@Test
public void testSeal() {

    // PREPARE
    final String version = "1.2.3";
    DownloadVersion testee = new DownloadVersion(version);
    final DownloadOSFamily linux = new DownloadOSFamily("Linux");
    final DownloadOSFamily windows = new DownloadOSFamily("Windows");
    testee.addOSFamily(linux);
    // TEST & VERIFY
    testee.seal();
    try {
        testee.addOSFamily(windows);
        fail();
    } catch (final IllegalStateException ex) {
        assertThat(ex.getMessage()).isEqualTo("The instance is sealed");
    }
}