add

The add method does something substantial for empty trees.

public SearchTree add(Comparable item) {
    return new NodeTree(item);
}

This constructs a new NodeTree with the item in its data field, with empty left and right subtrees, and returns it. In fact every insertion is ultimately done in this way. For the NodeTree class we have

public SearchTree add(Comparable item) {
    int compare = item.compareTo(data);
    if (compare < 0) {
        left = left.add(item);
    } else if (compare > 0) {
        right = right.add(item);
    }
    return this;
}

Note that ultimately this is returned, i.e. the reference is unchanged, but that the left or right subtrees may have changed. Assuming the item is not already present in the tree, eventually left or right will be empty and the add method called will be from the EmptyTree class. If the item is already present, nothing happens and the original tree is returned. (Other approaches are possible, but this is the simplest.)