将地图转换为地图列表

Question

I have a requirement where i want to convert a map into list of map.

I tried the following code,

List<Map<String, Object>> response = new ArrayList<>();
Set<Long> categories = new HashSet<>();

List<CCP> ccpList = ccpRepository.findByPIdIn(ids);
Map<Integer, List<Integer>> categoriesByProduct = ccpList.stream()
                  .collect(Collectors.groupingBy(ccp -> ccp.getPId(), Collectors.mapping(ccp-> ccp.getCId(), Collectors.toList())));
for(Entry<Integer, List<Integer>> en : categoriesByProduct.entrySet()) {
    Map<String,Object> responseObject = new HashMap<>();
    responseObject.put("pid", en.getKey());
    responseObject.put("categories",en.getValue());
    response.add(responseObject);
}

Is there any way in which i can modify the following code in java 8 style:

for(Entry<Integer, List<Integer>> en : categoriesByProduct.entrySet()) {
    Map<String,Object> responseObject = new HashMap<>();
    responseObject.put("pid", en.getKey());
    responseObject.put("categories",en.getValue());
    response.add(responseObject);
}

Answer 1

Put the for-loop-content in a function and use it with streams:

response = categoriesByProduct.entrySet()
                              .stream()
                              .map(e -> toMap(e))
                              .collect(Collectors.toList());

public static Map<String, Object> toMap(Map.Entry<Integer, List<Integer>> en) {
    Map<String,Object> responseObject = new HashMap<>();
    responseObject.put("pid", en.getKey());
    responseObject.put("categories",en.getValue());
    return responseObject;
}

Answer 2

You may do it like so,

List<Map<String, Object>> result = categoriesByProduct.entrySet().stream()
    .map(e -> Stream
        .of(new AbstractMap.SimpleEntry<String, Object>("pid", e.getKey()),
            new AbstractMap.SimpleEntry<String, Object>("categories", e.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)))
    .collect(Collectors.toList());