Question

I’m just learning a bit about threads in Java and I was wondering if some one could help me out.

I have created a list of 10 integers. What I want to do is have multiple threads go in, grab the integer at index 0, print it and remove it. I want this to happen until there are no more numbers in the list. This is my code so far.

public class SlothTest implements Runnable{

static ArrayList<Object> test = new ArrayList<>();
static int listSize;

public static void main(String[] args) {

    for (int i = 0; i < 10; i++){
        test.add(i);
    }

    SlothTest runner = new SlothTest();
    Thread alpha = new Thread(runner);
    Thread beta = new Thread(runner);
    alpha.setName("Alpha thread");
    beta.setName("Beta thread");
    alpha.start();
    beta.start();
}

@Override
public void run() {
    listSize = test.size();
    while (listSize > 0){
        getLink();
    }
}

private synchronized void getLink(){
    String threadName = Thread.currentThread().getName();
    System.out.println(threadName + " printed " + test.indexOf(listSize - 1));
    test.remove(0);
    listSize = test.size();
}

}

Can someone help point out everything I’m doing wrong, it’s probably a lot.

Answer 1

The test of listSize > 0 is not properly synchronized. The list could be non-empty when the check is performed but then actually be empty by the time the thread calls getLink(). It would probably be easier to have getLink() return boolean instead of void where true would indicate that the list is empty and false would indicate that it is not then each thread could just keep calling the method until it returns false. You would also have to check that the list is not empty at the top of of getLink() before you try to remove an item.

@Override
public void run() {
    while (getLink());
}

private synchronized boolean getLink(){
    String threadName = Thread.currentThread().getName();
    System.out.println(threadName + " printed " + test.indexOf(test.size() - 1));
    if(!test.isEmpty()){
        test.remove(0);
    }
    return !test.isEmpty()
}