simplification of function signature in java

You just need to use this as a reference to the curent object.

 public void add(int insert) {
    addRecursive(this, insert);
}

Also in Depth() you can do as following:

public int depth() => depth(this);

private int depth(Tree tree) {
    // Root being null means tree doesn't exist.
    if (tree == null)
        return 0;

    // Get the depth of the left and right subtree
    // using recursion.
    int leftDepth = depth(tree.lhs);
    int rightDepth = depth(tree.rhs);

    // Choose the larger one and add the root to it.
    if (leftDepth > rightDepth)
        return (leftDepth + 1);
    else
        return (rightDepth + 1);
}

}