Threading Basics

Advanced ~20 min read

Multithreading allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such program is called a Thread.

Creating a Thread

There are two main ways to create a thread in Java:

1. Extend Thread Class

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread running");
    }
}
// Start it:
MyThread t = new MyThread();
t.start();

2. Implement Runnable Interface (Preferred)

Better because you can still extend another class.

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Thread running");
    }
}
// Start it:
Thread t = new Thread(new MyRunnable());
t.start();

Thread Lifecycle

A thread goes through various states: New, Runnable, Running, Blocked/Waiting, Terminated.

Full Example

class Runner1 implements Runnable {
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Runner 1: " + i);
            try {
                Thread.sleep(500); // Sleep for 500ms
            } catch (InterruptedException e) {
                System.out.println(e);
            }
        }
    }
}

class Runner2 implements Runnable {
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Runner 2: " + i);
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                System.out.println(e);
            }
        }
    }
}

public class ThreadExample {
    public static void main(String[] args) {

// Create two threads
        Thread t1 = new Thread(new Runner1());
        Thread t2 = new Thread(new Runner2());

// Start them concurrently
        System.out.println("--- Starting Threads ---");
        t1.start();
        t2.start();

// Notice that the output will be interleaved
        // Because they run in parallel (depending on CPU availability)
    }
}

Output

Click Run to execute your code

Summary

Use start() to execute the run() method in a separate thread.
Directly calling run() does not start a new thread.
Runnable is generally preferred over extending Thread.

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