Re: Encapsulating HashMap bulding

Robert Klemme <>
Tue, 11 May 2010 07:52:17 +0200
On 11.05.2010 00:20, Roedy Green wrote:

Can anyone think of a way to write a method that takes an array of X,
and produces a HashMap<Key,X>

How would you specify the name of the key field/method?

Maybe you could do it by making X implement an interface that defines
the key.

Perhaps you could do it with reflection.

I'd rather provide an interface that is responsible for the conversion
from X to Key - this is more modular.

package util;

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class MapUtil {

   public interface Transformer<A, B> {
     B transform(A a);

   public static <K, V> Map<K, V> createHash(V[] i, Transformer<V, K> t) {
     return createHash(Arrays.asList(i), t);

   public static <K, V> Map<K, V> createHash(Iterable<V> i,
Transformer<V, K> t) {
     final Map<K, V> map = new HashMap<K, V>();
     fill(map, i, t);
     return map;

   public static <K, V> void fill(Map<K, V> m, Iterable<V> i,
Transformer<V, K> t) {
     for (V val : i) {
       m.put(t.transform(val), val);

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