我的代码，Eclipse提示有错，但是却能运行，关闭程序效果也有，我是新手，cls类哪里提示有错

import java.awt.*;
import java.awt.event.*;

public class Gridbagframe {
static Frame frame=new Frame("网格布局窗体");
static Button btn1;
static Button btn2;
static Button btn3;

public static void main(String args[]){
frame.setLayout(new GridBagLayout());
GridBagConstraints gbcs=new GridBagConstraints();
btn1=new Button("a");
btn2=new Button("b");
btn3=new Button("c");
gbcs.gridx=0;
gbcs.gridy=0;
gbcs.gridwidth=1;
gbcs.gridheight=1;
frame.add(btn1,gbcs);
frame.add(btn2,gbcs);
gbcs.gridx+=1;
frame.add(btn3,gbcs);
gbcs.gridx+=2;
frame.setVisible(true);
frame.setSize(200,200);
frame.addWindowListener(new cls());
}
}
class cls extends WindowAdapter{
public void windowClosing(WindowEvent e){
System.exit(0);
}
}