JAVA：对象数组仅打印最后一个元素

Question

My homework for a data & algorithms class tells us to create a System Task Workorder Program. It's asking us to do various things like add a new SystemTask object to an array, delete a task and list/print the sorted object array.

I've done most of the coding already, but I'm stuck in printing out my objects array. It seems that whatever the last data I add into the object array, it becomes the data for all the elements in my array.

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.

SystemTask Class

  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();
  }
}

WorkOrder Class

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());
    }
}    

There is another UserInput Class that I made. I didn't include it because it merely for my keyboard input. I have omitted some of the code/methods that has nothing to do with my question. I'm only concerned with the printing of the objects in my array.

The problem here is this is what I get for output when I print the list:

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

I don't know where the problem is coming from. I'm suspecting it's in the initializing of the arrays in the constructor but I can't seem to fix it. Or maybe it's in other place?

Answer 1

The problem is with the SystemTask class

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

You have declared all the member variables as static . There will be multiple instances of SystemTask but they are all storing their values in the same static fields. It's printing "the last SystemTask " because it was the last one to be created, overwriting the values set by the previous ones.

Simply remove the static declarations...

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

You have also defined all the accessor methods as static . Drop the static from these too, to allow them to access the object scope variables.

  public static String getMonth() {
    return month;
  }

Becomes...

  public String getMonth() {
    return month;
  }