Question

what’s the best way to make main thread to wait until all threads are finished?

for(int i=0;i<n;i++){
   Thread t=new Thread();
   t.start();

}

//wait for all threads to finish

Answer 1

Create a list and wait for the all.

List<Thread> threads = new ArrayList<Thread>();
for(int i=0;i<n;i++){
    Thread t=new Thread();
    t.start();
    threads.add(t);
}

for(Thread t: threads) t.join();

However using an ExecutorService can be a more elegant way to handle a pool of threads.

ExecutorService es = Executors.newCachedThreadPool();
for(int i=0;i<n;i++)
   es.submit(new Task(n));
es.shutdown();
es.awaitTermination(timeout, TimeUnit.SECONDS);