Difference Between sleep() and wait() in Java
💡 Concept: sleep() vs wait()
sleep() is a static method in Thread class that pauses the thread for a given time. wait() is an instance method of Object used for inter-thread communication.
📘 Quick Intro
sleep() is used to delay thread execution, while wait() is used to pause execution until another thread notifies it.
🧠 Analogy
sleep() is like putting an alarm clock and sleeping for a fixed time. wait() is like waiting for someone to tap your shoulder before you can continue.
🔧 Technical Explanation
sleep()does not release the monitor lock.wait()releases the lock and waits untilnotify()ornotifyAll()is called.sleep()is used for delaying execution.wait()is used for coordination between threads.wait()must be called within a synchronized context.
🎯 Use Cases
- ✅ Use
sleep()to pause execution unconditionally for a duration. - ✅ Use
wait()when a thread needs to pause until a condition is met by another thread. - ✅ Ideal for producer-consumer problem using synchronized blocks.
💻 Example: sleep() vs wait()
public class SleepVsWait {
public static void main(String[] args) {
Object lock = new Object();
Thread t1 = new Thread(() -> {
synchronized(lock) {
try {
System.out.println("Thread waiting...");
lock.wait();
System.out.println("Thread resumed!");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
Thread t2 = new Thread(() -> {
synchronized(lock) {
try {
Thread.sleep(1000);
lock.notify();
System.out.println("Thread notified waiting thread");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
t1.start();
t2.start();
}
}
❓ Interview Q&A
Q1: What is the purpose of sleep()?
A: Pauses thread for a fixed time without releasing lock.
Q2: What does wait() do?
A: Pauses and releases the monitor lock until notified.
Q3: Is sleep() part of Object class?
A: No, it's in Thread class.
Q4: Can wait() throw exceptions?
A: Yes, InterruptedException.
Q5: Can sleep() be called without synchronization?
A: Yes.
Q6: Can wait() be used outside synchronized block?
A: No, throws IllegalMonitorStateException.
Q7: Does sleep() release the lock?
A: No.
Q8: Can notify() work without wait()?
A: Yes, but it has no effect if no thread is waiting.
Q9: Is wait() overloaded?
A: Yes, with timeout and nanos.
Q10: Can both be interrupted?
A: Yes.
📝 MCQs
Q1. Which method releases the monitor lock?
- sleep()
- wait()
- join()
- notify()
Q2. What class contains sleep()?
- Object
- Thread
- Runnable
- Executor
Q3. Is synchronization needed for sleep()?
- Yes
- No
- Sometimes
- Only in threads
Q4. What exception can wait() throw?
- IOException
- NullPointerException
- InterruptedException
- IllegalStateException
Q5. Does sleep() pause for fixed time?
- No
- Yes
- Depends
- Only in threads
Q6. Can wait() be used outside synchronized?
- Yes
- No
- Sometimes
- With lock object
Q7. Which method is used for communication between threads?
- sleep()
- yield()
- wait()
- exit()
Q8. What does notify() do?
- Starts new thread
- Pauses thread
- Wakes a waiting thread
- Cancels thread
Q9. Does sleep() release CPU?
- No
- Yes temporarily
- Forever
- Only on wait
Q10. Does wait() block forever without notify()?
- No
- Yes
- Sometimes
- If lock is free
💡 Bonus Insight
Use Condition objects from java.util.concurrent.locks for advanced wait-notify patterns with better control than synchronized blocks.
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