java实现画图和清屏功能.

import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;/**
* @author Administrator
*
*/
@SuppressWarnings("serial")
public class DrawTest extends JFrame { final DrawPanel pnlImg = new DrawPanel();

public DrawTest() {
init();
}

private void init() {
setLayout(new GridLayout(2, 1));
add(pnlImg);
final JButton btnDraw = new JButton("画图");
btnDraw.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
pnlImg.setDrawRadius(200);
pnlImg.setDrawOval(true);
}
});
final JButton btnClear = new JButton("清屏");
btnClear.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
pnlImg.setDrawOval(false);
}
});
final JPanel pnlButton = new JPanel(new GridLayout(1, 2));
pnlButton.add(btnDraw);
pnlButton.add(btnClear);
add(pnlButton);
setSize(800, 600);
setLocationRelativeTo(null);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public static void main(String [] args) throws Exception {
javax.swing.SwingUtilities.invokeAndWait(new Runnable() {
public void run() {
new DrawTest().setVisible(true);
}
});
}

class DrawPanel extends JPanel {

boolean flag;
int radius = 100;

public void setDrawOval(boolean flag) {
this.flag = flag;
repaint();
}

public void setDrawRadius(int radius) {
this.radius = radius;
}

public void paintComponent(Graphics g){
super.paintComponent(g);
Rectangle rect = getBounds();
if (flag && rect.width > radius && rect.height > radius) {
g.drawOval((rect.width - radius) / 2, (rect.height - radius) / 2, radius, radius);
}
}
}
}

清屏很简单的，只要用背景色画一个和屏一样大的矩形就可以了。

另一方法： 清屏的办法在AWT和swing中可以这样实现:g.clearRect(0, 0, width, height); g 就是 Graphics 对象