List of comma separated String to Map of list using java 8

Question

I have a list of comma separated strings -

List: format = <unique id, label>
----- values -----
ab123,X
cd123,Y
ab123,Y
cd123,Z
------------------

I want to convert this list to Map<String, List<String>> using java 8 where key would be the unique id and value would be the list of labels (Map<unique-id, List<label>>).

Example -

Map[
ab123=List[X, Y],
cd123=List[Y, Z]
]

Could you please help me here so that I could implement this using java 8.

Also instead of Map, if I want to use dto class -

Class A {
 private String id;
 private List<String> labelList;
 // Getters and Setters methods
}

and I expect to create a list of class A, for example -

List[
A [id="ab123", labelList = List[X, Y],
A [id="cd123", labelList = List[Y, Z]
]

How could I get that?

Eugene · Answer 1 · 2018-02-10T08:58:28.743

4

yourList.stream()
        .map(x -> x.split(",", 2))
        .collect(Collectors.groupingBy(
            x -> x[0],
            Collectors.mapping(x -> x[1], Collectors.toList())

));

edited Feb 10 '18 at 08:58

answered Feb 09 '18 at 10:40

Eugene

117,005
15
201
306

2

You should use `x.split(",", 2)` here… – Holger Feb 09 '18 at 14:41
@Holger I know it exists, I don't know what *exactly* it does to be honest. Ill read... – Eugene Feb 09 '18 at 14:50
1

Think about what happens, if the string contains more than one comma… – Holger Feb 09 '18 at 14:55
@Holger thank you for yet another lesson, understood it now and edited – Eugene Feb 10 '18 at 08:58

score 0 · Answer 2 · answered Feb 09 '18 at 10:47

Using regex matching to filter all string that don't match given pattern:

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.Collectors;

public class ListToMapExample {

    public static void main(String[] args) {
        final List<String> list = Arrays.asList("ab123,X", "cd123,Y", "ab123,Y", "cd123,Z", "incorrect string", "", "a");
        final Pattern pattern = Pattern.compile("^([^,]+),(.*)$");

        final Map<String, List<String>> map = list.stream()
                .map(pattern::matcher)
                .filter(Matcher::matches)
                .collect(Collectors.groupingBy(
                        matcher -> matcher.group(1), 
                        Collectors.mapping(
                                matcher -> matcher.group(2), 
                                Collectors.toList()
                        ))
                );

        System.out.println(map);
    }
}

Output:

{cd123=[Y, Z], ab123=[X, Y]}

List of comma separated String to Map of list using java 8

2 Answers2

Related