java - Synchronise ArrayList over two threads -

i'm having difficult time understanding how synchronise arraylist on 2 threads. basically, want 1 thread appending objects list , other 1 reading list @ same time.

here class deploys threads:

public class main {     public static arraylist<good> goodlist = new arraylist();     public static void main(string[] args) {         thread thread1 = new thread(new goodcreator());         thread thread2 = new thread(new weightcounter());         thread1.start();         thread2.start();     } } 

then 2 runnable classes:

this 1 reads lines of 2 values text file , appends new objects.

public class goodcreator implements runnable{     private arraylist<good> goodlist = main.goodlist;     private static scanner scan;     @override     public void run() {         system.out.println("thread 1 started");         int objcount = 0;         try {             scan = new scanner(new file(system.getproperty("user.home") + "//goods.txt"));         } catch (filenotfoundexception e) {             system.out.println("file not found!");             e.printstacktrace();         }         while(scan.hasnextline()){             string line = scan.nextline();             string[] words = line.split("\\s+");             synchronized(goodlist){                 goodlist.add(new good(integer.parseint(words[0]), integer.parseint(words[1])));                 objcount++;             }             if(objcount % 200 == 0) system.out.println("created " + objcount + " objects");         }     }  } 

this iterates on arraylist , supposed sum 1 of fields.

public class weightcounter implements runnable{     private arraylist<good> goodlist = main.goodlist;     @override     public void run() {         system.out.println("thread 2 started");         int weightsum = 0;         synchronized(goodlist){             for(good g : goodlist){                 weightsum += g.getweight();             }         }         system.out.println(weightsum);      }  } 

no matter input, weightsum never gets incremented , stays 0

thread 1 started thread 2 started 0 

any appreciated

this called producer-consumer task. can arraylist, it's not right way approach problem.

luckily, java provides collections, blockingqueue collections, designed reason;

//the collection stuff in static blockingqueue<object> items = new blockingqueue<object>(); //(there few different types of blocking queues, check javadocs. //you want linked or array blocking queue  //what happens on reader thread public void producer() {     //read data collection     (all data in file)     {        //add next item        items.put(/* next item file or w/e */);         //stop if necessary        if (atendoffile) stillreadingdata = false;         //etc     } } 

now need read data out of queue - luckily easy enough;

//what happens on other threads public void consumer() {      //keep thread alive long there data process    //or long there might more data process    while (stillreadingdata || !items.isempty())    {        //get next item list        //while list empty, sleep "timeout" timeunits,        //then while-loop repeat, , on        object o = items.poll(long timeout, int units);        if (o != null) //process    } } 

in way, can continuously add items queue producer thread, , items processed consumer thread free (this approach scales lots of consumer threads). if still need collection items, should make second collection , add them after have been processed.

as side note, may still need synchronize oprations occur while processing items. example, need synchronize increments on "weightsum" (or alternately use atomicinteger).