How to print a count in Collections.forEach()

Question

So I've got a list as follows, with the lambda expression:

ArrayList<String> array = new ArrayList<>();
...
StringBuffer buf = new StringBuffer("");
array.stream().forEach((item) -> {
    buf.append("Counter here: " + item.toString() + "\n");
});
return buf.toString();

But i can't instantiate and modify a variable within the forEach lambda expression because it has to be final. Is there a way i can do this?

MutableInteger is available in apache-lang that has less overhead than AtomicInteger. Also avoid using StringBuffer as a local variable use StringBuilder (StringBuffer is synchronized and StringBuilder is not, unless you are going to update that buffer with multiple threads you are incurring the extra cost without benefit). — AlexC, Apr 18 '16 at 14:21

Alex · Accepted Answer · 2016-04-18T14:24:17.123

2

Use atomics for this:

AtomicInteger cnt = new AtomicInteger(0);
StringBuilder buf = new StringBuilder("");
array.stream().forEach((item) -> {
    buf.append("Counter here: " + cnt.incrementAndGet() + "\n");
});
return buf.toString();

edited Apr 18 '16 at 14:24

answered Apr 18 '16 at 14:18

Alex

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Wouldnt this code throw error for making `buf` `final` or _effectively final_ ?? – Lalit Rao Apr 19 '16 at 11:56
1

@LalitRao `cnt` and `buf` are effectively final, no error here. It'll work fine on Java 8. – Alex Apr 19 '16 at 11:59
Would u please explain me this in detail? I mean we are mutating the state of `buf` inside lambda right? So how it remains effectively final then ? – Lalit Rao Apr 19 '16 at 12:04
1

@LalitRao In few words: you're allowed to modify internal state of the objects you're referencing from lambda expression. But you're not allowed to change the object references themself, e.g. `buf.toString()` and `cnt.incrementAndGet()` are allowed but `buf = buf.append("test")` or for example `i = i + 1`, `i++` are not – Alex Apr 19 '16 at 12:25
Thanks mate :) Well explained – Lalit Rao Apr 19 '16 at 12:28

Bohemian · Answer 2 · 2016-04-18T14:36:51.763

0

Use an int[] of size 1 for the counter:

ArrayList<String> array = new ArrayList<>();
int[] count = {0};
return array.stream().map(item -> count[0]++ + ":" + item)
  .collect(Collectors.joining("\n", "", "\n"));

The int[] is effectively final. It's contents don't have to be.

Note how you don't even need the StringBuilder if you use the joining() collector.

edited Apr 18 '16 at 14:36

answered Apr 18 '16 at 14:30

Bohemian

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``I wish there was a feature that allowed folks to mark the place in the answer that is "not useful" when downvoting`` – Bohemian Apr 18 '16 at 14:42
2

`` I wish moderators and Java gold badge holders could vote to close duplicates appropriately.`` – Savior Apr 18 '16 at 14:47
@Pillar hmmm, well this answer isn't in the dupe, but ok.... – Bohemian Apr 18 '16 at 14:52
Isn’t the policy to add the answer there instead of not closing a dupe question? Anyway, you should consider that there might be a good reason for such an answer not being there… – Holger Apr 18 '16 at 16:42

How to print a count in Collections.forEach()

2 Answers2