1、Collections.sort()

//升序排列
Collections.sort(list, new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        return s1.getAge().compareTo(s2.getAge());
    }
});

//降序排列
Collections.sort(list, new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        return s2.getAge().compareTo(s1.getAge());
    }
});

//多条件-先年龄升序、后分数升序
Collections.sort(list, new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        int i = s1.getAge().compareTo(s2.getAge());
        if(i == 0) {
            i = s1.getScore().compareTo(s2.getScore());
        }
        return i;
    }
});

JDK1.8环境下可使用
2、list.sort()

//升序排列
list.sort((x,y)->Integer.compare(x.getAge(), y.getAge()));
//降序排列
list.sort((x,y)->Integer.compare(y.getAge(), x.getAge()));

3、list.stream()

//升序排列
list = list.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
list = list.stream().sorted(Comparator.comparingInt(Student::getAge)).collect(Collectors.toList());
//使用lambda表达式
list = list.stream().sorted(Comparator.comparing(e -> e.getAge())).collect(Collectors.toList());
//降序排列
list = list.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());
多条件-先年龄升序、后分数升序
list = list.stream().sorted(Comparator.comparing(Student::getAge).thenComparing(Comparator.comparing(Student::getScore))).collect(Collectors.toList());