Collections.sort()方法为List集合内对象进行排序

此处算是做一个笔记吧，我怕忘了。

对于List排序我们用的还是不少，我们平常都是用的Collections.sort()的方法，对于我们所需要排序的List内的对象是有要求的。那就是必须要实现Comparable这个接口，重写compare()这个方法。因此这种情况下我们一般会有两种方法来进行排序。我这儿用的是一个Student类，有String类型的name字段，int类型的year字段，String类型的day字段。这儿解释一下day字段，由于这儿的比较需要，我是用的“月-日”来表示的，此地需要才这样写，一般情况下不建议。

第一种方法：

Student类：

package com.uitrs.game.reg.common;/*** @author 作者 BealHe* @date 创建时间：2019年4月22日 下午2:44:06* @explain 类说明 ：学生类*/
public class Student {/* 姓名 */private String name;/* 出生年 */private int year;/* 出生月-日（此地需要才这样写，一般情况下不建议） */private String day;public Student() {super();}public Student(String name, int year, String day) {super();this.name = name;this.year = year;this.day = day;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getYear() {return year;}public void setYear(int year) {this.year = year;}public String getDay() {return day;}public void setDay(String day) {this.day = day;}@Overridepublic String toString() {return "Student [name=" + name + ", year=" + year + ", day=" + day + "]";}
}

下面是主方法和排序方法：

public static void main(String[] args) throws Exception {List<Student> list = new ArrayList<>();Student s1 = new Student("张三", 1992, "2-5");list.add(s1);s1 = new Student("李四", 1992, "12-5");list.add(s1);s1 = new Student("王五", 1992, "2-20");list.add(s1);s1 = new Student("万一", 1992, "12-20");list.add(s1);s1 = new Student("开心", 1993, "3-12");list.add(s1);s1 = new Student("幸福", 1993, "3-5");list.add(s1);s1 = new Student("刘二", 1990, "2-5");list.add(s1);s1 = new Student("书桌", 1991, "2-5");list.add(s1);for (Student student : list) {System.out.println(student.toString());}System.out.println("----------------------分割线----------------------");sort(list);for (Student student : list) {System.out.println(student.toString());}}/*** 学生年龄大小比较，首先比较年份，其次比较月份和日期* * @param list*/private static void sort(List<Student> list) {Collections.sort(list, new Comparator<Student>() {@Overridepublic int compare(Student o1, Student o2) {Integer year1 = o1.getYear();Integer year2 = o2.getYear();// 比较年份if (0 == year1.compareTo(year2)) {String[] arr1 = o1.getDay().split("-");String[] arr2 = o2.getDay().split("-");Integer month1 = Integer.parseInt(arr1[0]);Integer month2 = Integer.parseInt(arr2[0]);// 比较月份if (0 == month1.compareTo(month2)) {Integer day1 = Integer.parseInt(arr1[1]);Integer day2 = Integer.parseInt(arr2[1]);// 比较日期return day1.compareTo(day2);} else {return month1.compareTo(month2);}} else {return year1.compareTo(year2);}}});}

然后是排序前后输出结果对照：

Student [name=张三, year=1992, day=2-5]
Student [name=李四, year=1992, day=12-5]
Student [name=王五, year=1992, day=2-20]
Student [name=万一, year=1992, day=12-20]
Student [name=开心, year=1993, day=3-12]
Student [name=幸福, year=1993, day=3-5]
Student [name=刘二, year=1990, day=2-5]
Student [name=书桌, year=1991, day=2-5]
----------------------分割线----------------------
Student [name=刘二, year=1990, day=2-5]
Student [name=书桌, year=1991, day=2-5]
Student [name=张三, year=1992, day=2-5]
Student [name=王五, year=1992, day=2-20]
Student [name=李四, year=1992, day=12-5]
Student [name=万一, year=1992, day=12-20]
Student [name=幸福, year=1993, day=3-5]
Student [name=开心, year=1993, day=3-12]

上面就是第一种排序的方法，从结果可以看到，我们先以年为排序依据，其次再以月为依据，最后才以日为依据，排序结果没问题。接下来是第二种排序方法。

第二种方法：

实现Comparable的Student类：

package com.uitrs.game.reg.common;/*** @author 作者 BealHe* @date 创建时间：2019年4月22日 下午2:44:06* @explain 类说明 ：学生类*/
public class Student implements Comparable<Student> {/* 姓名 */private String name;/* 出生年 */private int year;/* 出生月-日（此地需要才这样写，一般情况下不建议） */private String day;public Student() {super();}public Student(String name, int year, String day) {super();this.name = name;this.year = year;this.day = day;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getYear() {return year;}public void setYear(int year) {this.year = year;}public String getDay() {return day;}public void setDay(String day) {this.day = day;}@Overridepublic String toString() {return "Student [name=" + name + ", year=" + year + ", day=" + day + "]";}@Overridepublic int compareTo(Student o) {Integer year1 = this.getYear();Integer year2 = o.getYear();// 比较年份if (0 == year1.compareTo(year2)) {String[] arr1 = this.getDay().split("-");String[] arr2 = o.getDay().split("-");Integer month1 = Integer.parseInt(arr1[0]);Integer month2 = Integer.parseInt(arr2[0]);// 比较月份if (0 == month1.compareTo(month2)) {Integer day1 = Integer.parseInt(arr1[1]);Integer day2 = Integer.parseInt(arr2[1]);// 比较日期return day1.compareTo(day2);} else {return month1.compareTo(month2);}} else {return year1.compareTo(year2);}}
}

下面是排序主方法：

public static void main(String[] args) throws Exception {List<Student> list = new ArrayList<>();Student s1 = new Student("张三", 1992, "2-5");list.add(s1);s1 = new Student("李四", 1992, "12-5");list.add(s1);s1 = new Student("王五", 1992, "2-20");list.add(s1);s1 = new Student("万一", 1992, "12-20");list.add(s1);s1 = new Student("开心", 1993, "3-12");list.add(s1);s1 = new Student("幸福", 1993, "3-5");list.add(s1);s1 = new Student("刘二", 1990, "2-5");list.add(s1);s1 = new Student("书桌", 1991, "2-5");list.add(s1);for (Student student : list) {System.out.println(student.toString());}System.out.println("----------------------分割线----------------------");Collections.sort(list);for (Student student : list) {System.out.println(student.toString());}}

最后是排序前后输出结果对照：

Student [name=张三, year=1992, day=2-5]
Student [name=李四, year=1992, day=12-5]
Student [name=王五, year=1992, day=2-20]
Student [name=万一, year=1992, day=12-20]
Student [name=开心, year=1993, day=3-12]
Student [name=幸福, year=1993, day=3-5]
Student [name=刘二, year=1990, day=2-5]
Student [name=书桌, year=1991, day=2-5]
----------------------分割线----------------------
Student [name=刘二, year=1990, day=2-5]
Student [name=书桌, year=1991, day=2-5]
Student [name=张三, year=1992, day=2-5]
Student [name=王五, year=1992, day=2-20]
Student [name=李四, year=1992, day=12-5]
Student [name=万一, year=1992, day=12-20]
Student [name=幸福, year=1993, day=3-5]
Student [name=开心, year=1993, day=3-12]

上面就是第二种排序的方法，从结果可以看到，和第一次的排序结果一致。

这就是我的两种排序方法，实现方法不同，一个new的Comparator类，一个实现Comparable类并重写compareTo()方法。