java – ExecutorService. Submit (callable) returns a null value for the future object

I am using executorservice to generate threads that perform different tasks When submitting the (callable < T >) method, it should return future < T > Object Instead, it returns null As a result, when calling future < T > Get() method, it will fail with NullPointerException

Has anyone ever encountered this problem? Or, what did I do wrong?

ArrayList<Boolean> resultsList = new ArrayList<Boolean> ();
ExecutorService excutorService  = Executors.newCachedThreadPool();
for(ClassImplementingCallable myClassImplementingCallable : listOfClassesImplementingCallable) {
    resultsList.add( excutorService.submit( myClassImplementingCallable ) );
}
excutorService.shutdown();
for ( Future< Boolean > result : resultList ) {
    result.get(); // getting exception here..
}

Solution

Surprisingly, you can compile code: you are trying to add some future < Boolean > to the list < Boolean >

ResultsList should be declared as:

List<Future<Boolean>> resultsList = new ArrayList<Future<Boolean>> ();

In addition, you should not use your code to receive NPE unless one of the myclassimplementingcallable in listofclassesimplementingcallable is null