<ul><li>When using .parallelStream() in Java, it is expected to achieve parallel execution across multiple threads.</li><li>However, there are cases where everything still runs in a single thread and no parallelism is observed.</li><li>The reason for this is that the flatMap function forcibly turns a parallel stream back into a sequential stream.</li><li>To achieve parallelism, it is recommended to flip the streams by looping over the products instead of users.</li></ul>

Why Your `.parallelStream()` Might Not Be Parallel at All

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