Initially, I was wondering if checkMaxBuckets(collectorCounts.size(), maxBuckets) might cause some of the tests earlier expecting exception to fail. But, all the tests with counts[i] == 0, use 1024 as maxBuckets which is comfortably over collectorCounts.size()

Maybe, we can specify 3 as maxBuckets for even stronger condition:

searcher.search(query, new HistogramCollectorManager("f", 4, 3));

While unrelated to this change, I am wondering if we should check eagerly to prevent unnecessary iterations:

if (counts[i] != 0) {
  collectorCounts.addTo(leafMinBucket + i, counts[i]);
  checkMaxBuckets(collectorCounts.size(), maxBuckets);
}

We are doing similar validation in other places:

if (bucket != prevBucket) {
  counts.addTo(bucket, 1);
  checkMaxBuckets(counts.size(), maxBuckets);
  prevBucket = bucket;
}

I think it's ok the way it is. The end goal is to prevent unbounded heap allocation. In this case, the amount of excess heap we may allocate is bounded by 1024 entries, so I'd err on the side of simplicity by not checking the number of buckets in the loop?

Sounds fair. I was expecting low bound like 1024, just wanted to confirm!