public interface Map<K,V> extends Function<K, V>

public interface Map<K,V> extends Function<K, V>, Predicate<K>

@Override default boolean test(K k) {
    return containsKey(k);
}

@Override default V apply(K k) {
    return get(k);
}

final MyMagicMap<String, Integer> map = new MyMagicHashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);
map.put("D", 4);

final Stream<String> strings = Arrays.stream(new String[]{"A", "B", "C", "D"});
final Stream<Integer> remapped = strings.map(map);

Function<K, V> f = map::get;

Predicate<Object> p = map::contains;