After groupingBy convert list of objects from Type A to Type B

Question

Map<Integer,List<ItemTypeA>> list = data.stream().collect(groupingBy(ItemTypeA::getId)); 

I have a function that converts ItemTypeA to ItemTypeB .

public ItemTypeB convert (ItemTypeA); 

How can I use that after groupingBy here so that that the end result is as shown below.

Map<Integer,List<ItemTypeB>> map = data.stream().collect(groupingBy(ItemTypeA::getId), 

How to invoke function to convert ItemTypeA to ItemTypeB?;

Answer 1

You can use Collectors.mapping:

Map<Integer,List<ItemTypeB>> output = 
    data.stream()
        .collect(Collectors.groupingBy(ItemTypeA::getId,
                 Collectors.mapping(a->convert(a),
                                    Collectors.toList())));