今天讀書遇到幾個問題
class Student {

String course, name, city;

public Student(String name, String course, String city) {
this.course = course;
this.name = name;
this.city = city;
}

public String toString() {
return course + ":" + name + ":" + city;
}

public String getCourse() {
return course;
}
}
List stds = Arrays.asList(
new Student("Jessy", "Java ME", "Chicago"),
new Student("Helen", "Java EE", "Houston"),
new Student("Mark", "Java ME", "Chicago"));
stds.stream()
.collect(Collectors.groupingBy(Student::getCourse))
.forEach((src, res) -> System.out.println(src));
這個分組出來會顯示
Java EE
Java ME
不管我怎麼換順序
都是根據字母排序

但是換成下面的程式碼就不是照字母了
public class Country {

public enum Continent {
ASIA, EUROPE
}
String name;
Continent region;

public Country(String na, Continent reg) {
name = na;
region = reg;
}

public String getName() {
return name;
}

public Continent getRegion() {
return region;
}
}
List couList = Arrays.asList(
new Country("Japan", Country.Continent.ASIA),
new Country("Italy", Country.Continent.EUROPE), new Country("Germany", Country.Continent.EUROPE));
/System.out.println(couList);/
Map<Country.Continent, List> regionNames = couList.stream().
collect(Collectors.groupingBy(Country::getRegion,
Collectors.mapping(Country::getName, Collectors.toList())));
System.out.println(regionNames);

這串在不加入註解部分時
不管我怎麼換順序 都會是先丟進去的那個組放後面
也就是顯示ASIA在後 EUROPE在前
但是我加入註解後
一樣的程式卻會變成ASIA在前 EUROPE在後

那我該如何判斷使用groupingBy後 它的排序問題?
拜託各位高手幫忙解惑