: Problem with Binary Search Tree

    Use the System.println method and add some println's in there to monitor the command line and see what object on that line could be null. NullPointerException indicates a reference is null...so when you find anything that is null, backtrack and make sure it is assigned properly (either to a valid reference or creating a new one). For future reference, posting an SSCCE with descent indenting helps
    Last edited by copeg; April 25th, 2012 at 09:02 PM.

  3. #3
    Super Moderator copeg's Avatar
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    Default Re: Problem with Binary Search Tree

    And please be forthright when crossposting - many (including myself) do not appreciate writing responses to posts which may have been answered or discussed elsewhere without their knowledge

    This thread has been cross posted here:

    http://www.java-forums.org/new-java/58889-problem-binary-search-tree.html

    Although cross posting is allowed, for everyone's benefit, please read:

    Java Programming Forums Cross Posting Rules

    The Problems With Cross Posting


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