Question

I find myself often writing such code:

Map < String, Set < String > > map = new TreeMap < String, Set < String > >();
String key;
String element;
/* ... */
Set < String > value = map.get(key);
if (value == null) {
  value = new TreeSet < String >();
  map.put(key, value);
}
value.add(element);

I hate the if statement above — how can I get rid of it in standard Java? If you can confirm that there is no standard Java solution, it would then be nice if you could suggest a non-standard library that addresses this need.

Answer 1

Apache Commons Collections has a MultiMap:

MultiMap mhm = new MultiHashMap();
mhm.put(key, "A");
mhm.put(key, "B");
mhm.put(key, "C");
Collection coll = (Collection) mhm.get(key);

Guava (used to be Google Collections) has one too, which supports generics, and has both a tree and hash version:

Multimap<String,String> mhm = new TreeMultimap<String,String>();
// etc.

To clarify, Multimap<T, S> is basically the same as a Map<T, Collection<S>>, and put automatically creates the collection if it needs to.

EDIT: Updated to link to Guava, since apparently Google Collections is deprecated.