JAVA:对象数组仅打印最后一个元素

Question

我的数据和算法类(class)作业告诉我们创建一个系统任务工单程序。它要求我们做各种事情，例如向数组添加新的 SystemTask 对象、删除任务以及列出/打印排序的对象数组。

我已经完成了大部分编码，但我一直在打印对象数组。看来无论我添加到对象数组中的最后一个数据是什么，它都会成为数组中所有元素的数据。

Some instructions:

• My WorkOrders() constructor must place the 4 default SystemTask objects in the array.
• listSystemTask() method lists all SystemTask objects in the array (in order) with a number in front.

NOTE: Please refrain from telling me to use List or any advance java methods. My homework has limitations to it.

系统任务类

  public class SystemTask{

  private static String month, message;
  private static int day, hour, minute;

  //constructor methods
  public SystemTask(){
    month = "Jan";
    day = 1;
    hour = 00;
    minute = 00;
    message = "Task Here";
  }

  public SystemTask(String mon, int d, int h, int mnt, String msg){
    month = mon;
    day = d;
    hour = h;
    minute = mnt;
    message = msg;

    /*setMonth(mon);
    setDay(d);
    setHour(h);
    setMinute(mnt);
    setMessage(msg);*/
  }

  //get and set methods
  public static String getMonth(){
    return  month;
  }
  public static int getDay(){
    return day;
  }
  public static int getHour(){
    return hour;
  }
  public static int getMinute(){
    return minute;
  }
  public static String getMessage(){
    return message;
  }
  public static void setMonth (String mon){
    if(mon.length()!= 3)
        System.out.println("Invalid 3 Letter Code. Try Again.");
    else
        month = mon;
  }
  public static void setDay(int d){
    if(d<1 || d>31)
        System.out.println("Invalid Day. Try Again.");
    else
        day = d;
  }
  public static void setHour(int h){
    if(h<0 || h>23)
        System.out.println("Invalid Hour. Try Again.");
    else
        hour = h;
  }
  public static void setMinute(int mnt){
    if(mnt<0 || mnt>59)
        System.out.println("Invalid Minute. Try Again.");
    else
        minute = mnt;
  }
  public static void setMessage(String msg){
    if(msg.length()<1 || msg.length()>40)
        System.out.println("Invalid Message. Try Again.");
    else
        message = msg;
  }

  @Override
  public String toString(){
    return getMonth()+" "+getDay()+", "+String.format("%02d",getHour())
            +":"+String.format("%02d",getMinute())+" "+getMessage();
  }
}

工单类

public class WorkOrders {

private final SystemTask[] ary = new SystemTask[20];  //declare private 20 objects array
//public static int index, empty;

public static void main(String args[]){
    WorkOrders object = new WorkOrders();       //create a WorkOrders object
    object.run();                               //call a run method
}

public WorkOrders(){
  //  for(int i = 0;i<ary.length;i++){
  //      ary[i] = new SystemTask();
  //  }

    /*SystemTask t1 = new SystemTask("Mar", 4, 21, 30, "Backup Users");
    ary[0] = t1;
    SystemTask t2 = new SystemTask("Apr", 1, 17, 0, "Upgrade Hard Drives");
    ary[1] = t2;
    SystemTask t3 = new SystemTask("May", 6, 10, 45, "Virus Scan");
    ary[2] = t3;
    SystemTask t4 = new SystemTask("Jun", 3, 9, 15, "Database Backup");
    ary[3] = t4;*/

    ary[0] = new SystemTask("Mar", 4, 21, 30, "Backup Users");
    ary[1] = new SystemTask("Apr", 1, 17, 0, "Upgrade Hard Drives");
    ary[2] = new SystemTask("May", 6, 10, 45, "Virus Scan");
    ary[3] = new SystemTask("Jun", 3, 9, 15, "Database Backup");
}

public void run(){
    char choice;
    do{
        System.out.println("SYSTEM WORKORDER PROGRAM:");
        System.out.println("A)dd SystemTask");
        System.out.println("D)elete SystemTask");
        System.out.println("L)ist SystemTask");
        System.out.println("E)xit");
        System.out.print("Select an option: ");
        choice = UserInput.getChar();

        switch (choice) {
            case 'a':
            case 'A':
                //addSystemTask();
                break;
            case 'd':
            case 'D':
                //deleteSystemTask();
                break;
            case 'l':
            case 'L':
                listSystemTask();
                break;
            case 'e':
            case 'E':
                System.out.println("You quit the program.");
                System.out.println("\nThanks for using System Workorder!");
                System.exit(0);
                break;
            default:
                System.out.println("\'" + choice + "\' does not exist. Try               again.");
                break;
        }
        System.out.println();
    }while(choice !='E'|| choice !='e');   
}

public void listSystemTask(){
    System.out.println("\nALL LISTED TASKS");
    for(int c=0;c<ary.length;c++){
        if(ary[c]!=null)
            System.out.println(c+1 + ": " + ary[c].toString());
    }
}    

我制作了另一个 UserInput 类。我没有包含它，因为它仅用于我的键盘输入。我省略了一些与我的问题无关的代码/方法。我只关心数组中对象的打印。

这里的问题是，这是我打印列表时得到的输出:

SYSTEM WORKORDER PROGRAM:
A)dd SystemTask
D)elete SystemTask
L)ist SystemTask
E)xit
Select an option: L

ALL LISTED TASKS
1: Jun 3, 09:15 Database Backup
2: Jun 3, 09:15 Database Backup
3: Jun 3, 09:15 Database Backup
4: Jun 3, 09:15 Database Backup

不知道问题出在哪里。我怀疑它是在构造函数中数组的初始化中，但我似乎无法修复它。或者也许它在其他地方？

Answer 1

问题出在 SystemTask 类

public class SystemTask {
  private static String month, message;
  private static int day, hour, minute;

您已将所有成员变量声明为静态。将有多个 SystemTask 实例，但它们都将其值存储在相同的 static 字段中。它打印“最后一个 SystemTask”，因为它是最后一个创建的，覆盖了之前设置的值。

只需删除 static 声明...

public class SystemTask {
  private String month, message;
  private int day, hour, minute;

您还将所有访问器方法定义为静态。也从其中删除 static，以允许它们访问对象范围变量。

  public static String getMonth() {
    return month;
  }

成为...

  public String getMonth() {
    return month;
  }