Question

我有一个类Student，它实现了Comparable，如下所示，因为我想将Student的对象放在优先级队列中，并让拥有较早注册日期的学生具有更高的优先级。

public class Student implements Comparable{
    private String fullName;
    private Date registrationDate;

public Student(String fullname){
    this.fullName = fullName;
}



public String getFullName() {
    return fullName;
}

public void setFullName(String fullName) {
    this.fullName = fullName;
}

public Date getRegistrationDate() {
        return registrationDate;
    }
    public void setRegistrationDate(Date registrationDate) {
        this.registrationDate = registrationDate;
    }

@Override
    public int compareTo(Object obj) {
        Student student = (Student) obj;

        if(getRegistrationDate().before(student.getRegistrationDate())){
            return 1;
        }else if(getRegistrationDate().after(student.getRegistrationDate())){
            return -1;
        }
        return 0;
    }
}

在main方法中，我正在创建学生对象，设置注册日期并将它们放入优先级队列

public class School {

    public static void main(String args[]) {

        //list of students
        Student student1 = new Student("John Doe");
        Date dateStudent1Joined = new GregorianCalendar(2014, Calendar.JULY, 1).getTime();
        student1.setRegistrationDate(dateStudent1Joined);

        Student student2 = new Student("Mary Penn");
        Date dateStudent2Joined = new GregorianCalendar(2014, Calendar.JULY, 2).getTime();
        student2.setRegistrationDate(dateStudent2Joined);

        Student student3 = new Student("George Coats");
        Date dateStudent3Joined = new GregorianCalendar(2014, Calendar.JULY, 3).getTime();
        student3.setRegistrationDate(dateStudent3Joined);

        Student student4 = new Student("Tony Case");
        Date dateStudent4Joined = new GregorianCalendar(2014, Calendar.JULY, 4).getTime();
        student4.setRegistrationDate(dateStudent4Joined);

        Student student5 = new Student("Ben Thomas");
        Date dateStudent5Joined = new GregorianCalendar(2014, Calendar.JULY, 5).getTime();
        student5.setRegistrationDate(dateStudent5Joined);

        //create a queue data structure to hold students
        PriorityQueue<Student> studentQueue = new PriorityQueue<Student>();
        //add students to queue
        studentQueue.offer(student1);
        studentQueue.offer(student2);
        studentQueue.offer(student3);
        studentQueue.offer(student4);
        studentQueue.offer(student5);

        //print names of people in queue
        for(Student student : studentQueue){
            String studentName = student.getFullName();
            System.out.println(studentName);
            System.out.println("");

        }

            studentQueue.poll();    

        for(Student student : studentQueue){
            String studentName = student.getFullName();
            System.out.println(studentName);
        }
    }
}

在School.java中的第一个for循环中，我打印出队列中所有学生的名字，以确保它们具有正确的优先级，即具有较早注册日期的学生位于队列前面。这里的结果就是我所期待的，因为John Doe加入了第一，Ben Thomas加入了最后。

John Doe
Mary Penn
George Coats
Tony Case
Ben Thomas

但是，当我在优先级队列上调用poll（）方法时，John Doe被删除但是优先级队列的顺序不再有意义，第二个for循环的输出是

Mary Penn
Tony Case
George Coats
Ben Thomas

Mary Penn处于正确的位置，但如果前一个优先级队列中的顺序仍然存在，那么George Coats应该来到Tony Case之前，我在这里做错了什么？

Answer 1

看看this。 PriorityQueue的迭代顺序无法保证排序。当您多次致电poll()时，您会发现订单实际上是正确的。

调查轮询方法后，Java优先级队列重新排列

1 个答案: