Java Thread Methods Explained (start, run, join, wait, notify…)

📌 What is a Thread in Java?

A thread is a lightweight unit of execution inside a Java program.
Java supports multithreading, allowing multiple tasks to run concurrently.

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🧭 Java Thread Lifecycle (Text Diagram)

        ┌──────────┐
        │  NEW     │  ← Thread object created
        └────┬─────┘
             │ start()
             ▼
        ┌──────────┐
        │ RUNNABLE │  ← Ready / Running
        └────┬─────┘
             │ sleep(), wait(), join()
             ▼
        ┌──────────┐
        │ WAITING  │
        │ /BLOCKED │
        └────┬─────┘
             │ notify(), timeout over
             ▼
        ┌──────────┐
        │ RUNNABLE │
        └────┬─────┘
             │ run() completes
             ▼
        ┌──────────┐
        │ TERMINATED│
        └──────────┘

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1️⃣ start() — Start a New Thread

🔹 Purpose

• Creates a new call stack

• Executes code concurrently

🔹 Example

Thread t = new Thread(() -> {
    System.out.println("Thread running");
});
t.start();

🔹 Key Point

✔️ start() calls run() internally
❌ Never call start() twice (IllegalThreadStateException)

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📌 Interview Tip

start() creates a new thread, run() does not.

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2️⃣ run() — Thread Execution Logic

🔹 Purpose

• Contains the business logic of the thread

🔹 Example

public void run() {
    System.out.println("Inside run()");
}

🔹 Important

Calling run() directly:

t.run();

❌ Runs like a normal method, NOT a thread

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🧠 Diagram

main thread
   |
   └── run()  ❌ (no new thread)

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3️⃣ sleep() — Pause Current Thread

🔹 Purpose

• Temporarily pauses execution

• Does not release lock

🔹 Example

Thread.sleep(1000); // 1 second

🔹 Diagram

RUNNING → sleep(1s) → RUNNABLE

🔹 Use Case

• Retry logic

• Polling

• Rate limiting

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4️⃣ join() — Wait for Thread to Finish

🔹 Purpose

• Makes one thread wait for another to complete

🔹 Example

Thread t = new Thread(() -> {
    System.out.println("Worker thread");
});
t.start();
t.join();
System.out.println("Main thread resumes");

🔹 Diagram

Main Thread
   |
   ├── start Worker
   |
   └── join() → WAIT
                 |
                 ▼
          Worker finishes
                 |
                 ▼
             Main resumes

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📌 Real-World Use

• Parallel tasks

• Batch processing

• Result aggregation

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5️⃣ wait() — Release Lock and Wait

🔹 Purpose

• Makes thread wait until notified

• Releases monitor (lock)

🔹 Must Be Called Inside

synchronized(obj) { ... }

🔹 Example

synchronized(lock) {
    lock.wait();
}

🔹 Diagram

Thread A (holds lock)
   |
   └── wait()
        ↓
   releases lock
        ↓
   WAITING

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6️⃣ notify() — Wake One Waiting Thread

🔹 Purpose

• Wakes one thread waiting on the same object

🔹 Example

synchronized(lock) {
    lock.notify();
}

🔹 Diagram

WAITING Threads
   |
   └── notify() → one thread wakes up

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7️⃣ notifyAll() — Wake All Waiting Threads

🔹 Purpose

• Wakes all threads waiting on the object

🔹 Example

synchronized(lock) {
    lock.notifyAll();
}

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🔄 wait() vs sleep()

Feature	wait()	sleep()
Releases lock	✅ Yes	❌ No
Needs synchronized	✅ Yes	❌ No
Wakes by notify	✅ Yes	❌ No
Used for	Inter-thread communication	Delay

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🔄 notify() vs notifyAll()

notify()	notifyAll()
Wakes one thread	Wakes all threads
Risk of starvation	Safer
Faster	Slightly slower

📌 Best practice: Prefer notifyAll() in complex systems.

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8️⃣ Real-World Producer–Consumer Example

class Shared {
    boolean dataReady = false;

    synchronized void produce() throws InterruptedException {
        while (dataReady) wait();
        System.out.println("Producing data");
        dataReady = true;
        notifyAll();
    }

    synchronized void consume() throws InterruptedException {
        while (!dataReady) wait();
        System.out.println("Consuming data");
        dataReady = false;
        notifyAll();
    }
}

🧠 Flow Diagram

Producer → produce() → notify()
Consumer → wait() → consume()

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⚠️ Common Mistakes (Interview Favorite)

❌ Calling wait() without synchronized
❌ Calling run() instead of start()
❌ Using notify() instead of notifyAll()
❌ Sleeping while holding locks

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🧠 Interview 30-Second Summary

start() creates a new thread, run() contains logic.
sleep() pauses without releasing locks.
join() waits for thread completion.
wait() and notify() enable inter-thread communication and must be used inside synchronized blocks.

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🧩 When to Use What?

Scenario	Method
Start parallel task	start()
Delay execution	sleep()
Wait for result	join()
Thread coordination	wait / notify
Multiple waiting threads	notifyAll

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🔑 Final Takeaway

Java threading is powerful but dangerous if misused.
Modern Java often prefers:

• ExecutorService

• CompletableFuture

• ForkJoinPool

…but interviews still expect core thread knowledge.