Maybe.java

abstract class Maybe<A> {

    Maybe() { }

    abstract A toNullableReference();
}

final class Just<A> extends Maybe<A> {

    private final A value;

    Just(A value) {
        this.value = value;
    }

    @Override
    A toNullableReference() {
        return value;
    }
}

final class Nothing<A> extends Maybe<A> {

    @Override
    A toNullableReference() {
        return null;
    }
}