java - PriorityQueue : Student cannot be cast to java. lang.Comparable

Question

我正在使用 PriorityQueue 结构来获取用户设置的一些字段，这是代码的一部分:

package gqg;

import java.util.Queue;

public class Student {
     //variables (ID, Name, ...), constructors, getters and setters...

Queue<Student> StudentQueue = new PriorityQueue<Student>();

public void Add() { //method to add the student at Queue
    for(int x=0; x<1; x++) {
        Student st = new Student();
        System.out.println("+---------------------------+\n"
                         + "|   Students Registration   |\n"
                         + "+---------------------------+");
        System.out.println("| Type the student's ID:");
        stu.setID(user.nextInt());
        System.out.println("| Type the student's name:");
        stu.setName(user.next());
        System.out.println("| Type the student's age:");
        stu.setAge(user.nextInt());
        //and other fields...

        StudentQueue.add(st);
    }
    System.out.println("Done. Student has been added successfuly\n");       
}

/* then I call Add(); in Menu();
 * this method also has Print(); Modify(); Eliminate(); those three works well
 * The only one problem is Add();
 */

public void Menu(){
    //... methods
}
}

当我只添加一个“学生”时没有问题，但是当我 try catch 第二个“学生”时应用程序抛出此异常

Exception in thread "main" java.lang.ClassCastException: gqg.Student cannot be cast to java.lang.Comparable     at
java.util.PriorityQueue.siftUpComparable(PriorityQueue.java:633)    at
java.util.PriorityQueue.siftUp(PriorityQueue.java:629)  at
java.util.PriorityQueue.offer(PriorityQueue.java:329)   at
java.util.PriorityQueue.add(PriorityQueue.java:306)     at
gqg.Student.AddQueue(Student.java:374)  at
gqg.Student.Menu(Student.java:592)  at
gqg.MainClass.main(MainClass.java:7)

有人可以向我解释问题出在哪里/为什么吗？我花了很多时间在网上寻找解决方案，但找不到，我在这里需要一些帮助...谢谢你帮助我

Answer 1

如果您不提供自定义 Comparator , PriorityQueue对其持有的对象使用自然排序。也就是说，它期望您的对象是 Comparable对彼此。你的Student类似乎没有实现 Comparable .

所以有两个选择:

• 实现并提供自定义 Comparator用于比较 Student对象
• 有你的Student类(class)工具Comparable<Student>具有适当的逻辑