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

import java.util.*;
import java.util.regex.*;
import java.util.stream.*;

public class Main {

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

    public static void main(String[] args) {
        List<String> locations = Arrays.asList("US:5423", "US:6321", "CA:1326", "AU:5631");

        // 使用 Pattern 和 Stream API 将列表转换为 Map<String, List<String>>
        Map<String, List<String>> locationMap = locations.stream()
                .map(DELIMITER::split)
                .collect(Collectors.groupingBy(
                        arr -> arr[0],  // 根据 ":" 分割后的第一个元素作为键
                        Collectors.mapping(
                                arr -> arr[1],  // 取 ":" 后面的部分作为值
                                Collectors.toList()
                        )
                ));

        // 打印结果
        System.out.println(locationMap);
    }
}

2､通过forEach循环转换

import java.util.*;

public class Main {

    public static void main(String[] args) {
        // 示例数据
        List<String> locations = Arrays.asList("US:5423", "US:6321", "CA:1326", "AU:5631");

        // 调用 groupByCountry 方法
        Map<String, Set<String>> map = groupByCountry(locations);

        // 打印结果
        System.out.println(map);
    }

    // 定义 groupByCountry 方法
    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;
    }
}

3、使用Stream API和split()

import java.util.*;
import java.util.stream.*;

public class Main {

    public static void main(String[] args) {
        List<String> locations = Arrays.asList("US:5423", "US:6321", "CA:1326", "AU:5631");

        // 使用 Java 8 Stream API 将列表转换为 Map<String, List<String>>
        Map<String, List<String>> map = locations.stream()
                .collect(Collectors.groupingBy(
                        location -> location.split(":")[0],  // 根据 ":" 分割，取第一个部分作为键
                        Collectors.mapping(
                                location -> location.split(":")[1],  // 取 ":" 后面的部分作为值
                                Collectors.toList()
                        )
                ));

        System.out.println(map);
    }
}