Implementing Set using BST

Question

Hi, I am implementing the Set using Binary Search tree but am stuck and I cannot go further. Could someone help me get this code working? I am trying to make the add method to work but it does not compile and it says it cannot find the compareTo(java.lang.Object) method. When i don't have the compareTo method, it says i have to override the compareTo. I think i can get the contains method working if the add method works. Code below:
import java.util.*;
import java.lang.*;
public class SetImplementation<E> implements SimpleSet <E> {
    private TreeNode root ;
    private int NodeCount ;
  public SetImplementation() {
      root = null ;
      NodeCount = 0 ;
  public boolean add(Object obj) {
     if(root == null) {
        root = new TreeNode(obj) ;
        NodeCount++ ;
        return true ;
   return recursiveAdd(root, obj) ;
public boolean recursiveAdd(TreeNode node, Object obj) {
     int compareResult = (node.getObj().compareTo(obj)) ;
         if(compareResult == 0) {
            System.out.println("You cannot duplicate values") ;
            return false;
         else if(compareResult < 0) {
            node.left = add( node.left, obj );
            NodeCount++ ;
            return true ;
         else if(compareResult > 0) {
            node.left = add( node.right, obj );
            NodeCount++ ;
            return true ;
  public boolean contains(Object o) { return false ;  }
  public boolean isEmpty() { return root == null ; }
  public int size() { return NodeCount ; }
  public void clear() { root = null ;  }
  public class TreeNode implements Comparable {
      Object obj ;
      TreeNode left , right ;
      public TreeNode(Object o) {
          obj = o ;
          left = null ;
          right = null ;
      public int compareTo(Object other) {
         return 1; //I don't know how to do the comparison. I hav never used this interface
      }             //I never used the comparable interface before, so i am just out of ideas
    }               //So i just try to return an integer to se if it works. BUt it does not.
 

Answer 1

You're comparing the Object references from your TreeNode classes but these are not Comparable. So, you should store Comparables in your TreeNode's. And you also want to add types to your tree that are Comparable, right?
If I were you, I'd first try to get this working without the generics (the strange brackets: <E>). When that works, extend your code using generics. Here's a small start:
class SetImplementation implements SimpleSet {
    public boolean add(Comparable obj) { // <-- Comparable here!
        return recursiveAdd(root, obj) ;
    private boolean recursiveAdd(TreeNode node, Comparable obj) { // <-- Comparable here! (and made it 'private')
        int compareResult = node.obj.compareTo(obj) ;
    public class TreeNode {
        Comparable obj ; // <-- Comparable here!
}