为什么我的图形代码不会运行，除非代码块中有System.out.println？

Question

I have this method paint() which receive a Graphics2D parameter. The weird thing that happen is that unless there is a System.out.println present(which i comment out in the block below), the canvas will not draw anything.

public class Map{

    public void paint(Graphics2D g){

        //fill background to black
        g.setColor(Color.black);
        g.fillRect(0, 0, TILE_SIZE*WIDTH, TILE_SIZE*HEIGHT);

        //draw the tiles and buildings

        for(int i=0;i<WIDTH;i++){
            for(int j=0;j<HEIGHT;j++){
                if(map[j][i] == CLEAR){
                    //System.out.println("");
                    g.setColor(Color.gray);
                    g.fillRect(i*TILE_SIZE, j*TILE_SIZE, TILE_SIZE, TILE_SIZE);
                    g.setColor(Color.red);
                    g.drawRect(i*TILE_SIZE, j*TILE_SIZE, TILE_SIZE, TILE_SIZE);

                }
            }
        }
    }
}

Here I use BufferStrategy to draw on Canvas and add it to a Frame. This method is in class Map which will be passed a Graphics2D from the getDrawGraphics() method from BufferStrategy(I hope many people are familiar with this stuff to understand what I'm doing).

public class MapTest extends Canvas{

    private Map map;

    public MapTest(){

        Frame frame = new Frame("MAP");
        frame.add(this);
        frame.setVisible(true);

        createBufferStrategy(2);
        strategy = getBufferStrategy();

        //draw the map

        Graphics2D g = (Graphics2D) strategy.getDrawGraphics();
        //g.translate(100, 100);
        map.paint(g);

        g.dispose();
        strategy.show();

    }
}

This code is from the Canvas class. As you can see the paint() method is separate from the Canvas class(which I name GameTest). So if I comment out the println statement then no graphics is shown in the canvas, otherwise it is displayed correctly. Anyone can help me???

Answer 1

You should use the SwingUtilities to switch to the Event Dispatch Thread(EDT), see below. This is required for almost all interactions with AWT and Swing classes.

  SwingUtilities.invokeLater(new Runnable(){
      public void run(){
          new MapTest();
      }    
   }

Notice that this uses a swing helper library, that should be fine for AWT, but even better is to start using Swing.