What is the difference between using Stream and ParallelStream when processing collections in Java?

In Java, Stream and ParallelStream are two ways to process collections, differing in how operations are executed:

• Stream — sequential processing of collection elements. All operations are performed in a single thread, which simplifies debugging and guarantees the order of processing.

• ParallelStream — parallel processing, where the collection is divided into parts, and each part is processed in a separate thread. This can speed up execution with large data volumes and resource-intensive operations.

Example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

// Sequential Stream
int sum = numbers.stream()
                 .mapToInt(Integer::intValue)
                 .sum();

// Parallel Stream
int parallelSum = numbers.parallelStream()
                         .mapToInt(Integer::intValue)
                         .sum();

It is important to consider that ParallelStream is not suitable for all tasks: if operations are inexpensive or the collection is small, parallelism may introduce overhead and slow down processing. Also, when processing in parallel, caution is needed regarding state and side effects.