Inverting Map<Integer, Integer> into a Map<key, list<Integer>> using Streams

Is it possible to convert a Java Map<Integer, Integer> m0 to a new Map<Integer, List<Integer>> m1
where in the m1 map to have key as value of m0 map and value as List<Integer> of keys of m0 map?

In other words, this task it is about the inverting key-value pairs grouped by values.

I have tried the following:

 public static void main(String[] args) {
        Map<Integer, Integer> m = new HashMap<>();
        m.put(1, 1);
        m.put(2, 1);
        m.put(3, 1);
        m.put(4, 2);
        m.put(5, 2);
        m.put(6, 2);
        m.put(7, 2);

        Map<Integer, List<Integer>> personByAge 
                = m.values()
                .stream()
                .collect(Collectors.groupingBy(Function.identity(), Collectors.toList()));

        Iterator it = personByAge.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<Integer, List<Integer>> me = (Map.Entry)it.next();
            System.out.println(me.getKey() + " <- key" );
            for (int i : me.getValue())
            {
                System.out.println("Values: " + i);
            }
        }
    }

But the values in the second map are wrong.

The values should be:

1 <- key Values: 1 Values: 2 Values: 3
2 <- key Values: 4 Values: 5 Values: 6 Values: 7

But I’m getting:

1 <- key Values: 1 Values: 1 Values: 1
2 <- key Values: 2 Values: 2 Values: 2 Values: 2

>Solution :

it is about the inverting key values pair grouped by values

If you want to invert the Map as you’ve said in the comments, you need to create a stream over the entries of the source map, not over the values:

Map<Integer, List<Integer>> personByAge = m.entrySet()
    .stream()
    .collect(Collectors.groupingBy(
        Map.Entry::getValue,
        Collectors.mapping(Map.Entry::getKey,
            Collectors.toList())));
        
personByAge.forEach((k, v) -> System.out.println(k + " -> " + v));

Output:

1 -> [1, 2, 3]
2 -> [4, 5, 6, 7]