GroupByExtractor implements WindowExtractor<T> { public static <T extends Serializable> Builder<T> builder() { return new Builder<>(); } private  GroupByExtractor(
            Map<String, Tuple2<Aggregation, Supplier>> aggregations,
            GroupAggregationFinalizer<T> finalizer
    ); @Override @SuppressWarnings("unchecked") // is insured to be safe void apply(MRecord record); @Override void finish(EvaluationContext<T> context); static Builder<T> builder();  }
@Test public void builder(){ Assert.assertNotNull(extractor); }