Starting out with Java2D/AWT

Newbie & Debugging Questions
#1

Hi,
I am trying to improve my knowledge of Java2D, and I started with a trivial example that for now is just drawing a line on a black background and exits when the user presses escape.

The code can be found here.

Yet even with this simple exaple I’m getting some strange problems:

  1. It doesn’t react on every key press - I normally have to press quite a few time until it registers a keypress. I’ve run it through a debugger and it indeed only enters the processKeyEvent function after several key-presses.
    2. The window is bigger than the picture, even though they are both defined to be the same size. this problem was because of a silly typo :frowning:

Any idea what I’ve done wrong?

-Ido.

#2

I’ve made several mistakes in that example, but I think I’ve now corrected all of them, tell me what you think:
New version.

#3

Usually I just add a key listener to the jframe and then override the three functions, the one for escaping the app is usually keypressed. Or perhaps even easier…

public class Example extends JFrame implements KeyListener{

Not sure I’ve seen it set up as you have.

check out… http://java.sun.com/docs/books/tutorial/uiswing/events/keylistener.html

public void keyTyped(KeyEvent e) {
    displayInfo(e, "KEY TYPED: ");
}

/** Handle the key pressed event from the text field. */
public void keyPressed(KeyEvent e) {
    displayInfo(e, "KEY PRESSED: ");
}

/** Handle the key released event from the text field. */
public void keyReleased(KeyEvent e) {
    displayInfo(e, "KEY RELEASED: ");
}
#4

As far as I know the 2 are the same, at least that’s what the documentation says.

My current code works well now.

#5

Yep, it’s the same, you’re doing it “the old way” which is fine as well. Listeners can be very handy though and it’s easy to implement. It also prohibits a lot of issues one normally can make while treating the events directly.

If you come from a C++ background it’ll take some getting used to but once you do it’ll be really easy.

#6

Imagine you have two threads:

  1. AWT Thread (which deals with mouse and key events) and
  2. Main Thread.

And imagine that this is what happens when you run your program:

Main Thread: new Example().mainLoop();
Main Thread: mainLoop();
Main Thread: while( !keys[KevEvent.VK_ESCAPE] ) { … }; // keys[KevEvent.VK_ESCAPE] is false, so continue looping.
Main Thread: while( !keys[KevEvent.VK_ESCAPE] ) { … }; // keys[KevEvent.VK_ESCAPE] is false, so continue looping.
Main Thread: while( !keys[KevEvent.VK_ESCAPE] ) { … }; // keys[KevEvent.VK_ESCAPE] is false, so continue looping.

Now imagine that the escape key is pressed:
AWT Thread: processKeyEvent(KeyEvent e) -> e.getID() is KeyEvent.KEY_PRESSED -> keys[KevEvent.VK_ESCAPE] is now true.
AWT Thread: processKeyEvent(KeyEvent e) -> e.getID() is KeyEvent.KEY_RELEASED -> keys[KevEvent.VK_ESCAPE] is now false.

Main Thread: while( !keys[KevEvent.VK_ESCAPE] ) { … }; // keys[KevEvent.VK_ESCAPE] is false, so continue looping.
Main Thread: while( !keys[KevEvent.VK_ESCAPE] ) { … }; // keys[KevEvent.VK_ESCAPE] is false, so continue looping.
Main Thread: while( !keys[KevEvent.VK_ESCAPE] ) { … }; // keys[KevEvent.VK_ESCAPE] is false, so continue looping.

Basically I am saying the code you have might have unpredictable results due to threading issues.

After a KEY_PRESSED event, IF mainLoop() is not called before the subsequent KEY_RELEASED event then mainLoop() will never see keys[KevEvent.VK_ESCAPE] as true.