public void add(int value) {
 root = addRecursive(root, value);
}

public void add(int value) {
 if (root == null) {
  root= new Node(value);
  }
// then i add left- and right-leafs recursively
if (value < root.value) {
   root.left = addRecursive(root, value);
} else if (value > root.value) {
  root.right= addRecursive(root, value);
 }
}

bt.add(2);
bt.add(1);
bt.add(3);

private Node addRecursive(Node current, int value) {
 if (current == null) {
  return new Node(value);
  }
 if (value < current.value) {
  current.left = addRecursive(current.left, value);
  } else if (value > current.value) {
    current.right = addRecursive(current.right, value);
  }
  return current;
}