Flickering while revalidating JPanel

Question

I have looked all round, and no one seems to have an answer for me. I have a JPanel where I add/remove labels from it while it;s running. The problem is that every time I add a new label to it I have to revalidate() it so the new label shows up. Every time I revalidate(), it I get a very annoying flicker, and all my labels ghost up to the top of my window for a millisecond then return to what they were doing. The only time I call revalidate() is after I add a label; I do not change the paint method or anything only did

public void paintComponent(Graphics page)
{
    super.paintComponent(page);
}

so that my page would not look bad. I could really use some help on this. I've made a asteroids game using JFrame, Jlabel and Jpanel game; it work great, but this flickering and component "ghosting quickly ghosting to top of panel" is just unbearable, and I know for a fact this is the revalidate() method.

for information, i have a label that represents a ship, bullets, and asteroids, i add and remove them to my panel as they are used shot or run out of life. using the add(component); and remove(conponent); then after i add i revalidate. — Elton, Feb 16 '13 at 10:30

score 2 · Answer 1 · answered Feb 16 '13 at 10:51

When I was having problems with filckering Swing components it turned out to be concurrency issues. Since you are doing a game, which means you are doing animation it may be the case here.

Take care to modify Swing components only from AWT-EVENT thread. Either use SwingUtilities.invokeLater or do your animation using javax.swing.Timer.

score 2 · Answer 2 · edited May 23 '17 at 10:32

As a concrete example, Asteroids doesn't flicker at all. Here are a few reasons:

It runs on the event dispatch thread.
It uses JPanel, which is double buffered by default.
It paces the animation using javax.swing.Timer, updating the game's model in actionPerformed().
It overrides paintComponent(), but can safely omit super.paintComponent() because the action listener repaints the entire panel.
The listener calls paintImmediately() to avoid any potential delay; substitute repaint() to see if there's any difference on your platform.

Addendum: As shown in this counter-example, revalidate() does not cause flickering. Instead of replacing labels, update the label's icon, as shown here.

score 2 · Answer 3 · answered Feb 16 '13 at 14:37

2

Have you considered reusing them - i.e. using setVisible(false) when they're not needed?

Not sure if adding and removing labels is the best way of achieving what you want for a game. I'd personally be tempted to manage the drawing myself.

answered Feb 16 '13 at 14:37

Sanjay Manohar

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i think im going to try this, i am still amateur coder so this seems simplest to me, thanks for the feed back guys. – Elton Feb 16 '13 at 19:00

Durandal · Accepted Answer · 2013-02-18T13:39:29.840

Using labels (or generally GUI components) to represent quickly animating, dynamically created/moved objects is not practical. Its like using a car to drive from the living room to the kitchen.

The Swing components come with a lot of nice features, e.g. they can determine the space they require and a layout manager can use that to automatically place them nicely. Unfortunately for a game, you neither need all the nice stuff, nor do you want to pay the processing overhead involved with it. Matters are further complicated by swings multi-platform capabilities which mandate that each component has a peer component (the peer deals with rendering the component to the active look).

To solve this, you do not represent individual game objects as swing components. Instead you use a single swing component (usually JPanel) as a canvas to paint your game objects on to. You simply call repaint() for that JPanel periodically (e.g. 30 or 60 times a second). Of course you will override paintComponent() on the JPanel to do paint all the game objects using your own code.

Edit: To outline how you do your own rendering

You need to decide how to organize your game related objects. A very simple skeleton could look like this (getters/setters omitted):

public class GameObject {
    public int x;
    public int y;
    public Image image;
}

In practice this class will have additional members (like a behavior that controls how the object moves, and maybe some states/counters for animation). It may also be abstract with a concrete subclass for each possible behavior (like ship, asteroid and bullet). All instances of GameObject are managed in some kind of collection, I assume you use a List.

When implementing the paintComponent it comes down to just loop through the list and render each objects's image:

public void paintComponent(Graphics g) {
    List<GameObject> list = ... // however you obtain it in your game
    for (GameObject gobject : list) {
        g.drawImage(gobject.image, gobject.x, gobject.y, (ImageObserver) null);
    }
}

More elaborate games may order the gameobjects to get control of how game objects overlap each other. But in principle its incredibly simple.

the only problem which i dont know how to get around because im a noob is that each thing i.e bullet, asteroid and ship, there are a good amount of things i dont know how to represent as graphics. — Elton, Feb 16 '13 at 19:03
@Elton I added some brief description how you create your own game objects and render them — Durandal, Feb 18 '13 at 13:40

Flickering while revalidating JPanel

4 Answers4