When I write a class in Java, I like to initialize the attributes which are set to a default value directly and attributes which are set by the caller in the constructor, something like this:

public class Stack<E> {
    private List<E> list;
    private size = 0;

    public Stack(int initialCapacity) {
        list = new ArrayList<E>(initialCapacity);
    }

    // remainder omitted
}

Now suppose I have a Tree class:

public class Tree<E> {
    private Node<E> root = null;

    // no constructor needed, remainder omitted
}

Shall I set the root attribute to null, to mark that it is set to null by default, or omit the null value?