Multithreading Basics

A Thread runs code concurrently with the rest of the program. Runnable describes what to run; Thread is what actually runs it.

Runnable task = () -> {
    for (int i = 0; i < 3; i++) {
        System.out.println("Working: " + i);
    }
};

Thread thread = new Thread(task);
thread.start();  // runs concurrently
thread.join();   // wait for it to finish before continuing

Common mistakes

• Calling thread.run() instead of thread.start() — run() just calls the method directly on the current thread, with no new thread created at all.
• Two threads modifying the same shared variable without synchronization, producing unpredictable results — the full treatment is in Concurrency in Depth, in Level 3.