Game loop - cause exception - graphics

Question

I have some problem. I am learning to program games and I want to try to program some game on my own. But I hit a snag just at beginning. I have 2 classes. The main class (call Frame) is only for create game frame:
public class Frame extends JFrame{
/**
*
*/
private static final long serialVersionUID = 1L;
Game game;
public Frame() {
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
game = new Game();
this.add(game.canvas);
this.pack();
this.setResizable(false);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new Frame().setVisible(true);;
}
});
}
}
It goes correctly. But I have created a second class (call Game), that is supposed to provide game loop:
public class Game implements Runnable{
public Canvas canvas;
Thread thread;
BufferStrategy bs;
Graphics g;
//Variables for game loop
private int fps, timePerTick;
double now, lastTime;
private double delta;
Game(){
initVariables();
canvas = new Canvas();
canvas.setPreferredSize(new Dimension(300, 300));
}
private void initVariables(){
//create and start thread
thread = new Thread(this);
thread.start();
}
private void tick(){
}
private void render(){
bs = canvas.getBufferStrategy();
if (bs == null){
canvas.createBufferStrategy(3);
return;
}
g = bs.getDrawGraphics();
g.drawRect(0, 0, 50, 50);
bs.show();
g.dispose();
}
private void initRun(){
delta = 0;
lastTime = System.nanoTime();
fps = 30;
timePerTick = 1000000000/fps;
}
#Override
public void run() {
initRun();
int timer=0;
int ticks=0;
while(true){
now = System.nanoTime();
delta += (now - lastTime) / timePerTick;
timer += (now - lastTime);
lastTime = now;
if(delta>=1){
tick();
render();
delta--;
ticks++;
}
if (timer>1000000000){
System.out.println("FPS: "+ticks);
timer = ticks = 0;
}
}
}
}
The problem was in value of variable call FPS. When I set FPS in method call initRun() to 10, everything goes correctly. (simply thanks to method run() program wrote "FPS: 60" every second, and in frame was draw a rectangle).
But when I set FPS to (for example) 30, there was the problem. There was exception call "Exception in thread "Thread-1" java.lang.IllegalStateException: Component must have a valid peer" in render() method, concretely at instruction "canvas.createBufferStrategy(3);".
(image of exception)
What can I do? Where is problem? I am not good in speaking english, so I don't understand this Exception and I don't know, what I can do...Please help me! Thanks.