Java invert map

Question

I need create inverse map - select unique values and for them find keys. Seems that only way is to iterate all key/value pairs, because entrySet returns set of so value not

Answer 1

I would give another approach to this problem giving an extra dimension: duplicate values in EntrySet.

public static void main(String[] args) {

    HashMap<Integer, String> s = new HashMap<Integer, String>();
    s.put(1, "Value1");
    s.put(2, "Value2");
    s.put(3, "Value2");
    s.put(4, "Value1");

    /*
     * swap goes here
     */
    HashMap<String,List<Integer>> newMap = new HashMap<String, List<Integer>>();
    for (Map.Entry<Integer, String> en : s.entrySet()) {
        System.out.println(en.getKey() + " " + en.getValue());

        if(newMap.containsKey(en.getValue())){
            newMap.get(en.getValue()).add(en.getKey());
        } else {
            List<Integer> tmpList = new ArrayList<Integer>();
            tmpList.add(en.getKey());
            newMap.put(en.getValue(), tmpList);
        }
    }

    for(Map.Entry<String, List<Integer>> entry: newMap.entrySet()){
        System.out.println(entry.getKey() + " " + entry.getValue());
    }
}

T result will be that:

1 Value1
2 Value2
3 Value2
4 Value1
Value1 [1, 4]
Value2 [2, 3]

Answer 2

With Guava

Multimaps.transformValues(Multimaps.index(map.entrySet(), Map.Entry::getValue),
        Map.Entry::getKey)

You'll get a multimap (basically a map of lists) in return.