import java.awt.*;
import java.awt.event.*;
class MyBtton extends Button implements ActionListener
{ int x=10,y=10,i=0;
Color color[]={Color.red,Color.yellow,Color.green};
Color c=color[0];
MyBtton()
{ setSize(38,85);
setBackground(Color.cyan);
addActionListener(this);
}
public void paint(Graphics g)
{ g.setColor(c);
g.drawOval(x, y, 20, 20);
}
public void update(Graphics g)
{ g.clearRect(x, y, 20, 20);
//paint(g) ;
}
public void actionPerformed(ActionEvent e)
{ i=(i+1)%3;
c=color[i];
y=y+23;
if(y>56)
y=10;
repaint();
}
}
class WindowCanvas extends Frame
{ WindowCanvas()
{ MyBtton button=new MyBtton();
setLayout(null);
add(button);
button.setLocation(30,30);
setBounds(60,125,100,200);
setVisible(true);
validate();
addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
}
}
public class PaintExa {
/**
* @param args
*/
public static void main(String[] args) {
new WindowCanvas();
// TODO Auto-generated method stub
}
}
上面的程序是实现交通指示灯的样式public void update(Graphics g)
{ g.clearRect(x, y, 20, 20);
paint(g) ;
}
repaint();
我把这三句删掉，程序任能实现该功能，这是因为事件处理之后，自动调用父类的方法吗？还有public void paint()重写父类方法怎么不调用就能运行？？
------解决方案--------------------
多态基础
请翻书...