import javax.swing.*;
import javax.swing.text.*;
import java.awt.event.*;
import java.awt.*;
import javax.swing.JOptionPane;
public class B
{  public static void main(String args[])
    { new Win();
    }
}
class DigitDocumnet extends PlainDocument
{  int count=1;
   public void insertString(int offset,String s,AttributeSet a)
    { char c=s.charAt(0);
      if(c<='9'&&c>='0'&&count<=10)
        {  try  {  super.insertString(offset,s,a);
                   count++;
                }
           catch(BadLocationException e){}
        }
    }
  public void remove(int offset,int length)
    { try
         { super.remove(offset,length);
           count--;
         }
      catch(BadLocationException e){}
    }

}
class Win extends JFrame implements ActionListener
{   JTextField text,text1;
    JLabel jlabel1,jlabel2;
    DigitDocumnet document;
    public Win()
    { setBounds(100,100,300,200);
      setVisible(true);
      text=new JTextField(20);
      text1=new JTextField(20);
      Container con=getContentPane();
      con.setLayout(new FlowLayout());
      jlabel1=new JLabel("学号");
      jlabel2=new JLabel("姓名");
      con.add(jlabel1);
      document=new DigitDocumnet();
      text.setDocument(document);
      con.add(text);
      con.add(jlabel2);
      con.add(text1);
      text1.addActionListener(this);
      text.requestFocus();
      con.validate();
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    public void actionPerformed(ActionEvent e)
    {   
        if(e.getSource()==text1)
        {
          if((text.getText()).equals("1234567890")&&(texe1.gettext()).equals("wangde"))
            JOptionPane.showMessageDialog(this,"登陆",JOptionPane.WARNING_MESSAGE);
          else if
            JOptionPane.showMessageDialog(this,"用户名或密码错误",JOptionPane.WARNING_MESSAGE);
         
        }   
    }
}

public class B
{  public static void main(String args[])
    { new Win();// 你没有先声明后使用
    }
}

1.有对话框出现没
2.Jframe窗体声明，引用没。
3.鼠标事件点击有反应没
4.类中的方法位置是不是先声明后引用呢。