Java 将List 转换成 Map>的几种方法

示例List<String>类型数据

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

将上面locations转换成Map<String, List<String>>，例如：

AU = [5631]
CA = [1326]
US = [5423, 6321]

1､通过stream()来转换

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

或

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

2､通过forEach循环转换

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;
}