不断检查布尔值

Question

I have 2 classes. The first class turns a boolean value to true when you click on a specific location, the second class calls frame.removeAll() and frame.add(new CanvasSelect()) If the boolean in the first class is true. It seems to only check the boolean value once.

Is there any way to make the second class constantly check the boolean value in the first class?

Answer 1

As suggested in the comments, you really need to post (a stripped-down version of) the code if you really want to know what's going on. I can think of three possible reasons this might be happening.

Possibility 1: You need to mark your boolean value as volatile . If there are multiple threads in play here then one of the threads might be seeing a stale value if the field is not marked as a volatile field. However, it sounds like you're doing GUI programming (probably with Swing), so all these events should be happening in the GUI event thread—therefore I doubt this is the problem. It still might be worth a try though.

Possibility 2: You are setting the event handler based on the boolean value rather than checking the boolean value within the event handler. In this case the value would only be read once (when the ActionEventHandler is set) rather than at the time of the click event.

Possibility 3: When you clear the frame you're clearing the items with the event handlers too, and you're not resetting them properly. I think this is the most likely problem.

Answer 2

Using busy waiting (as suggested in the other answer) in any form is a really bad idea, it's a programming anti-pattern (it should be used only under very special circumstances, like low-level CPU programming, and certainly never in Java):

1. You just consume all available power of one CPU for nothing.
2. With sleep ing it's better, but then your code will react to a change with a delay. And the shorter delay you make, the more CPU power you waste. Still, it's a bad programming pattern.

Without a code snippet or a more detailed explanatin it's not clear what exactly you want to achieve. Some general suggestions:

• If you want to perform some action when you receive user's input, you could simply do that in your event handler without using any flag or variable. However, this is possible only if your action is very short, otherwise you'll block the event dispatching thread .
• If the action you need to perform takes even just a little bit longer, you should start a new thread from your event handler, which will perform the action and when it finishes, updates the GUI. The most direct way is to use SwingWorker . Better read the whole chapter Concurrency in Swing to see all the possibilities.
• If you need to communicate between two independent threads in general, like if you need one thread to wait until some flag changes, use one of Java's concurrency mechanisms instead of busy waiting. The most direct way how to replace busy waiting is discussed in Guarded Blocks .

Answer 3

you can use code snippets like give below and put them into a thread which always run parallel to your main code

while(true)
{
  if(_val)  //_val is your boolean variable
  {
      // your logic
  }
  Thread.sleep(100);

}

Or an event can be raised on the click of that particular control which checks value of _val and performs actions