java8 Stream list to Map key 重复 value合并到Collectio的操作

java8 Stream list to Map key 重复 value合并到Collectio

关于把list转换成key value的map有很多博客上都有实现，这里是一个把value放入到集合中去

List<String> list = Lists.newArrayList("1", "2", "3", "1");
       Map<String, List<String>> map = list.stream().collect(Collectors.toMap(key -> key,
               value -> Lists.newArrayList(value),
               (List<String> newValueList, List<String> oldValueList) -> {
                   oldValueList.addAll(newValueList);
                   return oldValueList;
               }));
       System.out.println(JSON.toJSONString(map));

类似的，你的list泛型可以是一个bean，取bean的属性当key或者value，转换成一个集合

结果：

{"1":["1","1"],"2":["2"],"3":["3"]}

list中Map合并重复数据（map中key相等的value值相加）

public static void main(String[] args) {
List<Map<String, Object>> list = new ArrayList<Map<String, Object>>();
Map<String, Object> map1 = new HashMap<String, Object>();
map1.put("id", 1);
map1.put("value", 2);
list.add(map1);

Map<String, Object> map2 = new HashMap<String, Object>();
map2.put("id", 1);
map2.put("value", 5);
list.add(map2);

Map<String, Object> map3 = new HashMap<String, Object>();
map3.put("id", 2);
map3.put("value", 5);
list.add(map3);

Map<String, Object> map4 = new HashMap<String, Object>();
map4.put("id", 2);
map4.put("value", 4);
list.add(map4);

Map<String, Object> map5 = new HashMap<String, Object>();
map5.put("id", 1);
map5.put("value", 4);
list.add(map5);

//id相等value相加（合并id相同数据）
System.out.println("原始数据："+list);

//方法一
Map<String, Object> result1 = new HashMap<String, Object>();
for(Map<String, Object> map : list){
String id = map.get("id").toString();
Long value = Long.parseLong(map.get("value").toString());
if(result1.containsKey(id)){
Long temp = Long.parseLong(result1.get(id).toString());
value += temp;
}
result1.put(id, value);
}
System.out.println("合并后的数据："+result1);

//方法二
Map<String, Map<String, Object>> result2 = new HashMap<String, Map<String,Object>>();
for(Map<String, Object> map : list){
String id = map.get("id").toString();
Long value = Long.parseLong(map.get("value").toString());
if(result2.containsKey(id)){
Long temp = Long.parseLong(result2.get(id).get("value").toString());
value += temp;
result2.get(id).put("value", value);
continue;
}
result2.put(id, map);
}
System.out.println("合并后的数据2："+result2);

}

来源：https://blog.csdn.net/abc123maxiaolong/article/details/84906381