non-interference requirement on Java 8 streams

Question

I am a beginner in Java 8.

Non-interference is important to have consistent Java stream behaviour. Imagine we are process a large stream of data and

Answer 1

Well the oracle example is self-explanatory here. First one is this:

List<String> l = new ArrayList<>(Arrays.asList("one", "two"));
 Stream<String> sl = l.stream();
 l.add("three");
 String s = l.collect(Collectors.joining(" "));

If you change l by adding one more elements to it before you call the terminal operation (Collectors.joining) you are fine; but notice that the Stream consists of three elements, not two; at the time you created the Stream via l.stream().

On the other hand doing this:

  List<String> list = new ArrayList<>();
  list.add("test");
  list.forEach(x -> list.add(x));

will throw a ConcurrentModificationException since you can't change the source.

And now suppose you have an underlying source that can handle concurrent adds:

ConcurrentHashMap<String, Integer> cMap = new ConcurrentHashMap<>();
cMap.put("one", 1);
cMap.forEach((key, value) -> cMap.put(key + key, value + value));
System.out.println(cMap);

What should the output be here? When I run this it is:

 {oneoneoneoneoneoneoneone=8, one=1, oneone=2, oneoneoneone=4}

Changing the key to zx (cMap.put("zx", 1)), the result is now:

{zxzx=2, zx=1}

The result is not consistent.