What is the importance of wait(), notify() and notifyAll() methods in Java?

Threads can communicate with each other through the wait(), notify() and notifyAll() methods in Java. These are the final methods defined in the Object class and can only be called from a synchronized context. The wait() method causes the current thread to wait until another thread calls the object's notify() or notifyAll() method. notify() Method Wakes up a single thread that is waiting for this object's monitor. notifyAll() Method Wake up all threads that are waiting for the object monitor. A thread waits for an object's monitor by calling one of the wait() methods. These methods may throw IllegalMonitorStateException if the current thread is not the owner of the object monitor.

wait() method syntax

public final void wait() throws InterruptedException

notify() method syntax

public final void notify()

NotifyAll() method syntax

public final void notifyAll()<strong>
</strong>

Example

public class WaitNotifyTest {
   private static final long SLEEP_INTERVAL<strong> </strong>= 3000;
   private boolean running = true;
   private Thread thread;
   public void start() {
      print("Inside start() method");
      thread = new Thread(new Runnable() {
         @Override
         public void run() {
            print("Inside run() method");
            try {
               Thread.sleep(SLEEP_INTERVAL);
            } catch(InterruptedException e) {
               Thread.currentThread().interrupt();
            }
            synchronized(WaitNotifyTest.this) {
               running = false;
               WaitNotifyTest.this.notify();
            }
         }
      });
      thread.start();
   }
   public void join() throws InterruptedException {
      print("Inside join() method");
      synchronized(this) {
         while(running) {
            print("Waiting for the peer thread to finish.");
            wait(); //waiting, not running
         }
         print("Peer thread finished.");
      }
   }
   private void print(String s) {
      System.out.println(s);
   }
   public static void main(String[] args) throws InterruptedException {
      WaitNotifyTest test = new WaitNotifyTest();
      test.start();
      test.join();
   }
}

Output

Inside start() method
Inside join() method
Waiting for the peer thread to finish.
Inside run() method
Peer thread finished.

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