On ActionEvents, listeners, and timers

Question

I am new to Java and am working on what looks like a standard intro project that creates several (<10) timers that can each be started and stopped independently. Once started, they actively count down from their initial start time once per second with a display update blah blah.

My current setup is working and appears to meet the project's requirements. I'm wondering about the general layout and if there's a generally more clever way to approach.

Currently, I have:

• one listener that waits for any of the timer buttons to be pressed. That includes "Start", "Stop," "Reset," and "Enter New Time". For X timers this means listening for 4X buttons. This is all caught in a single class with several if event.getSource == (thisbutton) statements.
• one listener PER TIMER that executes its "decrease time and update display" commands once per second. So all of them count down independently, depending on when the user presses Start.

My textbook gave an example of assigning one listener to see which button is pressed instead of making one listener per button, so I took that approach here. Any general advice you'd like to offer would be welcomed.

Answer 1

Several questions you need to ask your self...

How scalable is it? What are the performance considerations?

How scalable is it?

If you take a look at your current approach, you have a single ActionListener monitoring ALL the buttons. What happens if you need to increase the number of Timers to 20, 100, 1000? That's a lot of extra work and a very large if statement.

Now, I'm particular about maintaining levels of responsibility and having had to maintain a multi-level if statements within an ActionListener before, really dislike this approach...

Instead, you could put everything (the buttons for example) in some kind of List and loop over the list when the actionPerformed method is called, but even that becomes difficult to manage and may not meet the requirements of your design in the long run. You may or can not allow the ActionListener to know about your buttons or you can't access the List due to reference issues, etc...

Personally, I would create two classes, a StartActionListener and a StopActionListener. These would take a reference to (at least) a Timer.

This would mean, to add a new start button, you would simply need to create a new JButton and create a new instance of StartActionListener and register to the button. You would do a similar thing for the stop button, registering an instance of StopActionListener with it.

This divorce the listener design from the application design, as the listeners won't actually need to care about the buttons at all, only the Timer they are managing. It means that each listener is responsible for just one thing and provides a level of abstraction to the design, giving you freedom to choose how to actually implement the overall design/requirements.

You may also wish to take a look at How to use Actions which provides with the ability to provide configuration details for the action, making even easier to create controls.

What are the performance considerations?

Having multiple Timers running may not scale well and will introduce performance issues as you increase their numbers, to the point where the UI updates begin to suffer.

It might be better to use a single Timer which "ticks" at a regular interval and instead of creating a new Timer, you would simply register a ActionListener to a single Timer.

This would possibly mean that the ActionListener would need to maintain know when it was registered so it can determine the amount of time that has passed for each "tick", but this would a consideration of the context of the problem you are trying to solve. Equally, you could simply keep some kind of counter per ActionListener.