Group list elements in list of list in java

Question

I have a list of elements. I want to group the elements of this list into a list of list based on some condition. Is it possible to do it easily in java ?

public class CollectionTest {

    public static void main(String[] arg) {
        Target target0 = new Target();
        target0.setRisklevel("III");
        target0.setLocation("Combined");
        Target target1 = new Target();
        target1.setRisklevel("III");
        target1.setLocation("Combined");
        Target target2 = new Target();
        target2.setRisklevel("III");
        target2.setLocation("Combined");
        Target target3 = new Target();
        target3.setRisklevel("III");
        target3.setLocation("Combined");
        Target target4 = new Target();
        target4.setRisklevel("IV");
        target4.setLocation("Combined");
        Target target5 = new Target();
        target5.setRisklevel("IV");
        target5.setLocation("Combined");
        Target target6 = new Target();
        target6.setRisklevel("IV");
        target6.setLocation("Combined");
        Target target7 = new Target();
        target7.setRisklevel("II");
        target7.setLocation("Domestic");
        Target target8 = new Target();
        target8.setRisklevel("IV");
        target8.setLocation("Domestic");
        Target target9 = new Target();
        target9.setRisklevel("IV");
        target9.setLocation("Domestic");
        Target target10 = new Target();
        target10.setRisklevel("IV");
        target10.setLocation("Domestic");
        Target target11 = new Target();
        target11.setRisklevel("IV");
        target11.setLocation("Domestic");

        List<Target> ucrtargetList = new ArrayList<Target>();
        ucrtargetList.add(target0);
        ucrtargetList.add(target1);
        ucrtargetList.add(target2);
        ucrtargetList.add(target3);
        ucrtargetList.add(target4);
        ucrtargetList.add(target5);
        ucrtargetList.add(target6);
        ucrtargetList.add(target7);
        ucrtargetList.add(target8);
        ucrtargetList.add(target9);
        ucrtargetList.add(target10);
        ucrtargetList.add(target11);
        List<List<Target>> fullList = new ArrayList<List<Target>>();
    }
}

Here condition is RiskLevel and Location has to be same in each list in the list of List. So fullList should have 4 lists within it(1st III & Combined, 2nd IV & Combined, 3rd II & Domestic, 4th IV & Domestic).

I could loop through the list and set the values. Is there an easier way to do this with java8 or apache commons ?

Possible duplicate of [Group by multiple field names in java 8](https://stackoverflow.com/questions/28342814/group-by-multiple-field-names-in-java-8) — LuCio, Oct 22 '18 at 09:07

score 1 · Answer 1 · answered Oct 22 '18 at 09:05

1

Unable to comment yet, so adding an answer, to extend what @Schidu Luca has replied, 'groupingBy' clauses can be chained, like this

Map<String, List<Target>> collect = ucrtargetList.stream()
           .collect(Collectors.groupingBy(Target::getRisklevel, Collectors.groupingBy(Target::getLocation)));

answered Oct 22 '18 at 09:05

keybored

129
2

Now this returns a key that is RiskLevel and value as hashmaps which again has key as location. Any simple way to get a list of list from this hashmap ? – minusSeven Oct 22 '18 at 09:21
this is well explained here, in the answer by @Holger [link]https://stackoverflow.com/questions/38360276/convert-map-of-maps-into-lists-using-java-8-streams[link] – keybored Oct 22 '18 at 10:57

LuCio · Answer 2 · 2018-10-23T15:25:40.620

According to your description fullList should have 4 lists within it(1st III & Combined, 2nd IV & Combined, 3rd II & Domestic, 4th IV & Domestic) I assume your looking for this:

Function<Target, List<String>> riskLevelAndLocation = t -> List.of(t.getRiskLevel(), t.getLocation());
Map<List<String>, List<Target>> fullList = ucrtargetList.stream()
        .collect(Collectors.groupingBy(riskLevelAndLocation));

riskLevelAndLocation is a Function which retruns a List containing riskLevel and location. groupingBy a List works because List.equals(Object o)

Returns true if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal.

A quick check:

for (Entry<List<String>, List<Target>> entrySet : fullList.entrySet()) {
    System.out.println(entrySet.getKey().toString() + ": " + entrySet.getValue().size());
}

The output is:

[IV, Combined]: 3
[II, Domestic]: 1
[III, Combined]: 4
[IV, Domestic]: 4

If fullList has to have the order you've described then you could use a LinkedHashMap and sort it.

This works but this seems to be using java10 features. I need solution in java 8 or using external libraries like Apache Commons. — minusSeven, Oct 22 '18 at 13:46
The Java 9 code is `List.of`. You can use `Arrays.asList` instead. — LuCio, Oct 22 '18 at 14:25

Ilya · Answer 3 · 2018-10-22T09:10:31.877

List<List<Target>> fullList = ucrtargetList.stream()
                .collect(Collectors.groupingBy(Target::getRisklevel)) // Or another collector
                .entrySet()
                .stream()
                .map(Map.Entry::getValue)
                .collect(Collectors.toList());

If you want to remove the same objects from the result, you need to add methods 'equals' and 'hashcode' and collects to Set.

Group list elements in list of list in java

3 Answers3