How split arraylist to multiple lists by specific property object Java8 in one line

Question

Suppose I have

private String userId;
private String email;
private AWSRegion region;
private String total;
List<Prop> all = new ArrayList<>
all.add(new Prop("aaa", "dddd", "EU", total1));
all.add(new Prop("aaa1", "dddd", "US", tota2l));
all.add(new Prop("aaa2", "dddd", "AU", tota2l));
all.add(new Prop("aaa3", "dddd", "AU", tota3l));
all.add(new Prop("aaa3", "dddd", "EU", tota4l));....a lot of regions

I want in one line java8 to have list of lists by property "AWSRegion"

some think like....but not to run it as "filter predicate" because I have many of regions...

List<Prop> users = all.stream().filter(u -> u.getRegeion() == AWSRegion.ASIA_SIDNEY).collect(Collectors.toList());

RESULT should be list of lists:

LIST : {sublist1-AU , sublist2-US, sublist3-EU....,etc'}

thanks,

Eran · Accepted Answer · 2018-09-05 07:45:33Z

8

Use groupingBy to get a Map<AWSRegion,List<Prop>> and then get the values of that Map:

Collection<List<Prop>> groups =
    all.stream()
       .collect(Collectors.groupingBy(Prop::getRegion))
       .values();

If the output should be a List, add an extra step:

List<List<Prop>> groups = new ArrayList<>(
    all.stream()
       .collect(Collectors.groupingBy(Prop::getRegion))
       .values());

answered Sep 5, 2018 at 7:45

Eran

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Comments

ernest_k · Accepted Answer · 2018-09-05 07:46:18Z

2

You can create a map with <AWSRegion, List<Prop> by grouping your list by region:

Map<AWSRegion, List<Prop>> regionMap = all.stream()
   .collect(Collectors.groupingBy(Prop::getRegion, Collectors.toList()));

If you want that as a list, then you can simply call:

List<List<Prop>> lists = new ArrayList<>(regionMap.values());

answered Sep 5, 2018 at 7:46

ernest_k

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