Method to convert `List` to `Map>`

Question

How can I convert List<Employee> to Map<Integer,List<Employee>>.

Group List<Employee> based on depId present in employee Object, Map key is depId.

Is there any method in java.util.* or in Google Guava to convert it with out iterating through list.

http://stackoverflow.com/questions/20363719/java-8-listv-into-mapk-v seems similar — sashwat, Oct 25 '16 at 06:47
@sashwat It's not the same - here OP wants a multimap, i.e. `Map>`, not `Map`. — Grzegorz Rożniecki, Oct 25 '16 at 15:29

score 10 · Accepted Answer · answered Oct 25 '16 at 06:45

10

With Java 8+, you can use a stream and group by the id:

List<Employee> list = ...;
Map<Integer,List<Employee>> map = list.stream()
                                      .collect(Collectors.groupingBy(Employee::getDepId));

answered Oct 25 '16 at 06:45

assylias

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score 2 · Answer 2 · answered Oct 25 '16 at 10:24

2

If you use Guava and want to use proper collection type, here ListMultimap<Integer, Employee>, use Multimaps#index():

ListMultimap<Integer, Employee> m = Multimaps.index(list, Employee::getDepId);

answered Oct 25 '16 at 10:24

Grzegorz Rożniecki

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Method to convert `List` to `Map>`

2 Answers2