@Override
    public int hashCode() {
        // Since all undefined maybes are equal, we can hard-code their hashCode to -1.
        // Since all defined null maybes are equal, we can hard-code their hashCode to 0.
        return defined ? nullableValue == null ? 0 : nullableValue.hashCode() : -1;
    }
A container that may be empty, may contain null, or may contain a value.
Distinct from {@link java.util.Optional<T>}, since Optional cannot contain null.

@param <T>
streams/src/main/java/org/apache/kafka/streams/state/internals/Maybe.java