Why does java thread wait() work only with time limit in here?

I am trying to get familiar with Java threads for the SCJP and I had a question.

In the below-written code i simply created: two Runnables with a common data storage (an array) and a synchronized write() method to fill it with data successively leaving a letter as a mark for each Runnable (A and B) in sequence.

I know the code is rou开发者_如何学JAVAgh and could be better written but I was seeking the moral of the threads.

So now when I run it, it never terminates and the results stop at:

Still good. A0.

But when I change wait() to wait(100) it works just fine counting from 0 to 9 and it terminates normally. Could someone explain the reason behind that for me please?

Thank you.

public class ArrayThreads {

Object[] array = new Object[10];
boolean isA = true;

    int position = 0;

    int getIndex(){
        return position;
    }



class ThreadA implements Runnable{

            synchronized void write(String value){
                    while(!isA){
            try {
                wait();
            } catch (InterruptedException ex) {
                System.out.println("An error in" + value);
                ex.printStackTrace();
            }
        }
        array[position] = value + position;
        System.out.println(array[position]);
        position++;
        isA = !isA;
        notify();
    }

    public void run() {
        while(getIndex()<array.length){
            if (getIndex()==9) return;
            else
        write("A");}
    }
}

    class ThreadB implements Runnable{

                synchronized void write(String value){
                    while(isA){
            try {
                wait();
            } catch (InterruptedException ex) {
                System.out.println("An error in" + value);
                ex.printStackTrace();
            }
        }
        array[position] = value + position;
        System.out.println(array[position]);
        position++;
        isA = !isA;
        notify();
    }

    public void run() {
        while(getIndex()<array.length){
            if (getIndex()==9) return;
            else
        write("B");}
    }
}

    public static void main(String[] args){
        ArrayThreads threads = new ArrayThreads();
        Thread threadA = new Thread(threads.new ThreadA());
        Thread threadB = new Thread(threads.new ThreadB());
        System.out.println("Still good");

        threadB.start();
        threadA.start();
    }

}

Your threads are each waiting and notifying separate objects - so they're not communicating with each other at all. If you want them to effectively release each other, they'll need a shared monitor to synchronize, wait on and notify.

It's "working" when you specify a timeout because it's effectively turning the wait call into a sleep call... still nothing is really waiting/notifying usefully, because the two threads are still dealing with separate monitors.

your objects are not working in same monitor.

you need to either move the wait() and notify() to same object like: http://www.java-samples.com/showtutorial.php?tutorialid=306

or you can notify the target object: http://www.linuxtopia.org/online_books/programming_books/thinking_in_java/TIJ315_016.htm

when you set wait(100). you are setting a timeout. and definitely it will wake up after 100ms.