Question

Not sure how to title this…

So I’ve got three child classes of Event: WeightEvent, TimedEvent, RepEvent. Through whatever means, I get an object of one of the children. Now I want to send that child event to a method in another object so it can pull the data from it with the getSavedEvents() method. The method only exists in the children since pulling the data is specific to the type of event.

I started with

public void setEvent(Event e) {

but that cast my child object to an Event (parent) object.

Is there any way around this short of writing three different methods. One each for the children?

public void setEvent(WeightEvent e) {
public void setEvent(TimedEvent e) {
public void setEvent(RepEvent e) {

Thanks for any advice.

-John

Answer 1

Instead of switching on the type you should call a method on the event that’s defined differently for each type of event type. This is called the Template method pattern. (It has nothing to do with C++ templates, BTW)

Using this pattern, your EventTable class becomes something like this:

public class EventTable {
  public void setEvent(Event e) {
    int x = 0;
    columns = e.getFields();
    Event[] savedEvents = e.getSavedEvents();
    for(Event ev : savedEvents) {
      tempdata[x] = ev.getTempData();
      x++;
    }
  }
}

Note that the entire switch has been replaced with a single call to getTempData(). This method is then abstract in Event, just like getSavedEvents:

public abstract class Event {
  public Date getDate() { return(_date); }
  public abstract Event[] getSavedEvents();
  public abstract int[] getTempData();
  public int[] getFormattedDate() {
    ...

}

Then you define the getTempData() method in each subclass. For example:

public class WeightEvent extends Event {
  public int getWeight() { return(_weight); }
  public int getReps() { return(_reps); }
  public int[] getTempData() {
    return new int[]{
      getFormattedDate()[0],
      getWeight(),
      getReps()
    };
  }
}

public class TimedEvent extends Event {
  public String getTimeInHMS() { return(_timeString); }
  public int[] getTempData() {
    return new int[]{
      getFormattedDate()[0],
      getTimeInHMS()
    };
  }
}

public class RepEvent extends Event {
  public int getReps() { return(_reps); }
  public int[] getTempData() {
    return new int[]{
      getFormattedDate()[0],
      getReps()
    };
  }
}