Apache Commons Collections

http://commons.apache.org/proper/commons-collections/userguide.html

1. Utilities

• SetUtils
• CollectionUtils 
• MapUtils

2.Maps

• Map Iteration
  • 　　

    IterableMap map = new HashedMap();
    MapIterator it = map.mapIterator();
    while (it.hasNext()) {
      Object key = it.next();
      Object value = it.getValue();
      
      it.setValue(newValue);
    }

• Ordered Maps
  • 　　

    OrderedMap map = new LinkedMap();
    map.put("FIVE", "5");
    map.put("SIX", "6");
    map.put("SEVEN", "7");
    map.firstKey();  // returns "FIVE"
    map.nextKey("FIVE");  // returns "SIX"
    map.nextKey("SIX");  // returns "SEVEN"

• Bidirectional Maps
  • 　　

    BidiMap bidi = new TreeBidiMap();
    bidi.put("SIX", "6");
    bidi.get("SIX");  // returns "6"
    bidi.getKey("6");  // returns "SIX"
    bidi.removeValue("6");  // removes the mapping
    BidiMap inverse = bidi.inverseBidiMap();  // returns a map with keys and values swapped

3.Bags

Bag bag = new HashBag();
bag.add("ONE", 6);  // add 6 copies of "ONE"
bag.remove("ONE", 2);  // removes 2 copies of "ONE"
bag.getCount("ONE");  // returns 4, the number of copies in the bag (6 - 2)