private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
//// Action submit button for main form submit//
       
    String user_name=jTextField1.getText();
        String pasword=jPasswordField1.getText();
    if(user_name==" " || pasword==" ")
{
  JOptionPane.showMessageDialog(null,"Invalid User Name or Password","Error",JOptionPane.ERROR_MESSAGE);
}

if you want file upload as like text file.you can see this code for file chooser

private JFileChooser fileChooser;

try
           {
            fileChooser = new JFileChooser();
            int result= fileChooser.showOpenDialog(this);                   
            if(result==JFileChooser.CANCEL_OPTION)
                 {
                   JOptionPane.showMessageDialog(null,"You Cancel it.","Warning",JOptionPane.ERROR_MESSAGE);
                 }
             else
                 {
              your work
                }
catch(FileNotFoundException exception)
           {
           }