RFC: draft API for JEP 269 Convenience Collection Factories

[Ivan Gerasimov]

Hi Stuart!

> Most of the API is pretty straightforward, with fixed-arg and varargs 
> "of()" factories for List, Set, ArrayList, and HashSet; and with 
> fixed-arg "of()" factories and varargs "ofEntries()" factories for Map 
> and HashMap.
>

Has it been considered to separate a Map creation into two steps: 
specifying keys and then values?
Varargs methods can then be used in each step.

I understand, that keeping the key and the value close each other looks 
better in many cases.
However, separating them can allow some additional flexibility.

Here's a draft of what it might look like:

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import java.util.*;
import java.util.function.*;

public class MapOf {

     public static void main(String[] a) {
         Map<Integer,Character> m1 = MyCollections.<Integer,Character>
                  ofKeys( 1,   2,   3,   4,   5)
                 .vals(  'a', 'b', 'c', 'd', 'e');
         m1.forEach((k, v) -> System.out.println("m1[" + k + "] = " + v));

         Map<Integer,String> m2 = MyCollections.<Integer,String>
                  ofKeys( 1,   2,   3,   4,   5)
                 .calculateVals(k -> "#" + k + "#");
         m2.forEach((k, v) -> System.out.println("m2[" + k + "] = " + v));
     }
}


class MyCollections {
     public static <K,V> KeysHolder<K,V> ofKeys(K... keys) {
         return new KeysHolder<>(keys);
     }

     public static class KeysHolder<K,V> {
         K[] keys;
         KeysHolder(K[] keys) {
             this.keys = keys;
         }

         public Map<K,V> vals(V... vals) {
             if (vals.length != keys.length) {
                 throw new IllegalArgumentException();
             }

             Map<K,V> result = new HashMap<>();
             for (int i = 0; i < keys.length; ++i) {
                 result.put(keys[i], vals[i]);
             }
             return result;
         }

         public Map<K,V> calculateVals(Function<K,V> fun) {
             Map<K,V> result = new HashMap<>();
             for (int i = 0; i < keys.length; ++i) {
                 result.put(keys[i], fun.apply(keys[i]));
             }
             return result;
         }
     }
}
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Sincerely yours,
Ivan