java学习日记（1）

public void paint(Graphics g){
oldColor=g.getColor();
g.setColor(Color.red);
g.fillOval(x, y, 30, 30);
g.setColor(oldColor);
y+=5;
}
public void update(Graphics g) {
if (offImage==null){
offImage= this.createImage(800,600);
}

Graphics gOffscreen=offImage.getGraphics();

gOffscreen.setColor(Color.green);
gOffscreen.fillRect(0, 0, 800, 600);

paint(gOffscreen);
g.drawImage(offImage, 0, 0, null);

}