Java 8 Stream 多个字段排序的大坑

1. 自定义类

public static class Person {
        private String name;
        private Integer age;
        private Integer level;
        
        public Person() {
        }
        public Person(String name, Integer age, Integer level) {
            this.name = name;
            this.age = age;
            this.level = level;
        }
		// 此处省略 get、set
    }

2. 单个字段排序

降序 reversed() (默认是升序)

直接排序如果排序字段为null的话会出现NullPointerException所以需要排空

解决方式：

1. 过滤掉该字段为null的数据
2. 使用nullsFirst()或者/nullsLast()

public class TestMain {
    public static void main(String[] args) {
        Person xm = new Person("小明", 18, 1);
        Person xh = new Person("小红", 22, 2);
        Person xb = new Person("小白", 18, 2);
        Person xl = new Person("小蓝", 22, 1);

        List<Person> list = Arrays.asList(xm, xh, xb, xl);

        // 以年龄降序 reversed() (默认是升序)
        List<Person> collect = list.stream()
                .sorted(Comparator.comparing(Person::getAge, Comparator.nullsFirst(Integer::compareTo)).reversed())
                .collect(Collectors.toList());

        for (Person person : collect) {
            System.out.println(JSONObject.toJSONString(person));
        }
        /* 结果
        {"age":22,"level":2,"name":"小红"}
        {"age":22,"level":1,"name":"小蓝"}
        {"age":18,"level":1,"name":"小明"}
        {"age":18,"level":2,"name":"小白"}
         */
    }
}

3. 多个字段排序

先以年龄 age 进行降序, 如果年龄相同则以层级 level 进行降序。

这里要注意，主排序字段要放到最后，次排序字段要放在前面

比如这里 level 字段要放前面, age 字段要放后面

// 多个字段进行降序, 先以年龄进行降序, 如果年龄相同则以层级进行降序
List<Person> collect = list.stream()
        .sorted(Comparator.comparing(Person::getLevel).reversed())
        .sorted(Comparator.comparing(Person::getAge).reversed())
        .collect(Collectors.toList());

/* 结果

{"age":22,"level":2,"name":"小红"}
{"age":22,"level":1,"name":"小蓝"}
{"age":18,"level":2,"name":"小白"}
{"age":18,"level":1,"name":"小明"}

 */