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The Java Interview Handbook: 300+ Interview Questions

Synchronized

Explore how synchronization works in Java multi-threading and understand common errors like object reassignment leading to IllegalMonitorState exceptions. This lesson helps you identify synchronization mistakes and improve thread safety in your Java programs.

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1.

What is synchronized?

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Technical Quiz
1.

Consider the code snippet below:

class NewbieSynchronization {

    Boolean flag = new Boolean(true);

    public void example() throws InterruptedException {

        Thread t1 = new Thread(new Runnable() {

            public void run() {
                synchronized (flag) {
                    try {
                        while (flag) {
                            System.out.println("First thread about to sleep");
                            Thread.sleep(5000);
                            System.out.println("Woke up and about to invoke wait()");
                            flag.wait();
                        }
                    } catch (InterruptedException ie) {

                    }
                }
            }
        });

        Thread t2 = new Thread(new Runnable() {

            public void run() {
                flag = false;
                System.out.println("Boolean assignment done.");
            }
        });

        t1.start();
        Thread.sleep(1000);
        t2.start();
        t1.join();
        t2.join();
    }
}

What will be the output of invoking the example() method ?

A.

InterruptedException is thrown

B.

IllegalStateException is thrown

C.

Program runs without issues

D.

Thread t2 doesn’t synchronize on the same object as thread t1

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Java
class Demonstration {
    public static void main( String args[] ) throws InterruptedException {
        (new NewbieSynchronization()).example();
    }
}
class NewbieSynchronization {
    Boolean flag = new Boolean(true);
    public void example() throws InterruptedException {
        Thread t1 = new Thread(new Runnable() {
            public void run() {
                synchronized (flag) {
                    try {
                        while (flag) {
                            System.out.println("First thread about to sleep");
                            Thread.sleep(5000);
                            System.out.println("Woke up and about to invoke wait()");
                            flag.wait();
                        }
                    } catch (InterruptedException ie) {
                    }
                }
            }
        });
        Thread t2 = new Thread(new Runnable() {
            public void run() {
                flag = false;
                System.out.println("Boolean assignment done.");
            }
        });
        t1.start();
        Thread.sleep(1000);
        t2.start();
        t1.join();
        t2.join();
    }
}

A classic ...