Create an object?

[leon matthew]

Hi,
I want to read in strings, and store them in a binary tree, i know that they must be objects, the problem i'm having now is that i don't know how to write the insert method because it must have (Comparable object) has parameters and when i try to store a the word i read in as a string object - Object word = reader.readLine(); and then TRY to store it i get
TreeTest1.java:14: insert(java.lang.Comparable) in Tree cannot be applied to (java.lang.Object)
t.insert(word);
^
Any ideas?
Thanks.

[David O'Meara]

public final class String
extends Object
implements Serializable, Comparable
The String class represents character strings... etc

A String IS A Comparable class so it can be used as the input to the method. The method will treat it as Comparable class.

String word = reader.readLine();
//assuming readLine returns a String, it sounds like it should
t.insert(word);

Dave.

[leon matthew]

Thanks i tried that and now i'm getting a NullPointerException:
This is the code for my tree:
class Tree
{
// Constructs an empty Tree
public Tree()
{
root = null;
}
/*
Inserts a new node into the tree.
@param obj the object insert
*/
public void insert(Comparable obj)
{
Node newNode = new Node();
newNode.data = obj;
newNode.left = null;
newNode.right = null;
if (root == null)
root = newNode;
else
root.insertNode(newNode);
}
/*Prints the contents of the tree
in a sorted order
*/
public void print()
{
if (root != null)
root.printNodes();
}
private Node root;
private class Node
{
/*
Inserts a new node as a descendant of this node
@param newNode the node to insert
*/
public void insertNode(Node newNode)
{
if (newNode.data.compareTo(data) < 0)
{
if(left == null)
left = newNode;
else
left.insertNode(newNode);
}
else
{
if(right == null)
right = newNode;
else
left.insertNode(newNode);
}
}
/*
prints this node and ALL of its
descendants in sorted order
*/
public void printNodes()
{
if (left != null)
left.printNodes();
System.out.println(data);
if (right != null)
right.printNodes();
}
public Comparable data;
public Node left;
public Node right;
}
}
I must have made a really dumb and obvious mistake but i can't see it!

[Jim Yingst]

If you're getting a NullPointerException, you are also getting a stack trace. Study this carefully - it tells you exactly which line is throwing the stack trace. This is usually a good clue about what your problem is. If you can't work out the problem, at least tell us which line the stack trace reports as the source of the error.

[leon matthew]

Sorry. These are the errors:
>left.insertNode(newNode); insertNode method
at Tree$Node.insertNode(TreeTest1.java:90)
>root.insertNode(newNode); insert method

at Tree.insert(TreeTest1.java:56)
>t.insert(word);

at TreeTest1.main(TreeTest1.java:14)
The word being read in is through a BufferedReader reading a file specified at the command prompt.
I've been at this problem for several hours now...its frustrating.
[This message has been edited by leon matthew (edited October 25, 2001).]

[Peter den Haan]

if(right == null)
   right = newNode;
else
   left.insertNode(newNode);

Looks wrong.
- Peter

[leon matthew]

I'm such an idiot! I've been staring at this thing for ages and never noticed. thanks :-)