code: @Test public void filterEmptyTest() throws Exception {
    try {
      AbstractRegistry.filterEmpty(null,null);
      Assert.fail();
    }
   catch (  Exception e) {
      Assert.assertTrue(e instanceof NullPointerException);
    }
    List<URL> urls=new ArrayList<>();
    try {
      AbstractRegistry.filterEmpty(null,urls);
      Assert.fail();
    }
   catch (  Exception e) {
      Assert.assertTrue(e instanceof NullPointerException);
    }
    urls.add(testUrl.setProtocol(Constants.EMPTY_PROTOCOL));
    Assert.assertEquals(AbstractRegistry.filterEmpty(testUrl,null),urls);
    List<URL> testUrls=new ArrayList<>();
    Assert.assertEquals(AbstractRegistry.filterEmpty(testUrl,testUrls),urls);
    testUrls.add(testUrl);
    Assert.assertEquals(AbstractRegistry.filterEmpty(testUrl,testUrls),testUrls);
  }
  