java - ScheduledExecutorService not printing the exception stacktrace when the run method throws NPE -

in code given below, i.intvalue throwing npe. not printed. instead scheduledexecutorservice terminates silently cancelling subsequent executions . why?

import java.util.concurrent.*; import java.util.concurrent.atomic.*; import java.util.*; class concurr {     public static void main(string[] args)      {         scheduledexecutorservice sce = executors.newscheduledthreadpool(1);         runnable task = new runnable()         {             public void run()             {                 system.out.print(".");                 integer = null;                 i.intvalue();             }         };         final scheduledfuture<?> future = sce.scheduleatfixedrate(task,0,2,timeunit.seconds);         sce.schedule( new runnable()         {             public void run()             {                 future.cancel(true);             }         },10,timeunit.seconds);     } } 

an executorservice catches (and stores) exceptions thrown in runnable , callable instances manages.

the scheduledfuture has get() method throws executionexception containing exception if 1 thrown during execution of runnable. throws cancellationexception if execution cancelled.

do

system.out.println(future.get()); // returns null otherwise  

at end of main. following output:

.exception in thread "main" java.util.concurrent.executionexception: java.lang.nullpointerexception     @ java.util.concurrent.futuretask$sync.innerget(unknown source)     @ java.util.concurrent.futuretask.get(unknown source)     @ test.main.main(main.java:50) caused by: java.lang.nullpointerexception     @ test.main$1.run(main.java:38)     @ java.util.concurrent.executors$runnableadapter.call(unknown source)     @ java.util.concurrent.futuretask$sync.innerrunandreset(unknown source)     @ java.util.concurrent.futuretask.runandreset(unknown source)     @ java.util.concurrent.scheduledthreadpoolexecutor$scheduledfuturetask.access$301(unknown source)     @ java.util.concurrent.scheduledthreadpoolexecutor$scheduledfuturetask.run(unknown source)     @ java.util.concurrent.threadpoolexecutor.runworker(unknown source)     @ java.util.concurrent.threadpoolexecutor$worker.run(unknown source)     @ java.lang.thread.run(unknown source) 

you use , pass callable instead of runnable executorservice if asynchronous task should return value. value future.get().

each call future.get() returns result of 1 execution of runnable. example, you've scheduled task run every 5 seconds. if after 16 seconds, call

future.get(); future.get(); future.get(); future.get(); 

the code block on 4th call because others have finished , have returned result (unless 1 of them failed).