collections

public class StuComparator implements Comparator {

@Override
public int compare(Student stu1, Student stu2) {

    return stu2.getAge()-stu1.getAge();
}

}
public class Student {

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

public int getAge() {
    return age;
}

public void setAge(int age) {
    this.age = age;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

}
public class Test {

public static void main(String[] args) {
    ArrayList<Student> clazz=new ArrayList<>();

    clazz.add(new Student ("张三",18));
    clazz.add(new Student ("李四",17));
    clazz.add(new Student ("王五",10));
    clazz.add(new Student ("赵六",28));
    clazz.add(new Student ("王八",35));
    clazz.add(new Student ("孙七",56));
    clazz.add(new Student ("小米",78));
    ArrayList<Student> clazz2=new ArrayList<>();
    clazz2.add(new Student("大哥",12));
    Collections.sort(clazz,new StuComparator() );
    for(Student stu:clazz){
        System.out.println(stu.getName()+" "+stu.getAge());
    }
}