Java.Multithreading.What is the difference between “concurrency” and “parallelism”?

🧠 Quick Definition:

Term	Definition
Concurrency	Dealing with lots of tasks at once (not necessarily simultaneously)
Parallelism	Actually doing lots of tasks at the same time

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🔄 Concurrency: Switching Between Tasks

Think of it like a single chef cooking multiple dishes:

• The chef is switching between boiling water, chopping veggies, and baking.
• Only one thing happens at a time, but the chef manages multiple tasks efficiently.

📌 In code:

• You might use threads, coroutines, or async/await.
• You focus on structure, responsiveness, and handling many things.

// Threads taking turns (concurrently)
Thread A -> read DB
Thread B -> handle HTTP request
Thread C -> log user info

⚡ Parallelism: Doing Tasks Simultaneously

Now imagine multiple chefs, each cooking a different dish at the same time:

• They can chop, boil, and fry in parallel, using different hands or kitchens!

📌 In code:

• Requires multiple CPU cores
• True simultaneous execution

// Tasks run truly in parallel
ForkJoinPool pool = new ForkJoinPool(4);
pool.submit(() -> {
    task1(); // runs on core 1
    task2(); // runs on core 2
});

💬 A Famous Quote (Rob Pike):

“Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once.”

🧩 Summary Table

Feature	Concurrency	Parallelism
Focus	Structure, responsiveness	Speed, throughput
Can run on 1 CPU?	✅ Yes	❌ Needs multiple CPUs/cores
Example	Web server handling many requests	Matrix multiplication using 8 cores
Java Tools	Threads, Executors, Future	ForkJoinPool, parallelStream()
Analogy	One cook juggling dishes	Multiple cooks cooking simultaneously