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。
如果寒暄只是打个招呼就了事的话，那与猴子的呼叫声有什么不同呢?事实上，
