### Question:
ApproximateMostFrequentHistogram { public void merge(ApproximateMostFrequentHistogram<K> other) { List<Counter<K>> counters = other.streamSummary.topK(capacity); for (Counter<K> counter : counters) { add(counter.getItem(), counter.getCount()); } } ApproximateMostFrequentHistogram(
            int maxBuckets,
            int capacity,
            ApproximateMostFrequentBucketSerializer<K> serializer,
            ApproximateMostFrequentBucketDeserializer<K> deserializer);  ApproximateMostFrequentHistogram(
            Slice serialized,
            ApproximateMostFrequentBucketSerializer<K> serializer,
            ApproximateMostFrequentBucketDeserializer<K> deserializer); void add(K value); void add(K value, long incrementCount); Slice serialize(); void merge(ApproximateMostFrequentHistogram<K> other); void forEachBucket(BucketConsumer<K> consumer); @VisibleForTesting Map<K, Long> getBuckets(); long estimatedInMemorySize();  }### Answer:
@Test public void testMerge() { ApproximateMostFrequentHistogram<Long> histogram1 = new ApproximateMostFrequentHistogram<Long>(3, 15, LongApproximateMostFrequentStateSerializer::serializeBucket, LongApproximateMostFrequentStateSerializer::deserializeBucket); histogram1.add(1L); histogram1.add(1L); histogram1.add(2L); ApproximateMostFrequentHistogram<Long> histogram2 = new ApproximateMostFrequentHistogram<Long>(3, 15, LongApproximateMostFrequentStateSerializer::serializeBucket, LongApproximateMostFrequentStateSerializer::deserializeBucket); histogram2.add(3L); histogram2.add(4L); histogram1.merge(histogram2); Map<Long, Long> buckets = histogram1.getBuckets(); assertEquals(buckets.size(), 3); assertEquals(buckets, ImmutableMap.of(1L, 2L, 2L, 1L, 3L, 1L)); }