Re: Problem with Threads

"Daniel Pitts" <>
22 Jan 2007 13:25:26 -0800
Damo wrote:


I'm trying to write a program using Threads. I want to pass a value
into the run() method of the Thread and I want to return an ArrayList
from the run method. I'm passing the value in via the constructor of
the threaded class and then using a getValue() method in the the run()
method to acces the value passed in
To retrieve the ArrayList I've declared it as an instance variable, and
in the run() method I'm adding to the ArrayList. Then I have a
getList() method to get the populated List.

I start the Thread like this:

Thread t = new Thread(parameter); //parameter is the value I'm
passing in.
list = t.getList(); // This is the method I created to retrieve the
populated list

Its not returning a the list as I want. The list appears to be empty.
Is the above the right way to go about this. I'm particularly unsure
about the line list=t.getList() , after I call the start method.

Any help would be appreciated.

If you can use Java 1.5 (and I suggest that you should)
Look into the java.util.concurrent package.
Specifically, Instead of starting threads directly with Runnable
instances, use Callable and Executors

Baring that, you could do it this way:
class MyRunnable implements Runnable {
  private int value;

  private final List<MyResultClass> list = new
  boolean ready = false;
  public MyRunnable(int value) {
    this.value = value;

  public void run() {

     synchronize(list) {
       try {
       for (int i = 0; i < value; ++i) {
         list.add(new MyResultClass(i));
       } finally {
         ready = true;

  public List<MyResultClass> getList() throws InterruptedException {
     synchronize(list) {
        while (!ready) {

Then you could do this:
MyRunnable myRunnable = new MyRunnable(value);
Thread t = new Thread(myRunnable);

The only problem with this approach is that getList() waits (and it has
to wait anyway) for the thread to complete. This gives you nearly the
same effect as running this program without any threads.

In reality, you'd be better to have your worker (MyRunnable in my case)
fire some sort of event.

Hope this helps,

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