public class tt extends JPanel {
    private JTextField txtName = new JTextField();
    private JButton jButton1 = new JButton();
    private JTextField jTextField1 = new JTextField();

    public tt() {
        try {
            jbInit();
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
    private void jbInit() throws Exception {
        txtName.setText("jTextField1");
        jButton1.setText("jButton1");
        jTextField1.setText("jTextField1");
        this.add(txtName, null);
        this.add(jButton1, null);
        this.add(jTextField1, null);

        txtName.setInputVerifier(new SellerVerifier());
    }
    public static void main(String[] args) {
        JFrame window = new JFrame("test");
        tt tt1 = new tt();
        window.getContentPane().add("Center", tt1);
        window.setDefaultCloseOperation(window.EXIT_ON_CLOSE);
        window.setSize(300,200);
        window.setVisible(true);
    }

    class SellerVerifier extends InputVerifier {
      public boolean verify(JComponent input){
        JTextField si = (JTextField)input;
        String str = si.getText();
        if (str.trim().equalsIgnoreCase("")){
          JOptionPane.showMessageDialog(null, "输入不能为空!", "ViTop", JOptionPane.ERROR_MESSAGE);
          return false;
        }
        return true;
      }
    }
}

class SellerVerifier extends InputVerifier { 

     private boolean tempFocusLost = false;

     public boolean verify(JComponent input){ 

      if(tempFocusLost){//for jdk1.4
        tempFocusLost = false;
        return true;
      }

       JTextField si = (JTextField)input; 
       String str = si.getText();  
      if (str.trim().equalsIgnoreCase("")){

            tempFocusLost = true;

            JOptionPane.showMessageDialog(null, "输入不能为空!", "ViTop", JOptionPane.ERROR_MESSAGE);     
           return false;
      }   
      return true;
    }  
  }