Question

I have a problem with getting a new value of an object. I have a code like that:

...
TimeSchedule[] offspringScheduleOne = new TimeSchedule[AVAILABLE_CLASSROOMS];
...
offspringScheduleOne[i] = genes.get(geneOneIndex).getSchedule()[i];
...

After that assignment offspringScheduleOne[i] and genes.get(geneOneIndex).getSchedule()[i] points the same memory address. I want that: offspringScheduleOne[i] should get the value of the genes.get(geneOneIndex).getSchedule()[i], they musn’t be same, they just should have same values.

TimeSchedule class:

public class TimeSchedule extends AlgorithmParameters {
    public int[][] timetable = new int[DAYS][HOURS];//DAYS and HOURS are static final variables that comes from AlgorithmParameters 

    public int[][] getTimetable() {
        return timetable;
    }

    public void setTimetable(int[][] timetable) {
        this.timetable = timetable;
    }
}

How can I do that?

Answer 1

It actually is copying the value – but you need to understand what that value is.

The value of offspringScheduleOne[0] isn’t a TimeSchedule object. It’s a reference to a TimeSchedule object. No expression in Java has a value which is an object. It’s really important that you understand this.

Now, if you want a copy of the object, you’ll have to make that happen yourself. For example, you could include a clone() method in TimeSchedule, and write:

offspringScheduleOne[i] = genes.get(geneOneIndex).getSchedule()[i].clone();

In other words, create a clone of the existing object, and then set offspringScheduleOne[i] to be a reference to that newly created object. Of course, if any of the fields within TimeSchedule is a reference type field, you’ll need to consider whether or not you need to clone that object as well…

… or you could add a constructor and call that, or another method, etc. But you need to be absolutely clear that the assignment operator is copying the value, but that value is a reference.

EDIT: Okay, now that you’ve posted TimeSchedule, a few suggestions:

• Stop using public fields. What’s the point of having properties if the field is public?

• Rather than having properties returning the whole array, change them to access an individual hour, e.g.

  public int getTimetable(int day, int hour) {
      // TBD: Argument validation
      return timetable[day][hour];
  }
  
  // Similar for `setTimetable`
  

• Create a clone method like this:

  public TimeSchedule clone() {
      TimeSchedule copy = new TimeSchedule();
      for (int i = 0; i < timetable.length; i++) {
          copy.timetable[i] = timetable[i].clone();
      }
      return copy;
  }
  

  (That’s slightly wasteful in that it will create the subarrays and then discard them, but let’s get something which works first…)
  }

  public Test clone() {
  int[][] timetableCopy = new int[timetable.length][];
  for (int i = 0; i < timetable.length; i++) {
  timetableCopy[i] = timetable[i].clone();
  }
  return null;
  }