Re: Encapsulating HashMap bulding

Daniel Pitts <>
Tue, 11 May 2010 15:18:18 -0700
On 5/10/2010 3:20 PM, 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.

public class Table<R> {
    private final List<UniqueIndex<?, R>> indexes =
                      new ArrayList<UniqueIndex<?, R>>();

    public <K> UniqueIndex<K, R> addIndex(KeyExtractor<K, R> extractor) {
       // create index and add to indexes

    public void add(R item) {
        for(UniqueIndex<?, R> index: indexes) {

public class UniqueIndex<K, R> {
    private final KeyExtractor<K, R> extractor;
    private final Map<K, R> map = new HashMap<K, R>();
    public R add(R item) {
       return map.put(extractor.extract(item), item);
    public Map<K, R> getMap() { /* return copy */ }

public interface KeyExtractor<K, R> {
   K extract(R);

 > Perhaps you could do it with reflection.
Don't use reflection unless you have a really good reason. I would
prefer to use code generation before I use reflection in this situation.

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