A query regarding Java Threads

Question

Please let me know how I can print “After wait”; how can I notify main thread in the following code:

import java.util.*;  

public class Test {

      public static void main(String[] args) throws InterruptedException {
            ArrayList al = new ArrayList(6);
            al.add(0, "abc");
        al.add(1, "abc");
        al.add(2, "abc");
        synchronized(al){
            System.out.println("Before wait");
            al.wait();
            System.out.println("After wait");
        }       

      }

}

As the other answers explain more fully, `wait()` is not a sleep call. — Andrzej Doyle, Nov 15 '10 at 09:48

score 4 · Accepted Answer · edited May 23 '17 at 12:11

The wait() call is blocking until someone notify()s it... Basically, you would need to create a new thread that calls al.notify() when the main thread is blocking in wait().

This program prints Before wait, pauses for one second, and prints After wait.

import java.util.ArrayList;

public class Test {

    public static void main(String[] args) throws InterruptedException {

        final ArrayList al = new ArrayList(6);
        al.add(0, "abc");
        al.add(1, "abc");
        al.add(2, "abc");

        // Start a thread that notifies al after one second.
        new Thread() {
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                synchronized (al) {
                    al.notify();      // <-- this "releases" the wait.
                }
            }
        }.start();

        synchronized (al) {
            System.out.println("Before wait");
            al.wait();
            System.out.println("After wait");
        }
    }
}

Here is a link to one of my previous answers, explaining why wait() and notify() must be executed while holding the lock of the monitor.

Why must wait() always be in synchronized block

you have to add try catch around the al.wait() as well... but other than that, it's perfect. :) — dirbacke, Nov 15 '10 at 09:55
No, the OP already has `throws InterruptedException` in the main method. — aioobe, Nov 15 '10 at 10:02
So, the wait-notify works only if your thread is having the object monitor. Thanks for help — Gaurav Navgire, Nov 15 '10 at 10:04

score 2 · Answer 2 · answered Nov 15 '10 at 09:45

You're not creating any other threads, so it's hard to see how there is anything else to notify the main thread. However, if you did have another thread which had a reference to al, you'd use something like:

synchronized(al) {
    al.notify();
}

or

synchronized(al) {
    al.notifyAll();
}

(The difference between the two is that notify will only wake a single thread from waiting; notifyAll will wake all the threads waiting on that object.)

A query regarding Java Threads

2 Answers2