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

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

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

    Map> locationMap = locations.stream().map(str -> str.split(":"))
            .collect(() -> new TreeMap>(), (map, parts) -> {
                if (map.get(parts[0]) == null) {
                    List 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]}