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 during the process the source is changed. The result will be unpredictable. This is irrespective of the processing mode of the stream parallel or sequential.

The source can be modified till the statement terminal operation is invoked. Beyond that the source should not be modified till the stream execution completes. So handling the concurrent modification in stream source is critical to have a consistent stream performance.

The above quotations are taken from here.

Can someone do some simple example that would shed lights on why mutating the stream source would give such big problems?

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.