A suggestion: Map.checkAndGet

[Boaz Nahum]

The pattern
map.containsKey(k) ? map.get() : othervAlue
(walks on tree twice)

repeats itself so many times, as in Mappers:

 public static <T, U> Mapper<T, U> forMap(Map<? super T, ? extends U> map,
U defaultValue) {
        Objects.requireNonNull(map);

        return t -> map.containsKey(t) ? map.get(t) : defaultValue;
 }


Now that we have 'default methods', we can add to Map:

NullableOptional<V> checkAndget(K key) default {

        if (containsKey(key)) {
            return new NullableOptional<>(get(key));
        } else {
            return NullableOptional.empty();
        }
    }

**and** implement it in Map implementation in such way that will walk on
tree only once.
(Assume creating instance of NullableOptional is more  efficient )

forMap becomes:

public static <T, U> Mapper<T, U> forMap(MyMap<? super T, ? extends U> map,
U defaultValue) {
        Objects.requireNonNull(map);


        return t -> {
            NullableOptional<? extends U> o = map.checkAndget(t);
            return o.isPresent() ? o.get() : defaultValue;
            //return o.orElse(defaultValue);
        };
    }