Can we run two threads simultaneously?

Yes, you can run two threads simultaneously in Java, but it depends on the system's hardware and the nature of the threads. Specifically, whether or not the threads are running simultaneously (i.e., in parallel) depends on two factors:

1. Number of CPU Cores: If your system has multiple CPU cores, it is possible for two threads to run truly simultaneously on separate cores, meaning they will execute in parallel.
2. Thread Scheduling: If you only have a single CPU core, Java will still allow the threads to run concurrently (not simultaneously) by time-slicing, which means the threads will take turns executing on the same core. While it may appear that they are running simultaneously, they are actually being interleaved in small time slices.

Running Threads Simultaneously (Parallelism)

• If your system has multiple CPU cores (for example, a quad-core CPU), the operating system and Java Virtual Machine (JVM) will manage the threads in a way that each thread can run on a separate core. This is true parallelism and allows threads to execute at the same time.
• Modern multi-core processors allow this kind of parallel execution for CPU-bound tasks, where each thread executes in parallel on different cores.

Running Threads Concurrently (Time-slicing on a Single Core)

• On a single-core processor, only one thread can execute at any given time. However, Java still provides the illusion of simultaneous execution by using context switching—the operating system quickly switches between threads, giving the appearance that multiple threads are running at the same time. This is concurrency, not parallelism.
• The JVM, along with the operating system's thread scheduler, manages this switching so that each thread gets a small slice of time to execute before switching to the next thread. The threads execute one after another, but quickly enough that they appear to be running at the same time.

Example of Running Two Threads Simultaneously

Here’s an example of two threads running in Java:

class MyThread extends Thread {
    private String name;
    
    public MyThread(String name) {
        this.name = name;
    }

    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(name + " is running: " + i);
            try {
                Thread.sleep(500);  // Sleep for 500 milliseconds
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {
        // Create two threads
        MyThread thread1 = new MyThread("Thread 1");
        MyThread thread2 = new MyThread("Thread 2");

        // Start both threads
        thread1.start();
        thread2.start();
    }
}

Explanation:

• Concurrency: If you have a single CPU core, both threads will execute concurrently with the operating system switching between them quickly.
• Parallelism: If you have multiple cores, both threads will run in parallel on different cores.

How the JVM and OS Handle Thread Execution:

• Java does not control how threads are scheduled; the operating system and the JVM together decide when and how each thread gets CPU time.
• On multi-core systems, Java utilizes hardware parallelism by distributing threads across multiple CPU cores, allowing simultaneous execution.
• On a single-core system, Java uses time-slicing to alternate between threads quickly, providing the illusion of simultaneous execution (though only one thread is actually executing at a time).

Conclusion:

• Yes, you can run two threads simultaneously if your system has multiple CPU cores (true parallelism).
• On a single-core processor, threads will run concurrently (context switching), giving the illusion of simultaneous execution.

Recommended Courses

To deepen your understanding of multithreading and concurrency, and to prepare for interview questions, consider exploring the following courses from DesignGurus.io:

• Grokking Multithreading and Concurrency for Coding Interviews
• Grokking Data Structures & Algorithms for Coding Interviews
• Grokking the System Design Interview

These courses will provide a solid foundation in multithreading and concurrency, helping you prepare for technical interviews and real-world development challenges.