private void jButton1ActionPerformed(java.awt.event.ActionEvent evt)    {
  String user=txtUsername.getText();
  char[] pass=txtPassword.getPassword();

  if(user == null){
  JOptionPane.showMessageDialog(null,"Username Field is empty");
  }
  else if(pass==null){
  JOptionPane.showMessageDialog(null,"Password Field is empty");
  }
  else{
  String sql="select * from account where username=? and password=?";

     try{
            pst=conn.prepareStatement(sql);
            pst.setString(1,txtUsername.getText());
            pst.setString(2,txtPassword.getText());

            rs=pst.executeQuery();


            if(rs.next()){

                GridlockMain a=new GridlockMain();
                a.setVisible(true);

            }

            else{

                JOptionPane.showMessageDialog(null,"Your Username or Password is incorrect");
                txtUsername.setText(null);
                txtPassword.setText(null);

            }

        }
  catch(SQLException e){
           JOptionPane.showMessageDialog(null,e);

        }

String user=txtUsername.getText();// It return empty String ""
                                    // even no data is entered.

  if(user.isEmpty){
     JOptionPane.showMessageDialog(null,"Username Field is empty");
  }
  ......

if(!user.length() > 0){
  JOptionPane.showMessageDialog(null,"Username Field is empty");
  }
else if(!pass.length > 0){
  JOptionPane.showMessageDialog(null,"Password Field is empty");
}

txtUsername.getText().equals("");

if (user.length() < 0) {
        JOptionPane.showMessageDialog(null,"Username Field is empty");
    }
    else if(pass.length() < 0) {
        JOptionPane.showMessageDialog(null,"Password Field is empty");
    }