java - synchronize on array is not showing outcome as expected -

public class threaddemo implements runnable { integer ar [] =  new integer[100];   @override public void run() {   synchronized (ar) {     system.out.println("start: in run method");     for(int =0; i<100;i++)     {         ar[i]=i;     }      for(int i=0;i<100; i++)     {         ar[i]= ar[i]*1000;         system.out.println(ar[i]+"\t");     }     system.out.println("end:in run method"); }    } 

other driver calls created 2 threads thread pool

executor task   =  executors.newfixedthreadpool(10);     task.execute(new threaddemo());     task.execute(new threaddemo()); 

output :

start: in run method start: in run method 0    0    1000     1000     2000     2000     3000     3000     

why 2 threads accessing array simultaneously when array synchronized. ideally when 1 thread access array, acquires lock on array , in case, other thread should wait till first thread doesnot leaves lock on array.

synchronized triggered when 2 or more threads try access same data. in case, 2 threads both run seperate instance of threaddemo class , hence seperate ar array. behaviour seeing expected.

try replacing this:

executor task   =  executors.newfixedthreadpool(10); task.execute(new threaddemo()); task.execute(new threaddemo()); 

by this:

executor task   =  executors.newfixedthreadpool(10); threaddemo td = new threaddemo(); task.execute(td); task.execute(td); 

in case both threads operating on same data.