Question

I’m having a bit of difficulty understanding how to synchronize ArrayList’s between threads in java. Currently my code looks like:

Public class Runner {

    public static void main(String argv[]) {   
       Connect f = new Connect(irc.freenode.net, 6667);
       Thread ft = new Thread(f);
       ft.start();
       Connect q = new Connect(irc.quakenet.org, 6667);
       Thread qt = new Thread(q);
       qt.start();
       MessageParser mp = new MessageParser(f);
       MessageParser mp = new MessageParser(q);
       f.addMessage("Hello!");
       q.addMessage("World!");
    }

}

public class Connect {

    public List<String> l = new ArrayList<String>();

    public static void addMessage(String str) {
        l.add(str);
    }

}

The example is just to show what I’m doing, it isn’t meant to make sense heh. Anyway I wanted to see if it was possible to have my ArrayList ‘l’ synched up between both threads. So that running f.addMessage(“Hello!”); and q.addMessage(“World!”);, both messages will be readable by either class. I know I could just easily make a seperate class that handles the ArrayList and pass it to both Connect classes but I wanted to see if there was an alternate way. I know about using a synchronizedList but I’m not very sure how that works and if it is applicable to my situation.

Thanks.

Answer 1

You are better off wrapping the List with Collections.synchronizedList.

List<String> l = Collections.synchronizedList(new ArrayList<String>());