I hava a list of Class A
like
class A {
private Integer keyA;
private Integer keyB;
private String text;
}
I want to transfer aList
to nested Map
mapped by keyA
and keyB
So I create below code.
Map<Integer, Map<Integer,List<A>>> aMappedByKeyAAndKeyB = aList.stream()
.collect(Collectors.collectingAndThen(Collectors.groupingBy(A::getKeyA), result -> {
Map<Integer, Map<Integer, List<A>>> nestedMap = new HashMap<Integer, Map<Integer, List<A>>>();
result.entrySet().stream().forEach(e -> {nestedMap.put(e.getKey(), e.getValue().stream().collect(Collectors.groupingBy(A::getKeyB)));});
return nestedMap;}));
But I don't like this code.
I think If I use flatMap
, I can better code than this.
But I don't know How use flatMap
for this behavior.
Seems that you just need a cascaded groupingBy
:
Map<Integer, Map<Integer,List<A>>> aMappedByKeyAAndKeyB = aList.stream()
.collect(Collectors.groupingBy(A::getKeyA,
Collectors.groupingBy(A::getKeyB)));
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