Button

import java.awt.*;
import javax.swing.*;
public class D extends JApplet {public void init() {
Container c=getContentPane();
FlowLayout flow=new FlowLayout();
flow.setAlignment(FlowLayout.LEFT);
flow.setHgap(20);
flow.setVgap(40);
c.setLayout(flow);
add(new Button("OK"));
for(int i=1;i<=12;i++)
{
c.add(new Button("i am "+i));
}
setBackground(Color.BLUE);
setVisible(true);
}
}
为什么这个程序的窗口中没有按钮？