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How to automatically fill in a value in Map <String, Collection> on request

I often write code like this:

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 instruction ifabove - how can I get rid of it in standard Java? If you can confirm that there is no standard Java solution, it would be nice if you proposed a non-standard library that satisfies this need.

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2 answers

Apache Commons collections have MultiMap :

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

Guava (usually Google Collections) also has a value that supports generics, and has a tree and hash version:

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

, Multimap<T, S> Map<T, Collection<S>>, put , .

EDIT: Guava, , , Google Collections .

+2

... multimap- Brendan, / java.util:

public static <K,V> Set<V> getTreeSet(Map<K,Set<V>> map, K key) {
    Set<V> set = map.get(key);
    if (set == null) {
        set = new TreeSet<V>();
        map.put(key, set);
    }
    return set;
}

:

Map < String, Set < String > > map = new TreeMap < String, Set < String > >();
String key;
String element;
/* ... */
MapUtils.getTreeSet(map, key).add(element);

, , .

+1

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