Converting Map<K, V> to Map<V,List<K>>

Question

I have map as below

Map<String, String> values = new HashMap<String, String>();
values.put("aa", "20");
values.put("bb", "30");
values.put("cc", "20");
values.put("dd", "45");
values.put("ee", "35");
values.put("ff", "35");
values.put("gg", "20");

I want to create new map in the format Map<String,List<String>> , sample output will be as

"20" -> ["aa","cc","gg"]
"30" -> ["bb"]
"35" -> ["ee","ff"]     
"45" -> ["dd"]

I am able to do by iterating through entity

Map<String, List<String>> output = new HashMap<String,List<String>>();
    for(Map.Entry<String, String> entry : values.entrySet()) {
        if(output.containsKey(entry.getValue())){
            output.get(entry.getValue()).add(entry.getKey());

        }else{
            List<String> list = new ArrayList<String>();
            list.add(entry.getKey());
            output.put(entry.getValue(),list);
          }
    }

Can this be done better using streams?

Answer 1

groupingBy can be used to group the keys by the values. If used without a mapping Collector, it will transform a Stream of map entries (Stream<Map.Entry<String,String>>) to a Map<String,List<Map.Entry<String,String>>, which is close to what you want, but not quite.

In order for the value of the output Map to be a List of the original keys, you have to chain a mapping Collector to the groupingBy Collector.

Map<String,List<String>> output =
    values.entrySet()
          .stream()
          .collect(Collectors.groupingBy(Map.Entry::getValue,
                                         Collectors.mapping(Map.Entry::getKey,
                                                            Collectors.toList())));
System.out.println (output);

Output :

{45=[dd], 35=[ee, ff], 30=[bb], 20=[aa, cc, gg]}