[java] How to know if other threads have finished?

I guess the easiest way is to use ThreadPoolExecutor class.

1. It has a queue and you can set how many threads should be working in parallel.
2. It has nice callback methods:

Hook methods

This class provides protected overridable beforeExecute(java.lang.Thread, java.lang.Runnable) and afterExecute(java.lang.Runnable, java.lang.Throwable) methods that are called before and after execution of each task. These can be used to manipulate the execution environment; for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method terminated() can be overridden to perform any special processing that needs to be done once the Executor has fully terminated.

which is exactly what we need. We will override afterExecute() to get callbacks after each thread is done and will override terminated() to know when all threads are done.

So here is what you should do

1. Create an executor:

   private ThreadPoolExecutor executor;
   private int NUMBER_OF_CORES = Runtime.getRuntime().availableProcessors();    
   
   
   
   private void initExecutor() {
   
   executor = new ThreadPoolExecutor(
           NUMBER_OF_CORES * 2,  //core pool size
           NUMBER_OF_CORES * 2, //max pool size
           60L, //keep aive time
           TimeUnit.SECONDS,
           new LinkedBlockingQueue<Runnable>()
   ) {
   
       @Override
       protected void afterExecute(Runnable r, Throwable t) {
           super.afterExecute(r, t);
               //Yet another thread is finished:
               informUiAboutProgress(executor.getCompletedTaskCount(), listOfUrisToProcess.size());
           }
       }
   
   };
   
       @Override
       protected void terminated() {
           super.terminated();
           informUiThatWeAreDone();
       }
   
   }
   

2. And start your threads:

   private void startTheWork(){
       for (Uri uri : listOfUrisToProcess) {
           executor.execute(new Runnable() {
               @Override
               public void run() {
                   doSomeHeavyWork(uri);
               }
           });
       }
       executor.shutdown(); //call it when you won't add jobs anymore 
   }
   

Inside method informUiThatWeAreDone(); do whatever you need to do when all threads are done, for example, update UI.

NOTE: Don't forget about using synchronized methods since you do your work in parallel and BE VERY CAUTIOUS if you decide to call synchronized method from another synchronized method! This often leads to deadlocks

Hope this helps!