一个学生类的实体类

@Data
public class Student {
    private Long id;
    private String name;
    private int age;
    private Double height;
    public Student(Long id, String name, int age, Double height) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.height = height;
    }

然后我们测试下三种排序方式：

• 按照id升序排列
• 按照id逆序排列
• 按照age排序，然后按照height排序

public class ListStreamSortTest {
    public static void main(String[] args) {
        // 一个集合中放入4个学生对象
        List<Student> list = new ArrayList<>();
        list.add(new Student(10002L, "ZhangSan", 19, 175.2));
        list.add(new Student(10003L, "LiSi", 18, 180.1));
        list.add(new Student(10004L, "Peter", 19, 170.8));
        list.add(new Student(10001L, "KangKang", 18, 167.4));
        // 打印默认顺序
        System.out.println("默认顺序：");
        list.stream().forEach(System.out::println);
        // 按照id排序
        System.out.println("id升序：");
        list.stream().sorted(Comparator.comparing(Student::getId))
                .forEach(System.out::println);
        // 按照id逆序排列
        System.out.println("id逆序：");
        list.stream().sorted(Comparator.comparing(Student::getId).reversed())
                .forEach(System.out::println);
        // 按照年龄排序，再按照升高排序
        System.out.println("age和height排序：");
        list.stream().sorted(Comparator.comparing(Student::getAge).thenComparing(Student::getHeight))
                .forEach(System.out::println);
    }
}

执行结果：

默认顺序：
Student{id=10002, name='ZhangSan', age=19, height=175.2}
Student{id=10003, name='LiSi', age=18, height=180.1}
Student{id=10004, name='Peter', age=19, height=170.8}
Student{id=10001, name='KangKang', age=18, height=167.4}
id升序：
Student{id=10001, name='KangKang', age=18, height=167.4}
Student{id=10002, name='ZhangSan', age=19, height=175.2}
Student{id=10003, name='LiSi', age=18, height=180.1}
Student{id=10004, name='Peter', age=19, height=170.8}
id逆序：
Student{id=10004, name='Peter', age=19, height=170.8}
Student{id=10003, name='LiSi', age=18, height=180.1}
Student{id=10002, name='ZhangSan', age=19, height=175.2}
Student{id=10001, name='KangKang', age=18, height=167.4}
age和height排序：
Student{id=10001, name='KangKang', age=18, height=167.4}
Student{id=10003, name='LiSi', age=18, height=180.1}
Student{id=10004, name='Peter', age=19, height=170.8}
Student{id=10002, name='ZhangSan', age=19, height=175.2}

需求： 需要对List中的每个student对象按照birthday顺序排序，时间由小到大排列

1. 刚开始用的是冒泡排序，出现数据覆盖的情况

for (int i = 0; i < list.size() - 1; i++) {
            for (int j = 0; j < list.size() - 1 - i; j++) {
                long time = list.get(j).getCreateAt().getTime();
                long time1 = list.get(j + 1).getCreateAt().getTime();
                if (time >time1) {
                    Object temp = list.get(j+1);
                    BeanUtils.copyProperties(list.get(j), list.get(j+1));
                    BeanUtils.copyProperties(temp, list.get(j));
                }
            }

2. 之后想到了Comparator比较器

public static <T> void sort(List<T> list，Comparator<? super T> )

这个里面就涉及到了Comparator 这个接口，位于java.util包下，排序是comparator能实现的功能之一,通俗地讲需要比较两个对象 谁排在前谁排在后，那么比较的方法就是：

• public int compare(String o1, String o2)：比较其两个参数的顺序

两个对象比较的结果有三种：大于，等于，小于。 
如果要按照升序排序， 则o1 小于o2，返回（负数），相等返回0，01大于02返回（正数） 
如果要按照降序排序 则o1 小于o2，返回（正数），相等返回0，01大于02返回（负数）

操作如下

package com.hbsi.test;
 
import java.text.SimpleDateFormat;
import java.util.*;
 
/**
 * @author lbq
 * @date 2020 10  15:07
 */
public class ListSort {
    static class Student {
        private String userName;
        private String birthday;
    }
 
    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = new Student();
        Student s3 = new Student();
        List<Student> list = new ArrayList<Student>();
        s1.setUserName("aa");
        s1.setBirthday("1997-01-08");
        s2.setUserName("bb");
        s2.setBirthday("1990-11-08");
        s3.setUserName("cc");
        s3.setBirthday("1957-05-08");
        list.add(s1);
        list.add(s2);
        list.add(s3);
        System.out.println("排序前:");
        for (Student o : list) {
            System.out.println(o);
        }
        listSort(list);
        System.out.println("排序后:");
        for (Student o : list) {
            System.out.println(o);
        }
    }
    private static void listSort(List<Student> list) {
        Collections.sort(list, new Comparator<Student>() {
            SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd");
 
            public int compare(Student o1, Student o2) {
                try {
                    Date dt1 = sf.parse(o1.getBirthday());
                    Date dt2 = sf.parse(o2.getBirthday());
                    if (dt1.getTime() > dt2.getTime()) {
                        return 1;
                    } else if (dt1.getTime() < dt2.getTime()) {
                        return -1;
                    } else {
                        return 0;
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
                return 0;
            }
        });
    }
}