stream and parallelStream

Question

I have a test code like this :

List list = new ArrayList<>(1000000);

for(int i=0;i<1000000;i++){
    list.add(i);
}

List

        

Answer 1

values.add(String) is not thread safe. When you invoke this method from different threads without synchronization it is no guarantee that it will work as expected.

To fix that you can:

• use thread-safe collection like Vector or CopyOnWriteArrayLis.
• Explicitly synchronize your code. For example put synchronize(this){values.add(new Date().toString())} into your code. Note i-> is outside synchronize block
• Or in this case map elments to get new stream like in @PeterLawrey answer: IntStream.range(0, 1_000_000).parallel().mapToObj(i -> new Date().toString()).collect(Collectors.toList());