Explain wait(), notify(), and notifyAll() in Java
💡 Concept: Inter-thread Communication
The methods wait(), notify(), and notifyAll() in Java are used for inter-thread communication, allowing threads to wait and notify each other using object monitors.
📘 Quick Intro
These methods are defined in the Object class and must be called inside synchronized blocks or methods. They help manage cooperation between threads rather than competition.
🧠 Analogy
Imagine workers at a loading dock. One waits for a shipment (wait), another signals when the shipment has arrived (notify), or informs all waiting workers at once (notifyAll).
🔧 Technical Explanation
wait(): Releases the lock and causes the current thread to wait until another thread callsnotify()ornotifyAll().notify(): Wakes up one randomly selected thread waiting on the object's monitor.notifyAll(): Wakes up all threads waiting on the object's monitor.- All three methods must be used within synchronized context, or an
IllegalMonitorStateExceptionis thrown.
🎯 Use Cases
- ✅ Implementing producer-consumer problems.
- ✅ Coordinating resource sharing among threads.
- ✅ Avoiding busy-wait loops by blocking until notified.
💻 Example: wait() and notify()
class SharedResource {
private boolean available = false;
public synchronized void produce() {
available = true;
System.out.println("Produced item");
notify();
}
public synchronized void consume() {
while (!available) {
try {
wait();
} catch (InterruptedException e) {}
}
System.out.println("Consumed item");
available = false;
}
}
public class WaitNotifyExample {
public static void main(String[] args) {
SharedResource resource = new SharedResource();
new Thread(resource::consume).start();
new Thread(resource::produce).start();
}
}
❓ Interview Q&A
Q1: What is the purpose of wait()?
A: To pause the current thread until it is notified.
Q2: Where can wait() be used?
A: Inside synchronized blocks or methods only.
Q3: What happens if wait() is called outside synchronization?
A: It throws IllegalMonitorStateException.
Q4: Difference between notify() and notifyAll()?
A: notify() wakes up one thread; notifyAll() wakes up all waiting threads.
Q5: Do these methods belong to Thread class?
A: No, they belong to Object class.
Q6: Can wait() be interrupted?
A: Yes, it throws InterruptedException if interrupted.
Q7: Is notify() deterministic?
A: No, the selected thread is not guaranteed.
Q8: Can you use notify() without wait()?
A: It's possible but meaningless if no thread is waiting.
Q9: How do these methods help efficiency?
A: They avoid CPU wastage by preventing busy-waiting.
Q10: Can these methods be used with static synchronization?
A: Yes, with proper locking on the Class object.
📝 MCQs
Q1. What does wait() do in Java?
- Terminates thread
- Causes the current thread to wait and release the lock
- Locks the object
- Executes immediately
Q2. Which class defines wait() and notify()?
- Thread
- Runnable
- Object
- Monitor
Q3. What happens if notify() is used without synchronized?
- Nothing
- IllegalMonitorStateException
- Thread dies
- Deadlock
Q4. What is the key difference between notify() and notifyAll()?
- No difference
- notify() wakes one thread, notifyAll() wakes all
- notifyAll() is deprecated
- notify() is faster
Q5. Can wait() throw exceptions?
- No
- Yes, InterruptedException
- Only in debug mode
- Only for IO threads
Q6. Which keyword must be used with wait()?
- static
- volatile
- synchronized
- final
Q7. Which method wakes up one waiting thread?
- notify()
- wake()
- signal()
- resume()
Q8. Are wait(), notify() blocking methods?
- No
- Yes
- Sometimes
- Only if static
Q9. Do these methods help prevent busy-waiting?
- No
- Yes
- Not always
- They cause busy-wait
Q10. What happens if no thread is waiting during notify()?
- Deadlock
- Error
- Nothing happens
- All threads start
💡 Bonus Insight
Use higher-level concurrency constructs like Condition, BlockingQueue, or CountDownLatch when possible for more readable and safer inter-thread communication.
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