使用lambda表达式创建嵌套的哈希图

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;

}

Answer 1

You can achieve this with nested Collector s:

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))));