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!
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.
}
}
You can use the subList http://docs.oracle.com/javase/6/docs/api/java/util/List.html#subList from List to split your ArrayList. The sublist will give you a view of the original list. If you really want to create a new list, separate from the old one, you could do something like:
int index = 0;
int increment = 1000;
while ( index < bigList.size() ) {
newLists.add(new ArrayList<Record>(bigList.subList(index,index+increment));
index += increment;
}
Note you'll have to check for off by one errors here. This is just a quick pseudocode sample.
ArrayLists containing 1000 records each. Not 5-60 ArrayLists that provide different views of the same array of 5000-60000 records.new ArrayList<>(bigList.subList(index, index+increment)).