nrNodes

The nrNodes method is simple for the EmptyTree class. An empty tree has no nodes, so we have

public int nrNodes() {
    return 0;
}

In the NodeTree class, however, we implement the nrNodes method recursively as follows:

public int nrNodes() {
    return left.nrNodes() + right.nrNodes() + 1;
}

In other words, the number of nodes in a NodeTree is one more than the sum of the numbers of nodes in its left and right subtrees.