使用Java8的Stream进行多字段排序以及剔除非空字段排序

　　使用 Stream的sort() 方法，并结合 Comparator 类 进行排序，主要解决以下两个问题：

　　1）当某个字段为空时，怎么进行排序？---使用 Comparator.nullsFirst() 或者 Comparator.nullsLast( ) 进行非空的剔除

　　2）如果比较两个字段？---使用  thenComparing( )

package com.demrystv.stonestrong.stone.streamTest;

import com.alibaba.fastjson.JSON;
import org.junit.Test;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class StreamController {

    @Test
    public void streamSortTest(){
        User user1 = new User();
        user1.setAge(36);
        user1.setUserName("Bryant");
        user1.setAddress("LA");
        user1.setPhone("11111");

        User user2 = new User();
        user2.setAge(37);
        user2.setUserName("James");
        user2.setPhone("22222");

        User user3 = new User();
        user3.setAge(39);
        user3.setUserName("Harden");
        user3.setAddress("Houston");
        user3.setPhone("33333");

        User user4 = new User();
        user4.setAge(28);
        user4.setUserName("Durant");
        user4.setAddress("NY");

        User user5 = new User();
        user5.setAge(29);
        user5.setUserName("Wade");
        user5.setAddress("Miami");
        user5.setPhone("55555");



        List<User> userList = Arrays.asList(user1, user2, user3, user4, user5);
        List<User> collect1 = userList.stream().sorted(Comparator.comparing(User::getAddress, Comparator.nullsFirst(String::compareTo))).collect(Collectors.toList());
        System.out.println("根据地址正序排序,null的在前面: ");
        System.out.println(JSON.toJSONString(collect1));
        System.out.println("=========================");
        System.out.println();

        List<User> collect2 = userList.stream().sorted(Comparator.comparing(User::getAddress, Comparator.nullsLast(String::compareTo))).collect(Collectors.toList());
        System.out.println("根据地址正序排序,null的在后面: ");
        System.out.println(JSON.toJSONString(collect2));
        System.out.println("=========================");
        System.out.println();

        List<User> collect3 = userList.stream().sorted(Comparator.comparing(User::getAddress, Comparator.nullsFirst(String::compareTo)).reversed()).collect(Collectors.toList());
        System.out.println("根据地址逆序排序,null的在后面: ");
        System.out.println(JSON.toJSONString(collect3));
        System.out.println("=========================");
        System.out.println();

        List<User> collect4 = userList.stream().sorted(Comparator.comparing(User::getAddress, Comparator.nullsFirst(String::compareTo)).
                thenComparing(User::getPhone, Comparator.nullsFirst(String::compareTo)).reversed()).collect(Collectors.toList());
        System.out.println("先根据地址正序排序,再根据电话逆序排序,null的在后面: ");
        System.out.println(JSON.toJSONString(collect4));
        System.out.println("=========================");
        System.out.println();

    }
}

　　运行结果如下：

根据地址正序排序,null的在前面: 
[{"age":37,"phone":"22222","userName":"James"},{"address":"Houston","age":39,"phone":"33333","userName":"Harden"},{"address":"LA","age":36,"phone":"11111","userName":"Bryant"},{"address":"Miami","age":29,"phone":"55555","userName":"Wade"},{"address":"NY","age":28,"userName":"Durant"}]
=========================

根据地址正序排序,null的在后面: 
[{"address":"Houston","age":39,"phone":"33333","userName":"Harden"},{"address":"LA","age":36,"phone":"11111","userName":"Bryant"},{"address":"Miami","age":29,"phone":"55555","userName":"Wade"},{"address":"NY","age":28,"userName":"Durant"},{"age":37,"phone":"22222","userName":"James"}]
=========================

根据地址逆序排序,null的在后面: 
[{"address":"NY","age":28,"userName":"Durant"},{"address":"Miami","age":29,"phone":"55555","userName":"Wade"},{"address":"LA","age":36,"phone":"11111","userName":"Bryant"},{"address":"Houston","age":39,"phone":"33333","userName":"Harden"},{"age":37,"phone":"22222","userName":"James"}]
=========================

先根据地址正序排序,再根据电话逆序排序,null的在后面: 
[{"address":"NY","age":28,"userName":"Durant"},{"address":"Miami","age":29,"phone":"55555","userName":"Wade"},{"address":"LA","age":36,"phone":"11111","userName":"Bryant"},{"address":"Houston","age":39,"phone":"33333","userName":"Harden"},{"age":37,"phone":"22222","userName":"James"}]
=========================