public static UpdateResult acknowledged(
     final long matchedCount, 
    @Nullable final Long modifiedCount,   // Marked as nullable.
    @Nullable final BsonValue upsertedId)

// Then in AcknowledgedUpdateResult its not marked nullable
AcknowledgedUpdateResult(final long matchedCount, final Long modifiedCount, @Nullable final BsonValue upsertedId) 

// The getter uses long
public long getModifiedCount()