What is the difference between 'competition' and 'parallelism' in the context of multithreading?

Concurrency is the ability of a system to handle multiple tasks simultaneously. These tasks can be executed on a single CPU core, switching between them over time (time-sharing principle). Not necessarily means executing tasks at the exact same physical moment.

Parallelism is the actual simultaneous execution of multiple tasks at the same physical moment in time, using multiple CPU cores or multiple processors.

Main difference:

• Concurrency: Managing multiple tasks that may or may not be executed simultaneously. Focus on structuring the program to handle multiple execution threads.
• Parallelism: Physical simultaneous execution of multiple tasks. Focus on utilizing available hardware resources to speed up execution.

// Example of concurrency (without guaranteed parallel execution)
public class ConcurrentExample {
    public static void main(String[] args) {
        Runnable task1 = () -> System.out.println("Task 1 started on thread: " + Thread.currentThread().getName());
        Runnable task2 = () -> System.out.println("Task 2 started on thread: " + Thread.currentThread().getName());

        new Thread(task1).start(); // May run on the same core
        new Thread(task2).start(); // May run on the same core
    }
}

// Example of parallelism (requires a multi-core processor)
import java.util.List;
import java.util.Arrays;
import java.util.stream.Stream;

public class ParallelExample {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

        numbers.parallelStream() // Uses ForkJoinPool for parallel execution
               .forEach(number -> System.out.println("Processing " + number + " on thread: " + Thread.currentThread().getName()));
    }
}

Aspect	Concurrency	Parallelism
Goal	Managing multiple tasks	Speeding up execution
Requirements	Multiple tasks/threads	Multi-core processor/multiple processors
Execution	Alternating execution	Simultaneous execution
Frequency	Can be achieved on a single core	Requires multiple cores/processors
Example	Multitasking on a single-core system	Processing a large data array in parallel