Question

My program creates an arraylist of 5000 to 60000 records depending on time of day. I want to split it into as many arraylists as possible that each arraylist will have 1000 records. I looked at many examples online and tried a few things but I ran into strange problems. Can you please show me an example of this?

Regards!

Answer 1

  public static <T> Collection<Collection<T>> split(Collection<T> bigCollection, int maxBatchSize) {
    Collection<Collection<T>> result = new ArrayList<Collection<T>>();

    ArrayList<T> currentBatch = null;
    for (T t : bigCollection) {
      if (currentBatch == null) {
        currentBatch = new ArrayList<T>();
      } else if (currentBatch.size() >= maxBatchSize) {
        result.add(currentBatch);
        currentBatch = new ArrayList<T>();
      }

      currentBatch.add(t);
    }

    if (currentBatch != null) {
      result.add(currentBatch);
    }

    return result;
  }

Here’s how we use it (assuming emails an a large ArrayList of email addresses:

Collection<Collection<String>> emailBatches = Helper.split(emails, 500);
    for (Collection<String> emailBatch : emailBatches) {
        sendEmails(emailBatch);
        // do something else...
        // and something else ...
    }
}

where emailBatch would iterate over the collection like this:

private static void sendEmails(Collection<String> emailBatch){
    for(String email: emailBatch){
        // send email code here.
    }
}