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