A suggestion: Map.checkAndGet

[Zhong Yu]

How can we add any default method to an interface, since its behavior
might conflict with some unknown subclass' unknown additional
contracts?

On Tue, Oct 30, 2012 at 11:49 AM, Mike Duigou <mike.duigou at oracle.com> wrote:
> Unfortunately such a method would introduce non-atomic behaviour for synchronized maps so we can't add it.
>
> Mike
>
> On Oct 30 2012, at 08:25 , Boaz Nahum wrote:
>
>> 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);
>>        };
>>    }
>>
>
>