What difficulties may arise when regularly applying Parallel Stream in Java?

When using Parallel Stream in Java regularly, the following difficulties may arise:

• Thread safety issues: if operations within the stream modify shared data without synchronization, it can lead to errors.
• Unpredictable performance: for small collections or simple operations, overhead from parallelization may outweigh the benefits.
• Processing order: Parallel Stream does not guarantee the order of elements, which can be critical for some tasks.
• Use of shared resources: if operations access external resources (e.g., databases), parallelism can cause deadlocks or overload.
• Debugging and testing: parallel code is more complex to debug due to potential race conditions and unpredictable behavior.

Example:

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

Here, parallelism is justified because the operation is pure and order-independent.