Explain thread lifecycle in Java
💡 Concept: Thread Lifecycle
The thread lifecycle in Java consists of several states that a thread passes through from creation to termination.
📘 Quick Intro
Java threads transition through states like New, Runnable, Running, Waiting, Timed Waiting, and Terminated.
🧠 Analogy
Think of a thread as a task moving through stages: waiting to start, executing, paused, and finished.
🔧 Technical Explanation
- New: Thread object created but not started.
- Runnable: Thread is ready to run and waiting for CPU.
- Running: Thread is actively executing.
- Waiting: Thread waiting indefinitely for a signal.
- Timed Waiting: Thread waits for a specified time.
- Terminated: Thread has completed execution.
🎯 Use Cases
- ✅ Understanding thread states aids debugging and optimization.
- ✅ Helps in managing concurrency effectively.
- ✅ Useful for designing responsive multi-threaded applications.
💻 Thread Lifecycle Example
public class ThreadStateDemo {
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(() -> {
System.out.println("Thread Running");
});
System.out.println("State after creation: " + t.getState());
t.start();
System.out.println("State after start: " + t.getState());
t.join();
System.out.println("State after completion: " + t.getState());
}
}
❓ Interview Q&A
Q1: What are the thread states in Java?
A: New, Runnable, Running, Waiting, Timed Waiting, Terminated.
Q2: What state is a thread in before start()?
A: New.
Q3: How to check thread state?
A: Using getState() method.
Q4: Can a thread move from waiting to runnable?
A: Yes, when notified.
Q5: What is terminated state?
A: Thread completed execution.
Q6: Difference between waiting and timed waiting?
A: Waiting is indefinite; timed waiting has timeout.
Q7: What method causes timed waiting?
A: sleep(), wait(timeout).
Q8: Can thread be restarted?
A: No.
Q9: What happens if start() called twice?
A: IllegalThreadStateException.
Q10: How to ensure thread safety during state transitions?
A: Use synchronization.
📝 MCQs
Q1. What are the thread states in Java?
- New only
- Running only
- New, Runnable, Running, Waiting, Timed Waiting, Terminated
- None
Q2. What state is a thread in before start()?
- Runnable
- New
- Running
- Terminated
Q3. How to check thread state?
- Using isAlive()
- Using getState() method
- Using start()
- Using run()
Q4. Can a thread move from waiting to runnable?
- No
- Yes
- Sometimes
- Never
Q5. What is terminated state?
- Running
- Waiting
- Terminated
- New
Q6. Difference between waiting and timed waiting?
- Same
- Waiting is indefinite; timed waiting has timeout
- Only for daemon threads
- None
Q7. What method causes timed waiting?
- start()
- sleep(), wait(timeout)
- join()
- notify()
Q8. Can thread be restarted?
- Yes
- No
- Sometimes
- Only with Executors
Q9. What happens if start() called twice?
- Nothing
- IllegalThreadStateException
- RuntimeException
- Error
Q10. How to ensure thread safety during state transitions?
- Ignore
- Use synchronization
- Use wait()
- Use notify()
💡 Bonus Insight
Understanding thread lifecycle is key to designing robust multi-threaded Java applications.
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