How to group and map string via lambda-CodePudding

I have a string as below:

String data = "010$$fengtai,010$$chaoyang,010$$haidain,027$$wuchang,027$$hongshan,027$$caidan,021$$changnin,021$$xuhui,020$$tianhe";

And I want to convert it into Map<String, List> like below(first split by , and split by $$,value before $$ is key need to be group,and value after $$ needs to put inside a list):

{027=[wuchang, hongshan, caidan], 020=[tianhe], 010=[fengtai, chaoyang, haidain], 021=[changnin, xuhui]}

I have use a traditional way to do it

private Map<String, List<String>> parseParametersByIterate(String sensors) {
    List<String[]> dataList = Arrays.stream(sensors.split(",")).map(s -> s.split("\\$\\$")).collect(Collectors.toList());
    Map<String, List<String>> resultMap = new HashMap<>();
    for (String[] d : dataList) {
        List<String> list = resultMap.get(d[0]);
        if (list == null) {
            list = new ArrayList<>();
            list.add(d[1]);
            resultMap.put(d[0], list);
        } else {
            list.add(d[1]);
        }
    }
    return resultMap;
}

But it seems more complicated and not very elegant,thus I want to use java8 lambda,such as grouping and map to meke it one-liner

What I have tried so far is below

Map<String, List<String>> result = 
    Arrays.stream(data.split(",")).collect(Collectors.groupingBy(s -> s.split("\\$\\$")[0]))

But the output is not what I want to have,can anyone help me to do it? Thanks in advance!

CodePudding user response：

You simply need to map the values of the mapping. You can do that by specifying a second argument to Collectors.groupingBy:

Collectors.groupingBy(s -> s.split("\\$\\$")[0], Collectors.mapping(s -> s.split("\\$\\$")[1],Collectors.toList()))

Instead of then splitting twice, you can split first and group afterwards:

Arrays.stream(data.split(",")).map(s -> s.split("\\$\\$")).collect(Collectors.groupingBy(s -> s[0], Collectors.mapping(s -> s[1],Collectors.toList())));

Which now outputs

{027=[wuchang, hongshan, caidan], 020=[tianhe], 021=[changnin, xuhui], 010=[fengtai, chaoyang, haidain]}