Should I always use a parallel stream when possible?

Question

With Java 8 and lambdas it\'s easy to iterate over collections as streams, and just as easy to use a parallel stream. Two examples from the docs, the second one using parall

Answer 1

Never parallelize an infinite stream with a limit. Here is what happens:

    public static void main(String[] args) {
        // let's count to 1 in parallel
        System.out.println(
            IntStream.iterate(0, i -> i + 1)
                .parallel()
                .skip(1)
                .findFirst()
                .getAsInt());
    }

Result

    Exception in thread "main" java.lang.OutOfMemoryError
        at ...
        at java.base/java.util.stream.IntPipeline.findFirst(IntPipeline.java:528)
        at InfiniteTest.main(InfiniteTest.java:24)
    Caused by: java.lang.OutOfMemoryError: Java heap space
        at java.base/java.util.stream.SpinedBuffer$OfInt.newArray(SpinedBuffer.java:750)
        at ...

Same if you use .limit(...)

Explanation here: Java 8, using .parallel in a stream causes OOM error

Similarly, don't use parallel if the stream is ordered and has much more elements than you want to process, e.g.

public static void main(String[] args) {
    // let's count to 1 in parallel
    System.out.println(
            IntStream.range(1, 1000_000_000)
                    .parallel()
                    .skip(100)
                    .findFirst()
                    .getAsInt());
}

This may run much longer because the parallel threads may work on plenty of number ranges instead of the crucial one 0-100, causing this to take very long time.