java - NoSuchElementException even after check - 


for (final arraylist<smartphone> smartphones : smartphonescluster) {     new thread(new runnable() {         @override         public void run() {             (smartphone smartphone : smartphones) {                 queue<smartphonetask> tasks = smartphone.getsystem()                                                         .gettaskque();                 smartphonetask task = null;                 assert tasks != null;                 try {                     while (!tasks.isempty()) {                         task = tasks.poll(); // line throwing exception (globalnetwork.java:118)                         assert task != null;                         task.execute();                         task.ontaskcomplete();                     }                 } catch (runtimeexception e) {                     e.printstacktrace();                 }             }         }     }).start(); }

and log:

java.util.nosuchelementexception     @ java.util.linkedlist.remove(linkedlist.java:788)     @ java.util.linkedlist.removefirst(linkedlist.java:134)     @ java.util.linkedlist.poll(linkedlist.java:470)     @ com.wtsang02.reu.botnet.network.globalnetwork$1.run(globalnetwork.java:118)     @ java.lang.thread.run(thread.java:662) java.lang.nullpointerexception exception in thread "thread-299" java.lang.assertionerror     @ com.wtsang02.reu.botnet.network.globalnetwork$1.run(globalnetwork.java:119)     @ java.lang.thread.run(thread.java:662)

line 118 points to:

task=tasks.poll();

how solve this? queue linkedlist implemenation if makes difference.

linkedlist not thread-safe, need external synchronization if access linkedlist on more 1 thread. synchronization on object (a synchronized method shorthand "synchronize on this"), , both gets , puts must synchronized on same object. you're doing here, since create new thread each smartphone, , access phone's linkedlist there.

if 1 thread puts list while synchronized on someobject1, , thread reads list while synchronized on someobject2, not count external synchronization -- code still broken.

even if used thread-safe collection, it'd possible hit exception if multiple threads emptying queue @ same time. instance, imagine this:

thread a: put e queue1 thread b: queue1.isempty()? no, go on thread c: queue1.isempty()? no, go on thread b: queue1.poll() // works thread c: queue1.poll() // nosuchelementexception

you should use blockingqueue, poll() method return null if there no more elements in list. keep pulling until null, , break loop.


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