Question about event handling in JComponents

Question

I have often found it useful to create a component that acts as an event handler for events the component generates itself. For example, a panel that listens for focus events that effect it and handle these events internally. (See below).
My question is: Can this practice cause synchronization issues or any other type of problem that I need to watch out for? Is it good/bad or neither. Are there any issues I should be aware of?
Thanks in advance for the help.
Example:
public class qPanel extends JPanel implements FocusListener
public qPanel () {
super.addFocusListener(this);
public void focusGained(FocusEvent e) {
//Do stuff
public void focusLost(FocusEvent e) {
//Do stuff
}

Answer 1

Hi,
Handling events this way is completely fine and saves on number of classes. Only thing you may want to watch out for is that the handler methods have to be public. This means that someone could use your component and call one of the methods. For example, I could write:
panel.focusGained(new FocusEvent(....))
when it's not really gaining focus. So, if you're writing this component for re-use you might want to be aware of this.
An alternative:
Use a single internal class to handle all events. It can then delegate to private methods of your component. Example:
class MyEventHandler implements FocusListener, MouseListener, etc... {
public void focusGained(FocusEvent fe) {
doFocusGained(fe);
public void mousePressed(MouseEvent me) {
doMousePressed(me);
Then your component could have:
private void doFocusGained(FocusEvent fe) {
private void doMousePressed(MouseEvent me) {
etc...
Just ideas :)
Thanks!
Shannon Hickey (Swing Team)
I have often found it useful to create a component
that acts as an event handler for events the component
generates itself. For example, a panel that listens
for focus events that effect it and handle these
events internally. (See below).
My question is: Can this practice cause
synchronization issues or any other type of problem
that I need to watch out for? Is it good/bad or
neither. Are there any issues I should be aware of?
Thanks in advance for the help.
Example:
public class qPanel extends JPanel implements
FocusListener
public qPanel () {
super.addFocusListener(this);
public void focusGained(FocusEvent e) {
//Do stuff
public void focusLost(FocusEvent e) {
//Do stuff