Implementing a three-valued logic operation with Boolean operators?

Question

I've created an enum class Proposition with three possible constants,

public enum Proposition {

    TRUE, FALSE, NULL;

} 

and I'd like to make it so that, when I write code using Boolean operators

Proposition propA = Proposition.FALSE;

Proposition propB = Proposition.NULL;

Proposition propC = propA && propB;

propC would become NULL, following Kleene's logic. https://en.wikipedia.org/wiki/Three-valued_logic

How would I implement that in Java? Do I HAVE to use a method or can I use Boolean operators?

Answer 1

You can implement this yourself using enum methods (something like what we get from Predicate); operators can't be overridden in Java. Here's an example:

enum Proposition {
    TRUE(Boolean.TRUE),
    FALSE(Boolean.FALSE),
    NULL(null) {
        @Override
        public Proposition and(Proposition other) {
            return this;
        }

        @Override
        public Proposition or(Proposition other) {
            return this;
        }
    };

    private final Boolean value;
    Proposition(Boolean value) {
        this.value = value;
    }

    public Proposition and(Proposition other) {
        if (NULL == other) return NULL;
        return of(this.value && other.value);
    }
    public Proposition or(Proposition other) {
        if (NULL == other) return NULL;
        return of(this.value || other.value);
    }

    public static Proposition of(Boolean bool) {
        return null == bool ? NULL : (bool ? TRUE : FALSE);
    }
}

And here's the call corresponding to your example:

Proposition propA = Proposition.FALSE;
Proposition propB = Proposition.NULL;

Proposition propC = propA.and(propB);
System.out.println(propC); // NULL

In other words, call .or() method for || and .and() method for &&.