create nested hashmap using lambda expression

Question

I want to create three layer HashMap using lambda expressions from an input ArrayList in Java. The three layers are year, month and week, and here is my code for first two layers. However, in the second layer I am getting an error (first layer works fine).

public HashMap<Integer,HashMap<Integer,HashMap<Integer,AbcDetails>>> createHashMapOfTimePeriod(List<AbcDetails> abcDetails){

    Map<Integer,List<AbcDetails>>result1=abcDetails.stream().collect(Collectors.groupingBy(AbcDetails::getYear));
    Map<Integer,Map<Integer,AbcDetails>>reult2=result1.entrySet().stream().collect(Collectors.groupingBy(e -> (e.getValue().stream().collect(Collectors.groupingBy(AbcDetails::getWeek)))));

    return null;

}

Eran · Accepted Answer · 2017-11-30 10:23:38Z

5

You can achieve this with nested Collectors:

Map<Integer,Map<Integer,Map<Integer,AbcDetails>>> groups = 
  abcDetails.stream ()
        .collect(Collectors.groupingBy (AbcDetails::getYear,
                                        Collectors.groupingBy (AbcDetails::getMonth,
                                                               Collectors.toMap (AbcDetails::getWeek, Function.identity()))));

Note that if there may be multiple AbcDetails instances having the same year, month and week, the inner Map will have multiple values for the same key, so the above code will fail. One way to resolve such a problem is to change your output to:

Map<Integer,Map<Integer,Map<Integer,List<AbcDetails>>>> groups = 
  abcDetails.stream ()
        .collect(Collectors.groupingBy (AbcDetails::getYear,
                                        Collectors.groupingBy (AbcDetails::getMonth,
                                                               Collectors.groupingBy (AbcDetails::getWeek))));

edited Nov 30, 2017 at 10:23

answered Nov 30, 2017 at 10:10

Eran

395k57 gold badges726 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

shridhar Over a year ago

but in second and third layer data is repeating only year is unique is that works here?

Eran Over a year ago

@shridhar The only issue may be with the inner most map. If there are multiple objects having the same year, month and week, you would have a duplicate key in the inner most map and must decide how to resolve it. You can change the structure to Map<Integer,Map<Integer,Map<Integer,List<AbcDetails>>>> to account for such duplicate keys.

Collectives™ on Stack Overflow

create nested hashmap using lambda expression

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related