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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