java8 listmap聚合后内容顺序不变

我就是我 2024-03-22 22:20 85阅读 0赞

学习了[groupingBy的用法][groupingBy]，在处理List<Map>的数据，顺序会发生变化：

### 直接聚合 ###

public static void main(String[] args){
            String data = "[{\"codeType\":\"ALRAM\",\"code\":\"1\",\"name\":\"普通告警\",\"sortId\":\"1\"},{\"codeType\":\"ALRAM\",\"code\":\"2\",\"name\":\"中级告警\",\"sortId\":\"2\"},{\"codeType\":\"ALRAM\",\"code\":\"3\",\"name\":\"严重告警\",\"sortId\":\"3\"},{\"codeType\":\"ALRAM\",\"code\":\"4\",\"name\":\"特级告警\",\"sortId\":\"4\"},{\"codeType\":\"NOTICE\",\"code\":\"1\",\"name\":\"短信通知\",\"sortId\":\"1\"},{\"codeType\":\"NOTICE\",\"code\":\"2\",\"name\":\"邮箱通知\",\"sortId\":\"2\"},{\"codeType\":\"NOTICE\",\"code\":\"3\",\"name\":\"微信消息通知\",\"sortId\":\"3\"}]";
            JSONArray items = JSON.parseArray(data);
            Map<String, List<JSONObject>> codeType = ListUtils.emptyIfNull(items).stream()
     .map(e -> (JSONObject) e)
     .collect(Collectors.groupingBy(e -> MapUtils.getString(e, "codeType")));
            System.out.println(JSON.toJSONString(codeType));
        }

**结果：**

{
    	"NOTICE": [{
    		"code": "1",
    		"codeType": "NOTICE",
    		"sortId": "1",
    		"name": "短信通知"
    	}, {
    		"code": "2",
    		"codeType": "NOTICE",
    		"sortId": "2",
    		"name": "邮箱通知"
    	}, {
    		"code": "3",
    		"codeType": "NOTICE",
    		"sortId": "3",
    		"name": "微信消息通知"
    	}],
    	"ALRAM": [{
    		"code": "1",
    		"codeType": "ALRAM",
    		"sortId": "1",
    		"name": "普通告警"
    	}, {
    		"code": "2",
    		"codeType": "ALRAM",
    		"sortId": "2",
    		"name": "中级告警"
    	}, {
    		"code": "3",
    		"codeType": "ALRAM",
    		"sortId": "3",
    		"name": "严重告警"
    	}, {
    		"code": "4",
    		"codeType": "ALRAM",
    		"sortId": "4",
    		"name": "特级告警"
    	}]
    }

这结果顺序发生了变化，那该如何处理，保持内容不变呢？

### LinkedHashMap ###

public static void main(String[] args){
            String data = "[{\"codeType\":\"ALRAM\",\"code\":\"1\",\"name\":\"普通告警\",\"sortId\":\"1\"},{\"codeType\":\"ALRAM\",\"code\":\"2\",\"name\":\"中级告警\",\"sortId\":\"2\"},{\"codeType\":\"ALRAM\",\"code\":\"3\",\"name\":\"严重告警\",\"sortId\":\"3\"},{\"codeType\":\"ALRAM\",\"code\":\"4\",\"name\":\"特级告警\",\"sortId\":\"4\"},{\"codeType\":\"NOTICE\",\"code\":\"1\",\"name\":\"短信通知\",\"sortId\":\"1\"},{\"codeType\":\"NOTICE\",\"code\":\"2\",\"name\":\"邮箱通知\",\"sortId\":\"2\"},{\"codeType\":\"NOTICE\",\"code\":\"3\",\"name\":\"微信消息通知\",\"sortId\":\"3\"}]";
            JSONArray items = JSON.parseArray(data);
            Map<String, ArrayList<JSONObject>> alram = ListUtils.emptyIfNull(items).stream()
                    .map(e -> (JSONObject) e)
                    .collect(Collectors.groupingBy(e -> MapUtils.getString(e, "codeType"),
                            LinkedHashMap::new,
                            Collectors.toCollection(ArrayList::new)));
            System.out.println(JSON.toJSONString(alram));
        }

**结果：**

{
    	"ALRAM": [{
    		"code": "1",
    		"codeType": "ALRAM",
    		"sortId": "1",
    		"name": "普通告警"
    	}, {
    		"code": "2",
    		"codeType": "ALRAM",
    		"sortId": "2",
    		"name": "中级告警"
    	}, {
    		"code": "3",
    		"codeType": "ALRAM",
    		"sortId": "3",
    		"name": "严重告警"
    	}, {
    		"code": "4",
    		"codeType": "ALRAM",
    		"sortId": "4",
    		"name": "特级告警"
    	}],
    	"NOTICE": [{
    		"code": "1",
    		"codeType": "NOTICE",
    		"sortId": "1",
    		"name": "短信通知"
    	}, {
    		"code": "2",
    		"codeType": "NOTICE",
    		"sortId": "2",
    		"name": "邮箱通知"
    	}, {
    		"code": "3",
    		"codeType": "NOTICE",
    		"sortId": "3",
    		"name": "微信消息通知"
    	}]
    }

### 总结： ###

对于List<Map>的数据使用groupingBy的时候，使用LinkedHashMap，保持内容顺序一致。

[groupingBy]: https://blog.csdn.net/qq_35461948/article/details/131992219?csdn_share_tail=%7B%22type%22%3A%22blog%22%2C%22rType%22%3A%22article%22%2C%22rId%22%3A%22131992219%22%2C%22source%22%3A%22qq_35461948%22%7D