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);
}以上为个人经验，希望能给大家一个参考，也希望大家多多支持编程网。
