实施PriorityQueue帮助

这里的典型学生，为作业的最后步骤寻求一些输入 . 任何帮助或提示指出我正确的方向非常感谢 .

我们的部分任务是实施急诊室PriorityQueue . 我坚持的部分细节如下：

写一个名为PatientQueue的课程：a . 默认的no-arg构造函数b . 两种公共方法：i . public void registerPatient（Patient p）ii . public Patient getNextPatient（）c . 内部使用PriorityQueue和PatientComparator

这是我到目前为止所拥有的：

在Patient.java中：

package blah;

import java.util.Date;

public class Patient {

// data fields
protected String name;
protected int category;
protected Date timeArrived;

// accessors and mutators
public String getName() {
    return name;
}
public void setName(String nameIn) {
    this.name = nameIn;
}
public int getCategory() {
    return category;
}
public void setCategory(int categoryIn) {
    this.category = categoryIn;
}

public java.util.Date getTimeArrived() {
    return timeArrived;
}

// default constructor
public Patient() {
    this.name = "Default Name";
    this.category = 5; // unclassified Patients go to the end of the queue
    this.timeArrived = new Date();
}

// overloaded constructor
public Patient(String nameIn, int categoryIn) {
    this.name = nameIn;
    this.category = categoryIn;
    this.timeArrived = new Date();
}

} // end Patient class

在PatientComparator.java中：

package blah;

import java.util.Comparator;

public class PatientComparator implements Comparator<Patient> {

public int compare(Patient p1, Patient p2) {
    if (p1.getCategory() < p2.getCategory())
        return -1;
    if (p1.getCategory() > p2.getCategory())
        return 1;
    else { if (p1.getTimeArrived().before(p2.getTimeArrived()))
        return -1;
           if (p1.getTimeArrived().after(p2.getTimeArrived()))
        return 1;
    }
    return 0;
}

} // end PatientComparator class

在PatientQueue.java中：

package blah;

import java.util.Comparator;
import java.util.PriorityQueue;

public class PatientQueue extends PriorityQueue<Patient> {

// default constructor
public PatientQueue() {

}

public void registerPatient(Patient p) {
            //NEED HELP IN THIS PART//
} // end registerPatient method

public Patient getNextPatient() {
    return (Patient)this.poll();
} // end getNextPatient method

} // end PatientQueue class

最后，在驱动程序EmergencyRoomSimulator.java中：package blah;

import java.util.Random; 

public class EmergencyRoomSimulator { 

private static final int WAIT_LIMIT = 3000; // 1000 = 1 second 

private PatientQueue pq = new PatientQueue(); 

private void t() { 
    try {  Thread.sleep(new Random().nextInt(WAIT_LIMIT));
    } catch (InterruptedException e) {
    } 
} // end t method

private void patientArrives(Patient p) { 
    pq.registerPatient(p); 
    System.out.println(" ARRIVAL: " + p.getName()); 
    System.out.println(" time arrived: " + p.getTimeArrived()); 
    System.out.println(" category: " + p.getCategory()); 
    System.out.println("------------------------------------"); 
    t(); 
} // end patientArrives method

private void doctorVisits() { 
    Patient p = pq.getNextPatient(); 
    System.out.println(" VISIT: " + p.getName()); 
    System.out.println(" time arrived: " + p.getTimeArrived()); 
    System.out.println(" category: " + p.getCategory()); 
    System.out.println("------------------------------------"); 
    t(); 
} // end doctorVisits method

private void simulate() { 
    System.out.println("------------------------------------"); 
    System.out.println("ER OPEN"); 
    System.out.println("------------------------------------"); 
    patientArrives(new Patient("John Paul Jones", 3)); 
    patientArrives(new Patient("Thomas Paine", 1)); 
    patientArrives(new Patient("Joseph Brant", 2)); 
    doctorVisits(); 
    patientArrives(new Patient("Ethan Allen", 2)); 
    patientArrives(new Patient("Henry Knox", 4)); 
    patientArrives(new Patient("Patrick Henry", 2)); 
    doctorVisits(); 
    doctorVisits(); 
    patientArrives(new Patient("Mary Draper", 1)); 
    patientArrives(new Patient("Samuel Adams", 3)); 
    doctorVisits(); 
    doctorVisits(); 
    doctorVisits(); 
    doctorVisits(); 
    doctorVisits(); 
    System.out.println("------------------------------------"); 
    System.out.println("ER CLOSED"); 
    System.out.println("------------------------------------"); 
} // end simulate method

public static void main(String[] args) { 
    EmergencyRoomSimulator er = new EmergencyRoomSimulator(); 
    er.simulate(); 
} // end main 

} // end class

我得到的错误是：

线程“main”java.lang.NullPointerException中的异常

在主要的第一个 doctorVisits() 电话 . 我知道'm missing the proper way to actually add and/or remove an object to/from the list, but I can't看到 PatientQueue 类需要发生什么来实际触发 PriorityQueue 被用来添加或"get"下一个病人 .

再次，非常感谢任何建议 . 谢谢！