JAVA: Object Array only prints last element

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?

Possible duplicate of [Why does my ArrayList contain N copies of the last item added to the list?](http://stackoverflow.com/questions/19843506/why-does-my-arraylist-contain-n-copies-of-the-last-item-added-to-the-list) — OneCricketeer, Feb 24 '17 at 03:41
You can't exactly "add" objects to an array, by the way. Arrays have fixed sizes. So, you rather "set" into the array — OneCricketeer, Feb 24 '17 at 03:43

slipperyseal · Accepted Answer · 2017-02-24T03:47:48.387

4

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;
  }

edited Feb 24 '17 at 03:47

answered Feb 24 '17 at 03:38

slipperyseal

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Thank you very much. This worked. I guess I need to learn more when to use the `static` keyword. – David Dredd de Jesus Feb 24 '17 at 22:32
@DavidDredddeJesus the general rule for using static is VERY RAIRLY :) they create "singletons" which are considered an anti-pattern. if you think you need a singleton, there are often ways to avoid them, or reduce how many you actually need. There is lots of debate and pros and cons of singleton on the web. – slipperyseal Feb 25 '17 at 04:58

JAVA: Object Array only prints last element

1 Answers1