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Message: [QUOTE="Tom Anderson, post: 4199783"] Yes, and i tried not to think about it, because it's smelly. How do you obtain these objects? Do you make them purely to use as key-holders? Does that mean you're going to have half-filled-out instances of your value class floating about the place? Does that mean you have to provide a constructor that allows such instances to be created, so abandoning the invariants that you could otherwise enforce through your constructors? Or do you need to do something like: public interface KeyHolder { public String getKey(); } public class Value implements KeyHolder { private final String key; // other fields public String getKey() { return key; } // other methods } public class Example implements KeyHolder { // so called for "query by example" private final String key; public Example(String key) { this.key = key; } public String getKey() { return key; } } Map<KeyHolder, Value> objects = new TreeMap<KeyHolder, Value>(new Comparator<KeyHolder, KeyHolder>(){ public int compare(KeyHolder a, KeyHolder b) { return a.getKey().compareTo(b.getKey()); } }); Value v = ...; String k = v.getKey(); objects.put(v, v); Value u = objects.get(new Example(k)); assert u == v; ? It can certainly be done, but all things considered, i lean towards extracting the key before hitting the map. The map could perhaps be wrapped in a simple wrapper: public class ValueStore { private Map<String, Value> map = new HashMap<String, Value>(); public void put(Value v) { map.put(v.getKey(), v); } public Value get(String key) { return map.get(key); } } So that calling code is not troubled with the details. tom [/QUOTE]

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