如何确保所有产生的线程都已完成，以便继续处理？

Question

I have a ConcurrentMap and I put elements from an ArrayList to the map for other threads to process. Once a thread finishes its processing it adds the result in the ConcurrentMap and calls a method in order for let's say the "main" thread to get and process all the results (from both the ConcurrentMap and the ArrayList ).
This approach works but has a result to call the method as many times as the number of elements in the list.
Question: What constructs can I use so that the method is called once, after the ConcurrentMap is fully populated by the results of all background threads?

Answer 1

You can use CountDownLatch initialize it with total items in Arraylist and every time a item in process count it down using countDown method,

Main thread can call await method on latch, await blocks the thread till counter reaches zero.

Answer 2

In Java 8 you can do this

ConcurrentMap<Key, Value> map = ...
listToProcess.parallelStream().forEach(e -> addElementToMap(e, map));

This will perform the processing of all the element using all the CPUs you have and only return when they are all done.