“Buffers have not been created” … whilst creating buffers

Question

I have (what I thought was) a straightforward BufferStrategy for a JFrame. It is created like so:

    // Buffer
    container.createBufferStrategy(2);           
    strategy = container.getBufferStrategy();

However, occassionally I receive the following error (which points to the first line of the preceeding snippet as the offending item) :

java.lang.IllegalStateException: Buffers have not been created

This error is peculiar as it comes and goes - sometimes it is triggered, sometimes not. I suspect this means it's a threading issue. Does anyone have any pointers as to what might be going on here? I'm a little at a loss, since I'm already trying to do what Java says it wants me to do!

edit: full trace:

Exception in thread "main" java.lang.IllegalStateException: Buffers have not been created
        at sun.awt.windows.WComponentPeer.getBackBuffer(WComponentPeer.java:877)
        at java.awt.Component$FlipBufferStrategy.getBackBuffer(Component.java:3815)
        at java.awt.Component$FlipBufferStrategy.updateInternalBuffers(Component.java:3800)
        at java.awt.Component$FlipBufferStrategy.createBuffers(Component.java:3791)
        at java.awt.Component$FlipBufferStrategy.<init>(Component.java:3730)
        at java.awt.Component$FlipSubRegionBufferStrategy.<init>(Component.java:4253)
        at java.awt.Component.createBufferStrategy(Component.java:3612)
        at java.awt.Window.createBufferStrategy(Window.java:3015)
        at java.awt.Component.createBufferStrategy(Component.java:3536)
        at java.awt.Window.createBufferStrategy(Window.java:2990)

Answer 1

The frame needs to be displayable when you call createBufferStrategy . Also as camickr has pointed out you need to call it from the EDT.

One way to ensure this is to extend JFrame and override addNotify :

class MyFrame extends JFrame {
    public void addNotify() {
        super.addNotify();
        // Buffer
        createBufferStrategy(2);           
        strategy = getBufferStrategy();
    }
}

Answer 2

Swing components are double buffered by default, so there is no need to play around with a BufferStrategy.

However when you get random errors like that its usually because code is not executed on the EDT. Read the section from the Swing tutorial on Concurrency for more information.