如何在 Java 中合并两个对象的属性？（用户定义的类的属性和 Map 的键值）

Question

我正在开发Spring启动应用程序，有两个对象，一个是自定义类列表，另一个是字符串键和字符串值的映射。例如。

DTO object with fixed fields fieldA,fieldB,fieldc. 

[{"fieldA": "lookupval1","fieldB": "val2","fieldc":"val3"},
{"fieldA": "lookupval1","fieldB": "val5","fieldc":"val6"}
{"fieldA": "lookupval2","fieldB": "val8","fieldc":"val9"}]

另外请注意，此字段A的前两个对象具有重复值

然后我有地图＜code＞地图

{
    lookupval1= {fieldA:"lookupval1",fieldD:"xxx",.............},
    lookupval2= {fieldA:"lookupval2",fieldD:"yyy",.............}.
    .
    .
    .
}

我想要的是，我想将列表中的每一项的字段与map中的字段相结合，我可以通过获取fieldA的查找值在Map中找到相应的键。

字段a:查找值1

然后我想把响应发送给REST客户端，那么有效的方法和数据结构是什么呢？

预期结果：

[
{"fieldA": "lookupval1","fieldB": "val2","fieldc":"val3",fieldD:"xxx"},
{"fieldA": "lookupval1","fieldB": "val2","fieldc":"val3",fieldD:"xxx"}
{"fieldA": "lookupval2","fieldB": "val8","fieldc":"val9",fieldD:"yyy"}]
]

更新:

下面是DTO对象的结构和Map的确切类型。

class DTO {
    int id;
    String name;
    String desc;
}

List<DTO> dtoList = getDTOList(); // returns list of DTO object

Map<String,Map<String,String> mapCollection
    = getMapCollection(); // return String, Map<String,String> pairs

• mapCollection中的here键始终等于DTO的id，并且DTO列表将具有重复的值

我已经发布了答案，我已经这样做了，但寻找更好的替代方案来减少计算。

Answer 1

假设

假设您的初始对象是项目类型，如下所述。

class Item {
    String fieldA,fieldB,fieldC;

    // Getters / Setter
}

假设< code>Item的初始列表称为< code>items

假设您的初始映射称为 map，并且类型为 Map

解决办法

向Item类添加方法以返回＜code＞Map

public Map<String, String> toMap() {
    return new HashMap<String, String>() {{
        put("fieldA", fieldA);
        put("fieldB", fieldB);
        put("fieldC", fieldC);
    }};
}

创建一个方法来获取给定字段的字段 D 值 A 值

private String getFieldDValueForFieldAValue(String fieldAValue) {
    return map.get(fieldAValue).get("fieldD");
}

创建将正确的 fieldD 值添加到项映射的方法

private Map<String, String> addFieldD(Map<String, String> itemMap) {
    itemMap.put("fieldD", getFieldDValueForFieldAValue(itemMap.get("fieldA")));
    return itemMap;
}

然后您可以使用正确的field dD值为每个对象生成映射列表

List<Map<String, String>> list = items.stream()
    .map(Item::toMap) // Converts all Items to Maps
    .map(m -> addFieldD(m)) // Adds the correct fieldD value to each Map
    .collect(Collectors.toList()); // make it a List

最后用它制作一个JSON字符串:

Type listType = new TypeToken<List<Map<String, String>>>() {}.getType();
Gson gson = new Gson();
String json = gson.toJson(list, listType);
System.out.println(json);

这将输出以下JSON：

[
  {"fieldA":"lookupval1","fieldC":"val3","fieldB":"val2","fieldD":"xxx"},
  {"fieldA":"lookupval1","fieldC":"val6","fieldB":"val5","fieldD":"xxx"},
  {"fieldA":"lookupval2","fieldC":"val9","fieldB":"val8","fieldD":"yyy"}
]

Answer 2

List<DTO> dtoList = getDTOList(); // returns list of DTO object

Map<String,Map<String,String> mapCollection
    = getMapCollection(); // return String, Map<String,String> pairs
List<Map<String,String>> result =  new HashMap<>();

// iterate over dto list
for(DTO singleDTO : dtoList) {
    Map<String,String> singleResult = new HashMap<>();

    // Add all fields in map
    singleResult.put("id",singleDTO.getId());
    singleResult.put("name",singleDTO.getName());
    singleResult.put("desc",singleDTO.getDesc());

    // fetch single map by id lookup
    Map<String,Strig> singleMap = mapCollection.get(singleDTO.getId());
    // fetch all entries from map
    Set<Map.Entry<String,String>> entrySet = singleMap.entrySet();
   // add them in hashmap
    for(Map.Entry<String,String> singleEntry : entrySet) {
        singleResult.put(singleEntry.getKey(),singleEntry.getValue());

    Add map in the list
    result.add(singleEntry);
}}

//result

 [
    {
        "id":"1", "name":"abc", "desc":"some desc abc", "ID":"1", "mapField1":"field1 value",  "mapField2":"field1 value"
    },
    {
        "id":"1", "name":"xyz", "desc":"some desc xyz", "ID":"1", "mapField1":"field1 value",  "mapField2":"field1 value"
    },
    {
        "id":"2", "name":"pqr", "desc":"some desc pqr", "ID":"2", "mapField1":"field1 value 2",  "mapField2":"field1 value2"
    }     
 ]