It’s not clear why you set up an example that does not address the actual question, you’re interested in. If you want to know, whether the processing is lazy when applying a short-circuiting operation like findFirst(), well, then use an example using findFirst() instead of forEach that processes all elements anyway. Also, put the logging statement right into the function whose evaluation you want to track:
Stream.of("hello", "world")
.flatMap(s -> {
System.out.println("flatMap function evaluated for \""+s+'"');
return s.chars().boxed();
})
.peek(c -> System.out.printf("processing element %c%n", c))
.filter(c -> c>'h')
.findFirst()
.ifPresent(c -> System.out.printf("found an %c%n", c));
flatMap function evaluated for "hello"
processing element h
processing element e
processing element l
processing element l
processing element o
found an l
This demonstrates that the function passed to flatMap gets evaluated lazily as expected while the elements of the returned (sub-)stream are not evaluated as lazy as possible, as already discussed in the Q&A you have linked yourself.
So, regarding your fetchDataFromInternet method that gets invoked from the function passed to flatMap, you will get the desired laziness. But not for the data it returns.
