Show progress of Java 8 stream processing

GnxR :

I have a Stream processing a few millions of elements. The Map-Reduce algorithm behind it takes a few milliseconds, so task completion takes about twenty minutes.

Stream<MyData> myStream = readData();
MyResult result = myStream
    .map(row -> process(row))
    .peek(stat -> System.out.println("Hi, I processed another item"))
    .reduce(MyStat::aggregate);

I'd like a way to display overall progress, instead of printing a line per element (which results in thousands of lines per second, takes time and doesn't provide any useful information regarding overall progress). I would like to display something similar to:

 5% (08s)
10% (14s)
15% (20s)
...

What would be the best (and/or easiest) way to do this?

Yassin Hajaj :

First of all, Streams are not meant to achieve these kind of tasks (as opposed to a classic data structure). If you know already how many elements your stream will be processing you might go with the following option, which is, I repeat, not the goal of streams.

Stream<MyData> myStream = readData();
final AtomicInteger loader = new AtomicInteger();
int fivePercent = elementsCount / 20;
MyResult result = myStream
    .map(row -> process(row))
    .peek(stat -> {
        if (loader.incrementAndGet() % fivePercent == 0) {
            System.out.println(loader.get() + " elements on " + elementsCount + " treated");
            System.out.println((5*(loader.get() / fivePercent)) + "%");
        }
    })
    .reduce(MyStat::aggregate);