Java - clase abstracta.

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {

    validar();

}

public void validar(){

    String dni = txtPass.getText();

    String user = txtUser.getText();

    if (txtUser.getText().equals("")||txtPass.getText().equals("")) {

        JOptionPane.showMessageDialog(this, "Faltan datos");

        txtUser.requestFocus();

    }else{

        ev = vdao.ValidarVendedor(dni, user);

        if (ev.getUser() != null && ev.getDni() != null) {

            Principal p = new Principal();

            p.setVisible(true);

            dispose();

        }

    }

}