package schemeFactory;

/**
 * Concrete IListFactory to manufacture AList as composites.
 * EmptyList and NEList are static nested classes and hidden from all external
 * client code.  The implementations for EmptyList and NEList are the same as
 * before but completely invisible to the outside of this factory.
 * @author D.X. Nguyen
 */
public class CompositeListFactory implements IListFactory {

    /**
     * Note the use of private static.
     */
    private static class EmptyList extends AList {
        public final static EmptyList Singleton = new EmptyList();

        private EmptyList() {
        }

        public Object getFirst() {
            throw new IllegalArgumentException("Empty list has no first!");
        }

        public AList getRest() {
            throw new IllegalArgumentException("Empty list has no first!");
        }
        public Object execute(IListAlgo algo, Object inp) {
            return algo.emptyCase(this, inp);
        }
    }

    /**
     * Note the use of private static.
     */
    private static class NEList extends AList {
        private Object _first;
        private AList _rest;

        public NEList(Object dat, AList tail) {
            _first = dat;
            _rest = tail;
        }

        public Object getFirst() {
            return _first;
        }

        public AList getRest() {
            return _rest;
        }

        public Object execute(IListAlgo algo, Object inp) {
            return algo.nonEmptyCase(this, inp);
        }
    }

    public AList makeEmptyList() {
        return EmptyList.Singleton;
    }

    public AList makeNEList(Object first, AList tail) {
        return new NEList(first, tail);
    }
}