code: @Test public void lookupTest() throws Exception {
    try {
      abstractRegistry.lookup(null);
      Assert.fail();
    }
   catch (  Exception e) {
      Assert.assertTrue(e instanceof NullPointerException);
    }
    List<URL> urlList1=abstractRegistry.lookup(testUrl);
    Assert.assertFalse(urlList1.contains(testUrl));
    List<URL> urls=new ArrayList<>();
    urls.add(testUrl);
    abstractRegistry.notify(urls);
    List<URL> urlList2=abstractRegistry.lookup(testUrl);
    Assert.assertTrue(urlList2.contains(testUrl));
  }
  