Enumerations

We now come to the implementation of the two enumeration methods. This is how they are specified in the SearchTree class definition:

public abstract Enumeration elementsInOrder();

public abstract Enumeration elementsLevelOrder();

In both cases (or for PreOrder, PostOrder etc) we have to write a method that returns an Enumeration, i.e. a member of a class implementing the interface

public interface Enumeration {
    boolean hasMoreElements();
    Object nextElement();
}

and we have to do this both for the EmptyTree class and for the NodeTree class.

For the EmptyTree class, this is a formality: it is never true that an empty tree has more elements, and a call for the next element should always throw an exception. In fact this is true of any empty structure, and we define a constant empty enumeration in Figure 8.3 together with the two SearchTree enumerations for the EmptyTree subclass.

Figure 8.3: The two SearchTree enumerations for the EmptyTree subclass.

private static Enumeration theEmptyEnumeration =
    new Enumeration() {
        public boolean hasMoreElements() {
            return false;
        }
        public Object nextElement() {
            throw new NoSuchElementException();
        }
    };

public Enumeration elementsInOrder() {
    return theEmptyEnumeration;
}

public Enumeration elementsLevelOrder() {
    return theEmptyEnumeration;
}