Java threads — creating and starting threads

Creating Threads

A thread is an independent path of execution within a program. Java threads map to OS threads and run concurrently on available CPU cores, enabling parallelism and responsive applications.

Three Ways to Create a Thread

// 1. Extend Thread
class PrintThread extends Thread {
    @Override
    public void run() { System.out.println("Thread: " + getName()); }
}

// 2. Implement Runnable
Runnable task = new Runnable() {
    @Override
    public void run() { System.out.println("Runnable task"); }
};

// 3. Lambda (preferred for simple tasks)
Runnable lambda = () -> System.out.println("Lambda thread");

Starting Threads

Thread t1 = new PrintThread();
Thread t2 = new Thread(lambda, "WorkerThread");

t1.start(); // new OS thread — calls run() concurrently
t2.start();

// WRONG: t1.run() — synchronous call, no new thread created

t1.join(); // current thread waits until t1 finishes
t2.join();
System.out.println("Both threads done");

Thread.sleep(1000); // pause current thread 1 second

Always call start(), never run() directly. Calling run() is just a regular method call on the current thread.

Daemon threads (setDaemon(true) before start()) die automatically when all non-daemon threads finish, making them suitable for background tasks like logging that should not block JVM shutdown. The main thread is non-daemon by default.

Use Thread.currentThread().interrupt() in catch blocks for InterruptedException to restore the interrupted status. Swallowing the interrupted flag silently prevents the calling code from knowing a shutdown was requested.