JFrame 突然找不到符号

emptyer3011
JFrame 突然找不到符号

代码是这样的:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/** Tiny example showing the main difference in using
*  JFrame instead of Frame: using the content pane
*  and getting the Java (Metal) look and feel.
*  1998-99 Marty Hall, [url]http://www.apl.jhu.edu/~hall/java/[/url]
*/

public class JFrameExample {
  public static void main(String[] args) {
    WindowUtilities.setNativeLookAndFeel();
    JFrame f = new JFrame("This is a test");
    f.setSize(400, 150);
    Container content = f.getContentPane();
    content.setBackground(Color.white);
    content.setLayout(new FlowLayout());
    content.add(new JButton("Button 1"));
    content.add(new JButton("Button 2"));
    content.add(new JButton("Button 3"));
    f.addWindowListener(new ExitListener());
    f.setVisible(true);
  }
}

这段代码昨天编译还好的,今天就怎么都通不过,报错就说找不到jframe这个类,请各位大侠帮忙~!