Multithreading: Doing Many Things at Once

In a standard Java program, code runs in a single "Thread" (the main thread). If one task takes a long time (like downloading a file or processing a heavy config), the whole program freezes. Multithreading allows you to spawn background workers to handle these tasks while the main program stays responsive.

1. Two Ways to Create a Thread

A. Extending the Thread Class

You create a subclass and override the run() method.

class MyWorker extends Thread {
    public void run() {
        System.out.println("Worker thread is running...");
    }
}

// To start:
MyWorker t1 = new MyWorker();
t1.start(); // This spawns the new thread

B. Implementing the Runnable Interface (Recommended)

This is more flexible because your class can still extend another parent class (like a BaseController).

class MyTask implements Runnable {
    public void run() {
        System.out.println("Runnable task is running...");
    }
}

// To start:
Thread t1 = new Thread(new MyTask());
t1.start();

2. Important Thread Methods

3. The synchronized Keyword (Safety First)

When two threads try to change the same variable at the exact same time, you get a "Race Condition" (data corruption). We use synchronized to lock a method so only one thread can enter at a time.

class Counter {
    int count = 0;

    // Only one thread can increment at a time
    public synchronized void increment() {
        count++;
    }
}

💻 Full Practical Example: Parallel Processing

Copy this to see how two threads run independently in the background:

public class MultiTasking {
    public static void main(String[] args) {
        
        Thread downloadThread = new Thread(() -> {
            for (int i = 1; i <= 5; i++) {
                System.out.println("Downloading: " + (i * 20) + "%");
                try { Thread.sleep(1000); } catch (InterruptedException e) {}
            }
            System.out.println("Download Complete!");
        });

        Thread musicThread = new Thread(() -> {
            while (downloadThread.isAlive()) {
                System.out.println("♪ Playing background music ♪");
                try { Thread.sleep(1500); } catch (InterruptedException e) {}
            }
        });

        downloadThread.start();
        musicThread.start();
    }
}

⚠️ Common Pitfall: start() vs run()

• If you call t1.start(), Java creates a new thread.
• If you call t1.run(), Java runs the code on the current thread (no multitasking happens).

💡 Challenge: The Thread Race

1. Create a Runnable task that prints its name and numbers 1 to 10.
2. Inside the loop, make the thread sleep for 500ms.
3. In main, create two threads with different names (e.g., "Thread-A" and "Thread-B").
4. Start both and watch how their output overlaps in the terminal!