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);

	}

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我们。