一个小问题，哪里出错了

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class Exam_9 extends Applet implements ActionListener
{
  Button left,center,right;
  public void init()
  {
    left=new Button("左对齐");
left.addActionListener(this);
add(left);
center=new Button("居中");
center.addActionListener(this);
add(center);
right=new Button("右对齐");
right.addActionListener(this);
add(right);
  }
  public void actionPerformed(ActionEvent e)
  {
    int align=FlowLayout.LEFT;
if(e.getSource==left)
  align=FlowLayout.LEFT;
else if(e.getSource==right)
   align=FlowLayout.RIGHT;
else if(e.getSource==center)
  align=FlowLayout.CENTER;
setLayout(new FlowLayout(align));
validate();
  }
  }