This is good, but why not...

public boolean isEmpty() {
  return highLowContainer.isEmpty();
}

This is good, but why not...

public boolean isEmpty() {
  return highLowContainer.isEmpty();
}

I was not sure of the invariance

All of the tests pass with that so I have pushed the suggestion

All mutating methods seems indeed to check for emptyness. There might be obscure edge-cases when deserializing (e.g. from non-Java implementations which might not prune empty containers).

All mutating methods seems indeed to check for emptyness. There might be obscure edge-cases when deserializing (e.g. from non-Java implementations which might not prune empty containers).

I han not considered the case of deserialisation from another form, or from a previous version that didnt prune

The code doesnt seems to expect this invariance (e.g. trim check for empty())
if would be simple to add a post deserialisation check

private void removeEmpty() {
    if (!highLowContainer.isEmpty()) {
      KeyIterator keyIterator = highLowContainer.highKeyIterator();
      while (keyIterator.hasNext()) {
        keyIterator.next();
        long containerIdx = keyIterator.currentContainerIdx();
        Container container = highLowContainer.getContainer(containerIdx);
        if (container.isEmpty()) {
          keyIterator.remove();
        }
      }
    }
  }

As a side note it looks like highKeyIterator doesnt implement the Iterator contract. It advances on hasNext not on next

Merged.