Grouping Java HashMap keys by their values

hashmap, java

Solution

With Java8 streams

Map<Integer, List<String>> valueMap = map.keySet().stream().collect(Collectors.groupingBy(k -> map.get(k)));

Output

{1=[E], 2=[C], 3=[A, F], 4=[B, D]}

using method ref

Map<Integer, List<String>> valueMap = map.keySet().stream().collect(Collectors.groupingBy(map::get));

to get the keys is sorted way

Map<Integer, List<String>> valueMap = map.keySet().stream().collect(Collectors.groupingBy(map::get, TreeMap::new, Collectors.toList()));

In Java7 and below

    Map<Integer, List<String>> valuesMap = new HashMap<>();
    for (String key : map.keySet()) {
        Integer val = map.get(key);
        if (valuesMap.get(val) == null) {
            List<String> values = new ArrayList<>();
            values.add(key);
            valuesMap.put(val, values);
        } else {
            valuesMap.get(val).add(key);
        }
    }

output

{1=[E], 2=[C], 3=[A, F], 4=[B, D]}

Problem

Suppose I have this HashMap: ``` (key: value) A: 3 B: 4 C: 2 D: 4 E: 1 F: 3 ``` and I want to convert it to this HashMap: ``` 1: {"E"} 2: {"C"} 3: {"A", "F"} 4: {"B", "D"} ``` How do I go about doing this?

Original source