Java 8: How to convert List to Map>?

Question

I have a List of String like:

List locations = Arrays.asList(\"US:5423\",\"US:6321\",\"CA:1326\",\"AU:5631\");

And I want to

Answer 1

You may do it like so:

Map<String, List<String>> locationMap = locations.stream()
        .map(s -> s.split(":"))
        .collect(Collectors.groupingBy(a -> a[0],
                Collectors.mapping(a -> a[1], Collectors.toList())));

A much more better approach would be,

private static final Pattern DELIMITER = Pattern.compile(":");

Map<String, List<String>> locationMap = locations.stream()
    .map(s -> DELIMITER.splitAsStream(s).toArray(String[]::new))
        .collect(Collectors.groupingBy(a -> a[0], 
            Collectors.mapping(a -> a[1], Collectors.toList())));

Update

As per the following comment, this can be further simplified to,

Map<String, List<String>> locationMap = locations.stream().map(DELIMITER::split)
    .collect(Collectors.groupingBy(a -> a[0], 
        Collectors.mapping(a -> a[1], Collectors.toList())));

Answer 2

What about POJO. It looks not complicated comparing with streams.

public static Map<String, Set<String>> groupByCountry(List<String> locations) {
    Map<String, Set<String>> map = new HashMap<>();

    locations.forEach(location -> {
        String[] parts = location.split(":");
        map.compute(parts[0], (country, codes) -> {
            codes = codes == null ? new HashSet<>() : codes;
            codes.add(parts[1]);
            return codes;
        });
    });

    return map;
}

Answer 3

Try this

Map<String, List<String>> locationMap = locations.stream()
            .map(s ->  new AbstractMap.SimpleEntry<String,String>(s.split(":")[0], s.split(":")[1]))
            .collect(Collectors.groupingBy(Map.Entry::getKey,
                     Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

Answer 4

You can just put the code in grouping by part where you put first group as key and second as value instead of mapping it first

Map<String, List<String>> locationMap = locations
            .stream()
            .map(s -> s.split(":"))
            .collect( Collectors.groupingBy( s -> s[0], Collectors.mapping( s-> s[1], Collectors.toList() ) ) );

Answer 5

Seems like your location map needs to be sorted based on keys, you can try the following

List<String> locations = Arrays.asList("US:5423", "US:6321", "CA:1326", "AU:5631");

    Map<String, List<String>> locationMap = locations.stream().map(str -> str.split(":"))
            .collect(() -> new TreeMap<String, List<String>>(), (map, parts) -> {
                if (map.get(parts[0]) == null) {
                    List<String> list = new ArrayList<>();
                    list.add(parts[1]);
                    map.put(parts[0], list);
                } else {
                    map.get(parts[0]).add(parts[1]);
                }
            }, (map1, map2) -> {
                map1.putAll(map2);
            });

    System.out.println(locationMap); // this outputs {AU=[5631], CA=[1326], US=[5423, 6321]}