Hi Rémi, Thanks for looking at the proposal.
We did consider this style of builder as one of the alternatives to what's in the JEP. The concern I have about builders such as this is that the number of entries isn't available at the time the map is to be built.
For HashMap, this means that a default size has to be chosen. It can either be too big, meaning space is wasted, or too small, which means that it has to be resized while being built.
For an unmodifiable map, in practical terms it means that the keys and values have to be accumulated in a temporary structure before the result map can be built. The reason is that we might want to choose different map implementations based on the size of the map.
(An additional, though unrelated issue is that in this example, the builder holds a reference to the map being constructed. If malicious code were to store this builder somewhere, it could be used to modify the result map at unforeseen times, leading to misbehavior. Careful coding could avoid such problems, possibly at the cost of making defensive copies.)
None of these problems are insurmountable, but it doesn't seem clear to me that this or similar builder approaches necessarily have any advantage over the varargs-of-entries approach outlined in the JEP. The varargs-of-entries approach of course has its own overhead. There is the possibility of the entry objects being replaced by value types in a future JDK (at least, Brian thinks this might be possible!) so that's what tipped us toward that approach.
s'marks On 9/25/15 12:32 AM, Remi Forax wrote:
Hi Mark, hi Stuart, hi all, for Map, i think a version with a builder will be cool too, something like: public interface EntryBuilder<K, V> { public EntryBuilder<K,V> entry(K key, V value); } and by example for HashMap: public static <K, V> HashMap<K, V> fromBuilder(Consumer<? super EntryBuilder<K,V>> consumer) { HashMap<K,V> map = new HashMap<>(); consumer.accept(new EntryBuilder<>() { public EntryBuilder<K,V> entry(K key, V value) { map.put(key, value); return this; } }); return map; } ... HashMap<String, Integer> map = HashMap.fromBuilder(b -> b .entry("foo", 1) .entry("bar", 2)); The builder pattern let us to avoid to create intermediary entry objects. regards, Rémi ----- Mail original -----De: "mark reinhold" <mark.reinh...@oracle.com> À: "stuart marks" <stuart.ma...@oracle.com> Cc: core-libs-dev@openjdk.java.net Envoyé: Jeudi 24 Septembre 2015 02:02:17 Objet: JEP 269: Convenience Factory Methods for Collections New JEP Candidate: http://openjdk.java.net/jeps/269 - Mark
