java List 排序问题

class User{
    private Student student;
    .....
}
class Student{
    private String name;
    private int age;
    private int score;
}

假设我有这样一个集合List <User>， 我想对这个集合按照属性Student的age降序，score升序进行排序 要如何实现？ 请高手指点不胜感激。

---

可以使用Collections的排序和Comparator来进行排序.
Java code:
import java.util.*;

public class Test {
public static void main(String[] args) {
List<Student> st = new ArrayList<Student>();
st.add(new Student("Hello", 20, 80));
st.add(new Student("Hi", 25, 80));
st.add(new Student("John", 22, 90));
st.add(new Student("Tomas", 24, 70));
st.add(new Student("Judo", 20, 85));

Collections.sort(st, new Comparator<Student>() {
public int compare(Student a, Student b) {
if (a.age > b.age){
return -1;
} else if (a.age < b.age){
return 1;
} else {
return a.score - b.score;
}
}
});

System.out.println(st);

}
}

class Student{