retrieve variable from a swing JLIST

I'm doing an application with java and swing and I need to recover the element marked by the user from a JList. I perform the same steps as in two checkboxes, but when doing

for (int i = 0; i < listadoEspecialidades.getModel().getSize(); i++) {
        String[] listado = new String[i+1];
        listado[i] = String.valueOf(listadoEspecialidades.getModel().getElementAt(i));
        rbEspecialidad.setActionCommand(listado[i]);

    }
    btgEspecialidades.add(rbEspecialidad);

Later, I try to recover btgEspecialidades

String especialidad =  btgEspecialidades.getSelection().getActionCommand();

But it comes null