Question

I have created a solution for optionals in Java.

public final class Optional<T> {

    private final T reference;

    private Optional(T reference) {
         this.reference = reference;
    }

    public T get() {
        if (!isPresent()) {
            throw new IllegalStateException("Cannot retrieve reference when absent!");
        }
        return reference;
    }

    public boolean isPresent() {
        return reference != null;
    }

    public static <T> Optional<T> of(T reference) {
        return new Optional<T>(reference);
    }

    public static <T> Optional<T> absent() {
        return of(null);
    }

    public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
        return of(nullableReference);
    }

}

But when I am using it in my production code, the compiler complains.

This is my production code:

public final class OnsetSequencer {

    private final Onset onset;
    private final Optional<EventManager> eventManager;

    public OnsetSequencer(Onset onset, Optional<EventManager> eventManager) {
        this.onset = onset;
        this.eventManager = eventManager;
    }

    public OnsetSequencer(Onset onset) {
        this(onset, Optional.absent());
    }

    public void sequence() {
        boolean present = eventManager.isPresent();
        if (present) {
            eventManager.get().dispatchEvent(new OnsetBeginEvent(onset));
        }
        onset.begin();
        if (present) {
            eventManager.get().dispatchEvent(new OnsetEndEvent(onset));
        }
        onset.end();
    }

}

The compiler complains at this(onset, Optional.absent()); saying: The constructor OnsetSequencer(Onset, Optional) is undefined

I have tried to fix the issue by changing it to this(onset, Optional<EventManager>.absent());
That syntax is wrong as well.

I am wondering how to fix this issue!

Answer 1

I think you want:

Optional.<EventManager>absent()

I’ve never liked the way of expressing type arguments for generic methods in Java, but such is life. See section 15.12 of the JLS for details.