DevoxxUK Lambdas Lab (original) (raw)

Stephen Colebourne scolebourne at joda.org
Wed Apr 3 05:50:59 PDT 2013

On 3 April 2013 13:31, Brian Goetz <brian.goetz at oracle.com> wrote:

ACTION: Add a static method to Map.Entry: public static <K, V> Entry<K, V> of(K key, V value) { return new AbstractMap.SimpleImmutableEntry<K, V>(key, value); } Been down this rathole, and we retreated. While you can make a stream out of Map.Entry, doing anything more than simple filtering and forEaching is unlikely to result in a happy outcome. If it hurts, don't do it.

The static method above is useful outside of stream-based development. SimpleImmutableEntry is a very hidden class that I didn't know about until I requested that the functionality was added. However SimpleImmutableEntry is also poor as it requires the generic parameters as well, whereas a static method can set them for you.

To me, this is a static method I've had to write on a number of occasions, and its really a no-brainer once you separate this request from its possible use in streams. Its a classic example of point lambdaification.

ACTION: Add a much better way to convert a stream of Entry to a Map. Perhaps: .collect(Collectors.toMap(entry -> getKey(), entry -> entry.getValue())); Or better yet, don't encourage people to make streams of Map.Entry.

I think thats hugely unrealistic. Maps are incredibly widely used in Java development, perhaps even more than collections. To say that maps can't be used with streams is a huge hole.

I really don't think the method above is a big ask, and it is generally useful for other cases. For example, processing a Stream with the goal of outputting Map<PersonId, Address>.

ACTION: Can there be some kind of reducingSum() method? It feels like a common use case, and might provide the source code for people to look at providing the example code for them to then learn and write more advanced reducers themselves. Do you mean something like: Collector<T, Integer> counting() { return reducing((T t) -> 1, Integer::sum); }

Yes. Although I named it reducingSum() partly so users could look at the source code and start to learn how reducing works by example.

Stephen

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