How to split a string into a map, grouping values by duplicate keys using streams?

Question

I want to convert the following

String flString="view1:filedname11,view1:filedname12,view2:fieldname21";

to a Map<String,Set<String>> to get the key/value as below:

view1=[filedname11,filedname12]
view2=[fieldname21]

I want to use Java 8 streams. I tried

Arrays.stream(tokens)
        .map(a -> a.split(":"))
        .collect(Collectors.groupingBy(
                a -> a[0], Collectors.toList()));

However the keys are also getting added to the value list.

Answer 1

You should use a Collectors::mapping to map the array to an element.

String flString = "view1:filedname11,view1:filedname12,view2:fieldname21";

Map<String, List<String>> map = Pattern.compile(",")
    .splitAsStream(flString)
    .map(a -> a.split(":"))
    .collect(
        Collectors.groupingBy(a -> a[0],
            Collectors.mapping(a -> a[1], Collectors.toList())
        )
    );

map.entrySet().forEach(System.out::println);

Output

view1=[filedname11, filedname12]
view2=[fieldname21]