Java Academy · Lesson

Thread Safety and Side Effects

Avoid shared mutable state.

Parallel Means Concurrent Access

In a parallel stream, multiple threads run your lambdas at the same time. Any shared mutable state they touch becomes a race condition.

import java.util.stream.IntStream;

public class Main {
    public static void main(String[] args) {
        int safe = IntStream.rangeClosed(1, 1000).parallel().sum();
        System.out.println(safe);
    }
}

The Danger: Mutating a Shared Variable

A plain int counter touched by many threads loses updates. Never accumulate into shared mutable state inside forEach.

import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.IntStream;

public class Main {
    public static void main(String[] args) {
        AtomicInteger counter = new AtomicInteger();
        IntStream.rangeClosed(1, 1000).parallel()
            .forEach(n -> counter.incrementAndGet());
        System.out.println(counter.get());
    }
}

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