What are the main challenges and potential errors that can occur when using Parallel Stream from Java Stream API?

When using Parallel Stream from Java Stream API, the following difficulties and errors may occur:

• Thread safety: operations that modify external states can lead to errors, as parallel streams execute simultaneously.
• Unpredictable order: if the order of elements is important, parallel processing may disrupt it.
• Overhead: for small data sets, parallelism can be less efficient due to the costs of creating and managing streams.
• Using inappropriate operations: operations with side effects or non-associative operations can lead to incorrect results.

Example of an error:

List<Integer> list = Arrays.asList(1, 2, 3, 4);
List<Integer> result = new ArrayList<>();
list.parallelStream().forEach(i -> result.add(i)); // Incorrect, ArrayList is not thread-safe

Instead, thread-safe collections or methods that return new collections should be used.