Concurrency and parallelism in Java

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 Concurrency and parallelism are two concepts in programming that deal with executing multiple tasks, but they differ in their approaches and purposes. Here’s a breakdown of each:

Concurrency

Concurrency is about dealing with multiple tasks at once but not necessarily executing them simultaneously. It allows a system to manage multiple tasks by interleaving their execution, giving the illusion that they are running in parallel.

  • Key Characteristics:

    • Task Interleaving: Multiple tasks make progress by being interleaved.
    • Context Switching: The system switches between tasks frequently.
    • Non-deterministic: The order of execution can vary.
    • Resource Sharing: Tasks share resources like CPU, memory, etc.

    In this example, thread1 and thread2 run concurrently, and the output will interleave the print statements from both threads.



public class ConcurrencyExample {
    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            for (int i = 0; i < 5; i++) {
                System.out.println("Thread 1: " + i);
            }
        });

        Thread thread2 = new Thread(() -> {
            for (int i = 0; i < 5; i++) {
                System.out.println("Thread 2: " + i);
            }
        });

        thread1.start();
        thread2.start();
    }
}


Parallelism

Parallelism is about performing multiple tasks simultaneously. It requires multiple processors or cores and is a subset of concurrency. In parallelism, tasks actually run at the same time.

  • Key Characteristics:

    • Simultaneous Execution: Tasks run at the same time on multiple processors or cores.
    • Deterministic Execution: Tasks can be executed in a predictable manner if properly managed.
    • Resource Utilization: Efficient use of multiple processors or cores.
    In this example, the threads are managed to run in parallel using a parallel stream in Java. This requires multiple cores to truly run in parallel.





public class ParallelismExample {
    public static void main(String[] args) {
        Runnable task1 = () -> {
            for (int i = 0; i < 5; i++) {
                System.out.println("Task 1: " + i);
                try {
                    Thread.sleep(100); // Simulate some work with a small sleep
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };

        Runnable task2 = () -> {
            for (int i = 0; i < 5; i++) {
                System.out.println("Task 2: " + i);
                try {
                    Thread.sleep(100); // Simulate some work with a small sleep
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };

        // Run tasks in parallel
        Arrays.asList(task1, task2).parallelStream().forEach(Runnable::run);
    }
}

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