Sharing an object between other objects?

Question

I have two classes, say DisplayPanel and ControlPanel. Both of these classes need to have access to a single instance of an object of type Data, say theData. I want to be able to set theData to a new object at any time, for example, theData = new KindOfData().
If I store a reference to theData in both objects, when I set theData to a new object in one class, the other will still reference the old object. Is there a way to share an object between two classes as I described?
I could create a superclass for DisplayPanel and ControlPanel that contains an instance of Data, however I may need to access this object in other classes and do not want to have the inflexibility of having to extend a possibly unrelated class just to access a single data member.
Is there any easy way to go about this? Thanks for your help.

Answer 1

Create a listener interface. Make each class implement that interface. When you create each class, register it as a listener to theData. Keep track of all listeners in a List such as an ArrayList. When you want to change theData, iterate through the List and call the interface's method to update theData in each class:
public interface SomeListener
   void updateData(KindOfData theData);
public class DisplayPanel implements SomeListener
  private KindOfData theData;
  public void updateData(KindOfData newData)
     theData = newData;
public class SomeOtherClass
   private List listenerList = new ArrayList();
   public void addDataListener(SomeListener listener)
        listenerList.add(listener);
   public void fireDataChange(KindOfData newData)
       for (Iterator listenerIter = listenerList.iterator();
              listenerIter.hasNext();
           ((DataListener)listenerIter.next()).updateData(newData);
}Look at how EventListeners are added and fired in Swing classes.