writerA.put("B", "B");
writerA.put("C", "C");
writerA.put("D", "D");
FirstCharExtractor extractor = new FirstCharExtractor();
Filter f = new EqualsFilter(extractor, "A");
Assert.assertEquals("[A]", writerA.keySet(f).toString());
Assert.assertEquals("[A]", dirtyA.keySet(f).toString());
Assert.assertEquals("[]", rrA.keySet(f).toString());
Assert.assertEquals("[]", rcA.keySet(f).toString());