Java 8, collect method, creating a new arraylist and mutation

Question

I just want to clarify something regarding the code below:

public static void ageOverTen(final List<Human> man) {
    final List<Human> byAge = man.stream()
            .filter(age -> age.getAge() > 10)
            .collect(ArrayList::new, ArrayList::add, ArrayList::addAll);

    System.out.println("People over 10 years: " + byAge);
}

In the Java world we all know ArrayList is not thread safe and it's mutable, so when you create a stream and use a collect() method, and in it we create an ArrayList. How does the library affect ArrayList thread safety and mutation, to make it comply with functional programming concept of immutability?

In particular, in the linked answer: "*This also applies to any of your own non-concurrent collectors that you might implement. They can be used safely with parallel streams, provided your collectors don't interfere with the stream source, are side-effect free, order independent, etc.*" — assylias, May 27 '14 at 17:43

score 2 · Accepted Answer · answered May 27 '14 at 17:52

Yes, collect() is designed to work even in parallel, even with non-thread-safe mutable data structures -- provided you follow the rules. These rules include non-interference -- that the behavioral parameters passed to the stream methods (lambdas) do not modify the source of the stream during the operation, and for most operations, statelessness -- that the lambdas do not access state that might change during the computation.

Java 8, collect method, creating a new arraylist and mutation

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