Java Multithreading: Thread Class, Runnable Interface & Concurrency

Multithreading is a core feature of Java that enables concurrent execution of two or more parts of a program for maximum utilization of modern multi-core CPU hardware. A Thread is the smallest lightweight unit of dispatchable execution within a process.

Process vs. Thread:

• Process: An independent operating system program with its own dedicated memory space (heavyweight).
• Thread: A lightweight path of execution inside a process. All threads inside a Java application share the Heap memory, but each thread maintains its own independent Call Stack and Program Counter (PC).

The 2 Traditional Ways to Create a Thread in Java

Approach 1: Extending the java.lang.Thread Class

class WorkerThread extends Thread {
    @Override
    public void run() {
        System.out.println("Thread running: " + Thread.currentThread().getName());
    }
}

public class ThreadDemo1 {
    public static void main(String[] args) {
        WorkerThread t1 = new WorkerThread();
        t1.setName("Worker-Alpha");
        t1.start(); // Spawns new thread on OS!
    }
}

Approach 2: Implementing java.lang.Runnable (Best Practice)

Because Runnable is a @FunctionalInterface, it can be written cleanly as a lambda expression:

public class RunnableDemo {
    public static void main(String[] args) {
        // Lambda implementation of Runnable
        Runnable task = () -> {
            for (int i = 1; i <= 3; i++) {
                System.out.println("Processing async step " + i + " on " + Thread.currentThread().getName());
            }
        };

        Thread worker = new Thread(task, "BackgroundWorker");
        worker.start();
    }
}