Java’s threading model can trip up even experienced developers. One of the most common sources of confusion? The difference between t.start() and t.run().

If you’ve ever written a multithreaded program in Java, you’ve likely encountered both. But what actually happens when you call t.start() instead of t.run() — and why does it matter?

Quick Answer: t.start() vs t.run()

• t.start(): Starts a new thread and executes the run() method in that new thread.
• t.run(): Calls the run() method in the current thread, just like a normal method.

Sounds simple, right? But the consequences are huge.

Let’s break it down with code examples and real-world implications.

A Little Threading Background

Java supports multithreading — the ability to run multiple threads (independent units of work) simultaneously. You can create threads by:

1. Extending the Thread class.
2. Implementing the Runnable interface.

In both cases, you override the run() method, which holds the code you want the thread to execute.

But here’s the twist: overriding run() doesn’t actually start the thread. That’s what start() is for.

t.start() vs t.run() in Action

public class Demo extends Thread {
    public void run() {
        System.out.println("Running in: " + Thread.currentThread().getName());
    }

public static void main(String[] args) {
        Demo t1 = new Demo();
        System.out.println("Calling t1.run()");
        t1.run();  // Executes in main thread
        System.out.println("Calling t1.start()");
        t1.start();  // Executes in new thread
    }
}

Output:

Calling t1.run()
Running in: main
Calling t1.start()
Running in: Thread-0

What’s Happening Here?

• t1.run() runs just like any regular method — in the main thread.
• t1.start() creates a new thread and calls run() in that thread.

That’s the real difference between t.start() vs t.run().

Why It Matters: Real-World Implications

1. Concurrency

Using t.start() enables true parallel execution. Your app can do multiple things at once.

Using t.run()? Everything runs sequentially in the same thread. No concurrency. Just like calling any other method.

2. Performance

On multi-core processors, t.start() lets Java take full advantage of your CPU. Each thread can run independently.

t.run()? No benefit — it runs synchronously, blocking other operations.

3. Thread Behavior

start() sets up everything: thread stack, scheduling, life cycle management.

run() bypasses that. You’re not creating a new thread; you’re just executing some code.

Common Mistakes to Avoid

Mistake 1: Using t.run() and expecting multithreading

Thread t = new Thread(() -> {
    System.out.println("Running in: " + Thread.currentThread().getName());
});

t.run(); // Looks like threading, but isn't!

You just called a method. No new thread is created. Use t.start() instead.

Mistake 2: Calling start() twice

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

t.start();  // OK
t.start();  // IllegalThreadStateException

Once a thread has been started, it can’t be restarted. You’ll get a runtime exception if you try.

Practical Tip: Always Use start() for Threads

If your goal is concurrency, always use start(). The only time you’d use run() directly is for testing or if you’re intentionally avoiding multithreading (rare).

Loop with Threads

for (int i = 0; i < 3; i++) {
    Thread t = new Thread(() -> {
        System.out.println("Running in: " + Thread.currentThread().getName());
    });
    t.start();  // Right choice
}

Want concurrency? Use start() — always.

t.start() vs t.run() in Java

When comparing t.start() vs t.run(), the key takeaway is this:

• start() kicks off a new thread.
• run() just runs the code in the current thread.

If you want real multithreading, go with t.start(). If you call t.run(), you’re just calling a method — no thread magic involved.

This difference is critical when building responsive, scalable Java applications — whether it’s for a web server, a game engine, or a background worker.

Key Takeaways

• t.start() creates a new thread and calls run() in it.
• t.run() runs like a normal method — no new thread.
• Always use start() to actually start threading.
• Don’t call start() more than once on the same thread.
• If it runs in the main thread, it’s not really multithreaded.

Conclusion

Understanding t.start() vs t.run() is foundational for writing efficient, concurrent Java programs. It’s not just about syntax — it’s about how your code executes in real time.

So next time you see run(), stop and ask: Am I actually starting a thread, or just calling a method?

If you want true multithreading, start() is your ticket.