Multithreading in Java

Run tasks concurrently with threads and executors, and protect shared data with synchronisation.

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ExecutorService pool = Executors.newFixedThreadPool(4);
Future<Double> result = pool.submit(() -> heavyCalculation());
System.out.println(result.get());   // waits for the result
pool.shutdown();

Shared mutable data must be protected — with synchronized, locks, or thread-safe classes like AtomicInteger and ConcurrentHashMap — to avoid race conditions. Java 21's virtual threads make huge numbers of concurrent tasks cheap.

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