Creating Threads in Java

Direct Answer

Java has two main ways to create a thread: extend Thread and override run(), or implement Runnable and pass it to a Thread (or an ExecutorService). Implementing Runnable is preferred because it separates the task from the execution and lets you reuse thread pools; extending Thread ties your class to a single execution and blocks subclassing anything else. In practice you rarely create threads directly — you submit Runnables/Callables to an ExecutorService, which manages a pool for you. There's also the modern Virtual Threads (Project Loom) approach for lightweight concurrency.

Code Example

// (1) EXTEND Thread (works, but ties you to a single execution).
class MyThread extends Thread {
  public void run() { System.out.println("running on " + getName()); }
}
new MyThread().start();

// (2) IMPLEMENT Runnable (PREFERRED: task separate from execution).
Runnable task = () -> System.out.println("running on " + Thread.currentThread());
new Thread(task).start();

// (3) PRACTICE: use an ExecutorService (pools threads, don't create directly).
ExecutorService pool = Executors.newFixedThreadPool(4);
pool.submit(task);                 // reuse a pooled thread
pool.submit(task);
pool.shutdown();                   // always shut down

// (4) MODERN: virtual threads (Project Loom) for lightweight I/O concurrency.
Thread.startVirtualThread(task);   // millions of cheap threads

Common Mistakes & Pitfalls

• Calling run() instead of start() — run() runs on the current thread, not a new one.
• Creating threads with new Thread().start() directly instead of using an ExecutorService pool.
• Extending Thread when you need to subclass something else (you've used your one superclass).