package com.company.comparator;

/**
* Created by cainiao
on 17/3/2.
*/
public class User {

    private  Long userId;
    private  Long userName;
    private  Integer age;

    public User(Integer age){
        this.age = age;
    }


    public Long getUserId() {
        return userId;
    }

    public void setUserId(Long userId) {
        this.userId = userId;
    }

    public Long getUserName() {
        return userName;
    }

    public void setUserName(Long userName) {
        this.userName = userName;
    }

    public Integer getAge() {
        return age;
    }

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


package com.company.comparator;

import java.util.Comparator;

/**
* Created by zhaowei on 17/3/2.
*/
public class AgeComparator implements Comparator<User> {

    @Override
    public int compare(User o1, User o2) {
        return Integer.compare(o1.getAge(),o2.getAge());
    }
}

package com.company.comparator;

import java.util.*;
import java.util.ArrayList;

/**
* Created by zhaowei on 17/3/2.
*/
public class UserComparatorTest {

    public static void main(String[] args) {

        List<User> list = new ArrayList();

        User u1 = new User(27);
        User u2 = new User(19);
        User u3 = new User(36);
        User u4 = new User(59);
        User u5 = new User(28);

        list.add(u1);
        list.add(u2);
        list.add(u3);
        list.add(u4);
        list.add(u5);

        Collections.sort(list,new AgeComparator());
        for(User user:list){
            System.out.println(user.getAge());
        }

    }
}

从上边的例子可以看出，默认是升序排列。如果需要降序排列，则可以修改
Integer.compare(o2.getAge(),o1.getAge());

总之：返回-1 是降序排列。返回1是升序排列。如果debug可以发现，O1 与 O2 是降序排列。

再看compare 源码可以发现默认按降序列。根据负负得正的理论，Comparator初始按照升序排列。
public static int compare(int x, int y) {
        return (x < y) ? -1 : ((x == y) ? 0 : 1);
    }