Tracking document position changes

I am using the Document method createPosition to add Positions to an array. These Positions change depending on the user inserting/removing lines.

How do I hook changes in my array? I need to know the new value and the value which it has changed from.

Interface Position has no method to add listener to monitor position change. So you need do it manually using wrapper for Position object.

public class PositionHolder {
  private int oldPosition;
  private Position position;
  private PropertyChangeSupport propertySupport = new PropertyChangeSupport(this);
  public PositionHolder(Position aPos) {
    position = aPos;
    oldPosition = aPos.getOffset();
  }
  public void addPropertyChangeListener(PropertyChangeListener aListener) {
    propertySupport.addPropertyChangeListener(aListener);
  }
  // same for remove listener
  public void verifyPosition() {
    // no custom if statement required - event is fiered only when old != new
    propertySupport.firePropertyChangeEvent("position", oldPosition, position.getOffset());
    oldPosition = position.getOffset();
  }
  public Position getPosition() {
    return position;
  }
}

public class PositionUpdater implements DocumentListener {
  private List<PositionHolder> holders;
  public PositionUpdater(List<PositionHolder> someHolders) {
    holders = someHolders;
  }

  public void insertUpdate(DocumentEvent e) {
    for (PositionHolder h : holders) {
      h.verifyPosition();
    }
  }
  public void removeUpdate(DocumentEvent e) {
    insertUpdate(e);
  }
  public void changeUpdate(DocumentEvent e) {
    insertUpdate(e);
  }
}

JTextComponent textComp = // your text component
List<PositionHolder> holderList = new ArrayList<>(); // your list of positions
textComp.getDocument().addDocumentListener(new PositionUpdater(holderList));

Now you will get notification when position is changed. You only need to fill the holderList with wrappers for your positions and register your event listener on these wrappers.