An easy way to group objects in java

In this example, we will be making grouping easy to use on Maps objects that contain collections as values.
For instance, we hava a Map of Integers, that has its values as an ArrayList of Strings :

Grouper<Integer, String> grouper = new ArrayListGrouper<>();
        grouper.put(1, "a");
        grouper.put(1, "b");
        grouper.put(1, "c");
        grouper.put(1, "c");
        grouper.put(2, "c");

The output will be :

{1=[a, b, c, c], 2=[c]}

All we have to do, is to define a grouping strategy, for example the already defined ArrayListGrouper class uses an ArrayList as its strategy.
We can always define a new Grouper that will use another
GroupingStrateg.

Let's change the ArrayList to a HashSet, so that the elements become unique :

public class HashSetGrouper<K, V> extends Grouper<K, V> {

    public HashSetGrouper() {
        super(HashSet::new);
    }
}

Testing it like :

@Test
    public void testHashSetGrouper() {
        Grouper<Integer, String> grouper = new HashSetGrouper<>();
        grouper.put(1, "a");
        grouper.put(1, "b");
        grouper.put(1, "c");
        grouper.put(1, "c");
        grouper.put(2, "c");
        System.out.println(grouper);

    }

The output will become :

{1=[a, b, c], 2=[c]}

The 1 key has now a set in which the "c" value is not repeated.

the code is hosted on Github : https://github.com/BelmoMusta/java-Grouper
Your feedback is welcomed.