start() vs run()

Thread t = new Thread(() -> {
    System.out.println("Running in thread");
});

t.run();   // ❌ normal method call (NO new thread)
t.start(); // ✅ creates new thread

Aspect	start()	run()
Purpose	Starts a new thread	Contains the task logic
Thread creation	✅ Creates a new thread	❌ No new thread
Execution	Asynchronous (parallel/concurrent)	Synchronous (normal method call)
Call stack	New thread stack created	Uses current thread stack
JVM involvement	Calls native method → OS thread	Just a normal method
Thread state change	NEW → RUNNABLE	No state change
Who executes	New worker thread	Calling thread (e.g., main)
Multiple calls	❌throws IllegalThreadStateException exception	✅ Can call multiple times
Performance	True concurrency	No concurrency
Real-world usage	Always used to start threads	Never called directly (almost always)

When you call start():

start()
   ↓
JVM calls native method
   ↓
Creates new OS thread
   ↓
Calls run() internally in new thread

So

You NEVER call run() directly — JVM does it for you.