title: Return message if no checkbox is selected
body: <p>How to implement more into this code so that it returns a message telling the user to 'select at least one checkbox' if he/ she does not select any?</p>
  
  <pre><code>public void verificaCheckBox() {
  
      Listcheck.clear();
  
      if (cbPapel.isChecked())
          Listcheck.add(cbPapel.getText().toString());
  
      if (cbPlastico.isChecked())
          Listcheck.add(cbPlastico.getText().toString());
  
      if (cbMetal.isChecked())
          Listcheck.add(cbMetal.getText().toString());
  
      if (cbVidro.isChecked())
          Listcheck.add(cbVidro.getText().toString());
  
      cbSelecionado = (Listcheck.toString());
  }
  </code></pre>