java - What's the dedicate way to use ExecutorService and Future? -

i have batch of items. , each item, can itemid, use id db lookup , generate transactions batch.

list<batchitem> batchitemslist; // assume list have list of items        list<transaction> results = new arraylist<>(); executorservice executor = executors.newfixedthreadpool(5); list<future<transaction>> tasks = new arraylist<>();  for(batchitem item: batchitemslist) {     mythread worker = new mythread(item);     future<transaction> future = executor.submit(worker);     tasks.add(future); }  for(future<transaction> t: tasks) {        results.add(t.get()); } 

mythread.java

public class mythread implements callable<transaction>{   private final batchitem batchitem;    public mythread(batchitem batchitem) {     this.batchitem = batchitem;   }    @override   public transaction call() throws exception {     string id = batchitem.getid();     // sql lookups....     transaction t = new transaction(sql lookup result values...);     return t;   } } 

question 1: need create mythread object? if have 15k items, means i'll create 15k mythread objects in memory... what's dedicate way solve problem?

question 2 need check if future done? if so, how should handle isdone()?