Priority Que 在 java 中返回 null 以及如何使用它的 comparable

Question

我是第一次使用 priorityQue，我很困惑为什么当我尝试打印它时它返回 null。这是一个作业，我正在制作一个简单的菜单来存储显示和删除（只是 head 值）。另外，如果有人可以解释可比较的 class 如何对 priorityQue 进行排序，因为我很确定我做错了。感谢任何帮助

主要Class

import java.util.PriorityQueue;
import java.util.Scanner;

public class  PatientManager{
    Patient p =new Patient(0, null, 0);

    PriorityQueue<Patient> waitingList= new PriorityQueue<Patient>();
    int order=0; 

    public static void main(String[] args){
        PatientManager pm=new PatientManager();
        pm.start();
    }
    void start(){
        boolean go=true;
        try{
            while(go){

                System.out.println("------------------");
                System.out.println("(1) New Patient.");
                System.out.println("(2) Next Patient.");
                System.out.println("(3) Waiting List.");
                System.out.println("(4) Exit.");
                System.out.println("------------------");
                System.out.print("Chose an item from the menu: ");
                Scanner input=new Scanner(System.in);
                int ask= input.nextInt();
                switch (ask) {
                case 1:  optionOne();
                         break;
                case 2:  optionTwo();
                         break;
                case 3:  optionThree();
                         break;
                case 4:  go=false;
                         break;
                default: System.out.print("(x) Wrong choice: ");
                         break;
                }
            }
        }catch(Exception e){
            System.out.print("(x) Wrong choice");
            e.printStackTrace();
            start();
        }
    }
    void optionOne(){
        System.out.print("Enter Patients name ");
        Scanner reader=new Scanner(System.in);
        String name= reader.next();
        p.setName(name);
        System.out.print("Enter Emergency[1 (low) to 5 (life-and-death)] ");
        boolean a=true;
        while(a){
            try{
                int b=reader.nextInt();
                if(1<=b && b<=5){
                    p.setEmergency(b);
                    a=false;
                }else System.out.print("(x) Wrong choice: ");
            }catch(Exception e){
                System.out.print("(x) Wrong choice");
                a=false;
            }
        }
        System.out.println(waitingList.add(new Patient(order,p.getName(),p.getEmergency())));
        System.out.println(waitingList.poll());
        order++;
    }
    void optionTwo(){
        System.out.println(waitingList.peek());
        waitingList.remove();
    }
    void optionThree(){
        System.out.println(waitingList.peek());
    }
}

getter setter class

import java.util.Comparator;

public class Patient implements Comparable<Patient>{
    //attributes
    private String name;
    private int order;      //order of arrival
    private int emergency; //1 is normal, 5 is life-and-death situation

    //constructor
    public Patient(int order, String name, int priority) {
        this.order = order;
        this.name = name;
        this.emergency = priority;
    }

    //getters and setters
    public int getOrder() {
        return order;
    }
    public void setOrder(int order) {
        this.order = order;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    public int getEmergency() {
        return emergency;
    }

    public void setEmergency(int emergency) {
        this.emergency = emergency;
    }

    public String toString() {
        return name;
    }
    public int compareTo(Patient a){
        if(emergency!=a.emergency && emergency>a.emergency) return emergency;
        else if(emergency!=a.emergency && emergency<a.emergency) return a.emergency;
        else if(emergency==a.emergency){
            if(order<a.order)return order;
            else return a.order;
        }
        return emergency=0;
    }
}

Answer 1

对于 Comparable，您需要这样做：

如果相同则 0 => 相同的紧急情况

if current > second = 1 current patient has higher priority/emergency

if current < second = -1 current patient has lower priority/emergency

在代码中：

 public int compareTo(Patient a){
    if(emergency==a.emergency)
        return 0;       
    else if(emergency>a.emergency)
        return 1;
    else
        return -1;          
}

您是否真的想在 optionOne() 中执行以下操作。你得到 null 的原因是因为一旦你将它添加到队列中你就会在下一行中删除它。

  System.out.println(waitingList.add(new Patient(order,p.getName(),p.getEmergency())));
  System.out.println(waitingList.poll()); //poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.

再读一遍民意调查的内容，因为我认为你误解了。

Priority Que 在 java 中返回 null 以及如何使用它的 comparable

Priority Que returning null in java and how to use comparable for it

java

priority-queue