Grouping and Sorting a Collection with Java 8 Streams

Question

I have a list of people which I would like to group based on their age (group1 has age < 20, group2 has age >=20) and sort each group based on name and age using Java 8: List of people:

Group 1:

Ane 12
Col 14

Group 2:

James 24
thomas 34
Xavier 55

I would like to use Java 8 to do something like this:

list.stream().collect(Collectors.groupingBy(...

@akaliza is the question about splitting the list into two groups, or grouping them by multiple age ranges? If it's about grouping by age range, you could update your question for clarity (and to let me fix my answer proposal :)) — Shastick
– Shastick, Commented Nov 14, 2016 at 10:52

Shastick · Accepted Answer · 2016-11-16 12:10:37Z

7

You can get to what you want by:

Sorting your whole collection of people by age (and then name).
Partitioning it.

Note that the order of each partition will be maintained:

import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.partitioningBy;

people.stream()
  .sorted(
     comparing(Person::getAge)
       .thenComparing(Person::getName))
  .collect(partitioningBy(p -> p.getAge() >= 20))

_{Thanks to Christoffer for his improvement proposal: Scandinavian Java must be as slick as a set of IKEA assembly instructions :)}

edited Nov 16, 2016 at 12:10

answered Nov 13, 2016 at 18:11

Shastick

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3 Comments

Christoffer Hammarström Over a year ago

You should be using Comparator.comparing(Person::getAge).

Shastick Over a year ago

Thanks, it's cleaner that way :)

Christoffer Hammarström Over a year ago

Also, to make it even cleaner, import static java.util.Collectors.partitioningBy and import static java.util.Comparator.comparing;

Joe C · Accepted Answer · 2016-11-13 18:07:05Z

2

You will want to use the partitioningBy collector instead. It will allow you to specify a predicate and return a map keyed by a boolean:

Map<Boolean, List<?>> partitioned = list.stream().sorted(...).collect(Collectors.partitioningBy(...));

answered Nov 13, 2016 at 18:07

Joe C

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7 Comments

akaliza Over a year ago

what if I want people between age 20 and 50? can I chain partitioningBy?

Joe C Over a year ago

Not really. In that case you'll need to write a function which returns a different value for each group, and then do a groupingBy with that function.

akaliza Over a year ago

I understand, we have a predicate, therefore I can do this p -> p.getAge() >= 20 && p.getAge() <50

Joe C Over a year ago

Ah, I thought you wanted three groups: a <20, a 20-50, and a >50. If you want one group of 20-50 and the other of everyone else, that will work.

Joe C Over a year ago

Basically, yes.

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