Can you explain what parallel streams mean in Java Stream API?

Using parallel streams in Java Stream API means that operations on collection elements are executed concurrently in multiple threads, which can speed up processing large volumes of data.

When you call the parallelStream() method instead of stream(), Java automatically splits the data into parts and processes them in parallel using a thread pool from ForkJoinPool.

Example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int sum = numbers.parallelStream()
                 .mapToInt(Integer::intValue)
                 .sum();

Here, the sum is calculated in parallel, which can be more efficient on multi-core processors.

However, it is important to consider that parallel streams are not suitable for all tasks — the overhead of managing threads can outweigh the benefits for small data volumes or operations with side effects.