Question

Here is a piece of code in my algorithm:

public void insert(int element) {
    _insert(element, root);
    System.out.println(root);
}

private void _insert (int element, Node t) {
    if (t == null) {
        t = new Node(null, element);;
        return;
    }
}

Node here is a predefined class.

When the public method insert is called, the private method will be called and check if the tree is empty. If so, a new node would be created at root position.

The output is supposed to be a node. However the actual output is null which means root is not updated though it’s passed as t into the private method.

This should work under C++ with pointers. Maybe I misunderstand something in Java?

Answer 1

Java always uses pass-by-value for method calls. When you pass a reference type to a method, a copy of the reference is made.

The following line changes t to refer to a new object, but it does not change the original variable root:

t = new Node(null, element);

---

You can change your method to return the inserted node, as follows:

private Node _insert (int element, Node t) {
    if (t == null) {
        t = new Node(null, element);
    }
    return t;
}

And call like this:

root = _insert(element, root);