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The Java Interview Handbook: 300+ Interview Questions

Wait() and Notify()

Explore the use of wait and notify methods in Java multi-threading to manage thread synchronization effectively. Understand how spurious wakeups occur and why waiting threads should recheck conditions in a loop. Learn about thread interruption handling to safely abort waiting operations when needed.

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1.

Explain the wait() method.

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Java
class Demonstration {
    
    static boolean flag = false;
    public static void main( String args[] ) throws InterruptedException {
        Object lock = new Object();
        Thread thread = new Thread(() -> {
            synchronized (lock) {
                while (!flag) {
                    try {
                        System.out.println("thread 1 is about to wait");
                        lock.wait();
                        System.out.println("thread 1 woken up with flag set to " + flag);
                    } catch (InterruptedException ie) {
                        // swallow
                    }
                }
            }
        });
        thread.start();
        Thread.sleep(1000);
        Thread thread2 = new Thread(() -> {
            synchronized (lock) {
                while (!flag) {
                    lock.notify();
                    System.out.println("thread 2 just notified");
                    flag = true;
                }
            }
            
            // try {
            //     Thread.sleep(100);
            // } catch (InterruptedException ie){
            //   // swallow
            // }
            synchronized (lock) {
                flag = false;
                System.out.println("thread 2 changed flag back to false");
            }
        });
        thread2.start();
        thread2.join();
        thread.join();
    }
}

  • Spurious wakeups are ...