java编写数据库查询执行器的问题？求帮忙看下

代码如下，调试了很久，没调好，帮忙看下，，，JAVA初学者，，谢谢了！明天要交这个作业，急死了，，，package quaryExcutor;
import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;//用于在已经建立数据库连接的基础上，向数据库发送要执行的SQL语句。
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class go extends JFrame 
{
private static final long serialVersionUID = 1L;
private JLabel l1;
private JLabel l2;
private JTextField sql;
private JButton ex;
private JFrame jf;public go()
{
   jf.setTitle("SQL查询执行器");
   jf.setSize(500, 150);
   jf.setVisible(true);
   jf.setLocationRelativeTo(null);///使窗口显示到中间
   jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//使用 System exit 方法退出应用程序  
  
   l1=new JLabel("请输入要执行的SQL语句：");
   l2=new JLabel("执行结果");
   sql=new JTextField();
   sql.addKeyListener(new KeyAdapter()
       {
                           public void keyPressed(KeyEvent e) 
       {
                            if(e.getKeyCode()==KeyEvent.VK_ENTER)
       {
                            
        } 
        }
        });
   ex=new JButton("执行");
   ex.addActionListener(new exlistener());
   JPanel inputpanel=new JPanel();
   JPanel buttonpanel=new JPanel();
   inputpanel.setLayout(new BoxLayout(inputpanel,BoxLayout.Y_AXIS));//垂直排列
   buttonpanel.setLayout(new FlowLayout());//居中对齐
  
   inputpanel.add(l1);
   inputpanel.add(sql);
   inputpanel.add(l2);
  
   buttonpanel.add(ex);
  
   this.getContentPane().add(inputpanel,BorderLayout.NORTH);
   this.getContentPane().add(buttonpanel,BorderLayout.SOUTH);
}
}
   try {
   
    Connection con=DriverManager.getConnection("user/db/schema");
         Statement stmt=con.createStatement();
         String SQL=sql.getText();
         stmt.execute(SQL);
         l2.setText("执行成功！");//将l2上面的文字设置为"执行成功！"
                                                                                                                  
   } catch (SQLException e) {
    JOptionPane.showMessageDialog(null, "SQL语句输入有误或者该操作非法！","警告",JOptionPane.WARNING_MESSAGE);
    l2.setText("无法执行！");
    e.printStackTrace();
  
   }
   
  
}public class exlistener implements ActionListener{
  
   public void actionPerformed(ActionEvent ev) {
    //excute();
   
   }
  
}
}