java使用compareTo实现一个类的对象之间比较大小操作

package cn.demo;
 
public class Student implements Comparable{
 private int number=0;  //学号
 private String name="";  //学生姓名
 private String gender="";  //性别
 public int getNumber(){
 return number;
 }
 public void setNumber(int number){
 this.number=number;
 }
 public String getName(){
 return name;
 }
 public void setName(String name){
 this.name=name;
 }
 public String getGender(){
 return gender;
 }
 public void setGender(String gender){
 this.gender=gender;
 }
 
 public int compareTo(Object obj){
 Student student=(Student)obj;
 if(this.number==student.number){ 
  return 0;  //如果学号相同，那么两者就是相等的
 }else if(this.number>student.getNumber()){ 
  return 1;  //如果这个学生的学号大于传入学生的学号
 }else{ 
  return -1;  //如果这个学生的学号小于传入学生的学号
 }
 }
}

package cn.demo;
 
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
 
public class Test {
 public static void main(String[] args) {
 Student student1=new Student();
 student1.setNumber(5);
 Student student2=new Student();
 student2.setNumber(2);
 Student student3=new Student();
 student3.setNumber(1);
 Student student4=new Student();
 student4.setNumber(4);
 ArrayList<Student> list=new ArrayList<Student>();
 list.add(student1);
 list.add(student2);
 list.add(student3);
 list.add(student4);
 System.out.println("-------排序前-------");
 Iterator<Student> iterator=list.iterator();
 while(iterator.hasNext()){
  Student stu=iterator.next();
  System.out.println(stu.getNumber());
 }
 //使用Collections的sort方法对list进行排序
 System.out.println("-------排序后-------");
 Collections.sort(list); 
 iterator=list.iterator();
 while(iterator.hasNext()){
  Student stu=iterator.next();
  System.out.println(stu.getNumber());
 } 
 
 }
}

-------排序前-------
5
2
1
4
-------排序后-------
5
4
2
1

//定义学生类，并实现Comparable接口<定义Student类型>
public class Student implements Comparable<Student>{

  private String name;
  private int age;
  private double grade;

  public Student() {
  }

  public Student(String name, int age, double grade) {
    this.name = name;
    this.age = age;
    this.grade = grade;
  }

  public String getName() {
    return name;
  }

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

  public int getAge() {
    return age;
  }

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

  public double getGrade() {
    return grade;
  }

  public void setGrade(double grade) {
    this.grade = grade;
  }

  @Override
  public String toString() {
    return "Student [姓名 ：" + name +
        " 年龄 ：" + age +
        " 分数" + grade + "]";
  }


  /**
   * 用Comparable接口对下列四位同学的成绩做降序排序，
   * 如果成绩一样，那在成绩排序的基础上按照年龄由小到大排序。请正确指定泛型
   * @param o
   * @return
   */
  @Override
  public int compareTo(Student o) {
    if(Double.compare(this.getGrade() , o.getGrade()) != 0){
      if(Double.compare(this.getGrade() , o.getGrade()) > 0){
        return 1;
      }else {
        return -1;
      }
    }
    return this.getAge() - o.getAge();
  }
}

public class Task {

  public static void main(String[] args) {

 //定义四个学生对象
    Student[] s1 = new Student[4];
    s1[0] = new Student("liusan" , 20 , 90.0);
    s1[1] = new Student("lisi" , 22 , 90.0);
    s1[2] = new Student("wangwu" , 20 , 99.0);
    s1[3] = new Student("sunliu" , 22 , 100.0);

    Arrays.sort(s1);
    System.out.println(Arrays.toString(s1));

    System.out.println("==================");

    //用Comparator实现按照姓名排序，请正确指定泛型
    Arrays.sort(s1, new Comparator<Student>() {
      @Override
      public int compare(Student o1, Student o2) {
        return o1.getName().compareTo(o2.getName());
      }
    });

    System.out.println(Arrays.toString(s1));
  }
}