[Stream使用(一)]对对象集合多字段进行排序，包括该字段为null的对象

测试类如下

package com;

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

public class StreamOrderTest {
    public static void main(String[] args) {
        User user1 = new User("a",10,999.0);
        User user2 = new User("b",11,300.0);
        User user3 = new User("c",12,null);
        User user4 = new User("d",null,100.0);
        List list = new ArrayList<>();
        list.add(user4);
        list.add(new User("b",11,100.0));
        list.add(user2);
        list.add(user1);
        list.add(user3);
        list.add(new User("e",null,200.0));
        list.add(new User("e",null,50.0));
        System.out.println(list);//[User{name='c', age=null}, User{name='b', age=11}, User{name='a', age=10}, User{name='c', age=12}]
        //1、对年龄进行升序排序，并且age为null的对象会放到最后
        List ageOrderList = list.stream().sorted(
                Comparator.comparing(User::getAge, Comparator.nullsLast(Integer::compareTo))).collect(Collectors.toList());
        System.out.println(ageOrderList);//[User{name='a', age=10}, User{name='b', age=11}, User{name='c', age=12}, User{name='c', age=null}]

        //2、对年龄进行降序排序
        List ageOrderDescList = list.stream().sorted(
                Comparator.comparing(User::getAge,Comparator.nullsFirst(Integer::compareTo)).reversed()).collect(Collectors.toList());
        System.out.println(ageOrderDescList);

        //2、对年龄进行降序排序 第二种写法  但都会把null放前面了.用nullsFirst解决！
        List ageOrderDescList2 = list.stream().sorted(
                Comparator.comparing(User::getAge,Comparator.nullsFirst(Integer::compareTo).reversed())).collect(Collectors.toList());
        System.out.println(ageOrderDescList2);

        //3、对年龄进行降序排序，并且age为null的对象会放到最后后 再对财富进行排序，包括年龄为null的那些对象
        List ageWealthOrderList = list.stream().sorted(
                Comparator.comparing(User::getAge, Comparator.nullsLast(Integer::compareTo))
                .thenComparing(User::getWealth, Comparator.nullsLast(Double::compareTo).reversed()))
                .collect(Collectors.toList());
        System.out.println(ageWealthOrderList);//[User{name='a', age=10, wealth=999.0}, User{name='b', age=11, wealth=100.0}, User{name='b', age=11, wealth=300.0}, User{name='c', age=12, wealth=null}, User{name='e', age=null, wealth=50.0}, User{name='d', age=null, wealth=100.0}, User{name='e', age=null, wealth=200.0}]
    }

    static class User{
        private String name;
        private Integer age;
        private Double wealth;

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

        public User(String name, Integer age, Double wealth) {
            this.name = name;
            this.age = age;
            this.wealth = wealth;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public Integer getAge() {
            return age;
        }

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

        public Double getWealth() {
            return wealth;
        }

        public void setWealth(Double wealth) {
            this.wealth = wealth;
        }

        @Override
        public String toString() {
            return "User{" +
                    "name='" + name + ''' +
                    ", age=" + age +
                    ", wealth=" + wealth +
                    '}';
        }
    }
}

• Stream表达式可以对某个字段进行升序排序且默认就是升序，为null的对象会放在最后。
• 可以追加多字段排序
• 要想降序排序且null还是在最后，可用nullsFirst认为先把null放在前面，再reversed()则null还是再最后且是按照这个字段降序排序了。
• 注意.reversed()的位置，如果在多字段的整个Comparator外面则会整体倒序！