Java中Comparator升序降序的具体使用

@Override
public int compare(CommentVo o1, CommentVo o2) {
           return o1.getTime().compareTo(o2.getTime());
}

package com.zhb.test;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
class A {
    int a;
    public A(int a) {
        this.a = a;
    }
    @Override
    public String toString() {
        return "[a=" + a + "]";
    }
}
class MyComparator implements Comparator<A> {
    @Override
    public int compare(A o1, A o2) {
        //升序
        //return o1.a - o2.a;
        //降序：后面会具体分析为什么降序
        return o2.a - o1.a;
    }
}
public class ComparatorTest {
    public static void main(String[] args) {
        A a1 = new A(5);
        A a2 = new A(7);
        List<A> list = new ArrayList<A>();
        list.add(a1);
        list.add(a2);
        Collections.sort(list, new MyComparator());
        System.out.println(list);
    }
}