java - How to access arrays from multi-thread? -

simply speaking, problem follows:

i have class named test , has 3 double arrays,named array1, array2,array3 respectively(assuming have same length).it has major function named estep, , need implement function multi-threading improve speed.

in estep, need modify array1 , array2, , array3 read.i define indicator variable named which_array indicator array modify.i passed 3 arrays estep , modify array1 , array2.

my code follows , test , it works fine modification of array1 , array2 in estep can seen in test class. still have doubt if absolutely right. i'm wondering if should add volatile 3 array1 in test class or should need synchronization mechanism?.

any advice appreciated!

public class test{      double[] array1;     double[] array2;     double[] array3;     //for simplifization, omitted code allocation , initialization of the arrays.     public void estepintest()     {         final countdownlatch countdown = new countdownlatch(2);      estep modifyarray1 = new estep(array1, array2, array3, 1,countdown);     thread t1 = new thread( firsthalf);     t1.start();      estep modifyarray2 = new estep(array1, array2, array3, 2,countdown);             thread t2 = new thread(secondhalf);     t2.start();      try{     countdown.await();     }catch (interruptedexception e) {         e.printstacktrace();     }          //do next things     }  }  class estep implements runnable{      double[] array1;     double[] array2;     double[] array3;     int which_array;      estep(double[] array1, double[] array2, double[] array3, int which_array, countdownlatch cd)     {         this.array1 = array1;         this.array2 = array2;         this.array3 = array3;         this.which_array = which_array;         this.cd = cd;     }      public void run(){          if( this.which_array == 1){            for( int = 0; < array1.length; ++i )            {                   array1[i] = array1[i] + array3[i];            }           }else{            for( int = 0; < array2.length; ++i )            {                   array2[i] = array2[i] + array3[i];            }         }         cd.countdown();    } 

it right. countdownlatch.await() guarantees done before returns happens-before done after has returned.

your code cleaner , shorter if passed array modify estep constructor, instead of passing 2 arrays, , magic number indicate 1 modify.

edit:

regarding access array3, it's safe well, because, explained in the documentation of concurrent package, call start on thread happens-before action in started thread. 2 threads see every modification made array3 has been made before starting them.

also, note making array volatile makes array reference volatile. doesn't make elements volatile.