In this example, we will be making grouping easy to use on Maps objects that contain collections as values.
For instance, we hava aMap ofIntegers, that has its values as anArrayList ofStrings :

Grouper<Integer,String>grouper=newArrayListGrouper<>();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 definedArrayListGrouper class uses anArrayList as its strategy.
We can always define a new Grouper that will use another
GroupingStrateg.

Let's change theArrayList to aHashSet, so that the elements become unique :

publicclassHashSetGrouper<K,V>extendsGrouper<K,V>{publicHashSetGrouper(){super(HashSet::new);}}

Testing it like :

@TestpublicvoidtestHashSetGrouper(){Grouper<Integer,String>grouper=newHashSetGrouper<>();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]}

The1 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.