import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {

  public static void main(String[] args) throws Exception {
    List<Person> persons =
        Arrays.asList(
            new Person("Max", 18),
            new Person("Peter", 23),
            new Person("Pamela", 23),
            new Person("David", 12));   
    Map<Integer, String> map = persons
        .stream()
        .collect(Collectors.toMap(
            p -> p.age,
            p -> p.name,
            (name1, name2) -> name1 + ";" + name2));

    System.out.println(map);
  }

}
class Person {
  String name;
  int age;

  Person(String name, int age) {
      this.name = name;
      this.age = age;
  }

  @Override
  public String toString() {
      return name;
  }
}

運(yùn)行結(jié)果如下: