Best Practice: Where to put Listeners?

Question

In a Swing application the typical way of handling events is to add a listener. So far, so good.
But where to locate the listeners? Obviously there are a lot of possible solutions:
- Scatter them all over your code, like using anonymous listeners.
- Implement all of them in a single, explicit class.
- Only uses windows as listeners.
- etc.
The intention of my question is not to get a rather long list of more ideas, or to get pros or cons of any of the above suggestion. My actual question is: Is there a best practice for where to locate a listener's implementation? I mean, after decades of Swing and thousands of Swing based applications, I am sure that there must be a best practice where to but listeners implementations.

Answer 1

mkarg wrote:
In a Swing application the typical way of handling events is to add a listener. So far, so good.
But where to locate the listeners? Obviously there are a lot of possible solutions:
- Scatter them all over your code, like using anonymous listeners.
- Implement all of them in a single, explicit class.
- Only uses windows as listeners.
- etc.
The intention of my question is not to get a rather long list of more ideas, or to get pros or cons of any of the above suggestion. My actual question is: Is there a best practice for where to locate a listener's implementation? I mean, after decades of Swing and thousands of Swing based applications, I am sure that there must be a best practice where to but listeners implementations.You've asked other similar questions about best practices. No matter how long Swing has been around, people still program in a variety of ways, and there are lots of areas where there are several equally correct ways of doing things. Each way has its pros and cons, and the specific situation drives towards one way or the other. One's best practice of using anonymous listeners will be another's code smell. One's best practice of using inner class will be another's hassle.
So you will probably only get opinions, and likely not universally recognized best practices.
That being said, here is my opinion (nothingmore than that, but it has a high value to me :o) :
In yous list of options, one thing that is more likely to form a consensus against it, is "only uses window as listeners". I assume you mean each frame is implemented as a MyCustomFrame extends JFrame , and add this as listener on all contained widgets.
This option is disregarded because
1) extending JFrame is generally not a meaningful use of inheritance (that point is open to debate, as it is quite handy)
2) register the same object to serve as a listener for several widgets makes the implementation of listener callbacks awkward (lots of if-then-else). See [that thread|http://forums.sun.com/thread.jspa?forumID=57&threadID=5395604] for more arguments.
Now, no matter what style of listeners you choose, your listeners shouldn't do too much work (how much is too much is also open to debate...):
if a listener gets complicated, you should simplify it by making it a simple relay, that transform low-level graphical events into functional events to be processed by a higher-level class (+Controller+). I find the Mediator pattern to be a best practice for "more-than-3-widgets" interactions. As the interactions usually involves also calls to the application model, the mediator becomes a controller.
With that in mind, sort anonymous listeners are fine: the heavy work will be performed by the mediator or controller, and that latter is where maintenance will occur. So "Scatter them all over your code" (sounds quite pejorative) is not much of an issue: you have to hook the widget to the behavior somewhere anyway, the shorter, the best.
For simpler behavior, see the previous reply which gives perfect advice.