I have tried this.setLocationRelativeTo(null);
only that he was not in the middle of the screen, I would say he was much more right and down than in the center.
public Principal() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setLocationRelativeTo(null);
setTitle("Menu Principal");
iniciarTela();
setSize(300, 200);
setVisible(true);
setResizable(false);
}