java Binary search tree insertion recursion seem to always return null root

Question

Hi I'm currently trying to build a Java BST with some online references, but I have a problem during my insertion as I realize it does not create a tree after I tried to do inOrder traversal, after some attempts I found issue when passing the root into my insertion method.

My Node class:

public class TreeNode
{
    String m_key;
    Object m_value;
    TreeNode m_left;
    TreeNode m_right;
    
    //constructor
    public TreeNode(String inKey, Object inVal)
    {
        if(inKey==null)
        {
            throw new IllegalArgumentException("Key cannot be null.");
        }
        
        m_key = inKey;
        m_value = inVal;
        m_left = null;
        m_right = null;
    }
}

My insert method:

public void insert(String key, Object data)
{
    m_root = insertRec(m_root, key, data);
}

private TreeNode insertRec(TreeNode x, String key, Object val)
{
    if(x == null)
    { 
        x = new TreeNode(key,val);
    }
    
    int cmp = key.compareTo(x.m_key);
    if(cmp<0)
    {
        x.m_left = insertRec(x.m_left, key, val);
    }
    else if(cmp>0)
    {
        x.m_right = insertRec(x.m_right, key, val);
    }
    else
    {
        x.m_value = val;
    }
    
    return x;     
}

print root:

public void printRoot()
{
    System.out.println("the root is: " + m_root.m_key);
}

My main class:

public static void main(String[] args)
{
   binaryTree bt = new binaryTree();
   bt.insert("a", "data a");
   bt.insert("b", "data b");
   bt.insert("c", "data c");
   bt.printRoot();
}

I got "a" as root result from printing root, and I tried to print root.m_left it shows null. Is there something I could've missed?

Answer 1

The recursive part after the first if-statement will always be executed. Also, given it's a BST, if comp <= 0 recur left:

if(x == null) { 
  x = new TreeNode(key,val);
} else {  
  int cmp = key.compareTo(x.m_key);
  if(cmp <= 0) {
    x.m_left = insertRec(x.m_left, key, val);
  } else {
    x.m_right = insertRec(x.m_right, key, val);
  }
}
return x;