Return a value from Java 8 forEach loop

Question

In the below example , someObjects is a set. I am trying to return true if a condition matches within the loop , however this doesn't seem to compile. However when I just add "return" it works fine.What is the issue that I need to fix?

public boolean find(){

    someObjects.forEach(obj -> {
       if (some_condition_met) {
          return true;
       }
    });

    return false;
}

Compilation Errors

The method forEach(Consumer) in the type Iterable is not applicable for the arguments (( obj) -> {})

What are the compiler errors? Add the output to the question. — francium, Oct 29 '17 at 21:54
It is impossible to return from the outer method inside the body of a lambda. Use a for loop instead — cpp beginner, Oct 29 '17 at 21:56
Thanks @cppbeginner. So when I add a return (without return value) , it just exits the loop? — Punter Vicky, Oct 29 '17 at 21:57
The method being implemented by the lambda is `public void accept(Object t)`, the `return` type is `void` so `return;` is valid but `return true;` is not. — d.j.brown, Oct 29 '17 at 21:59
Return inside the lambda passed to forEach is like continue in a normal loop. — cpp beginner, Oct 29 '17 at 21:59
Is there some reason you want to use `forEach`? Really, the best solution for this is the old-fashioned one. You don't need the Java 8 goodies here. — Dawood ibn Kareem, Oct 29 '17 at 21:59
I love this answer. https://stackoverflow.com/a/20177092/6253321 — cpp beginner, Oct 29 '17 at 22:05
Thanks @DawoodibnKareem. Appreciate your help , will switch to old fashioned for-loop. I didn't have any specific reason for using it. — Punter Vicky, Oct 29 '17 at 22:08

score 16 · Answer 1 · edited Oct 29 '17 at 21:58

16

I guess you want to do this:

public boolean find(){
    return someObjects.stream().anyMatch(o -> your_condition);
}

edited Oct 29 '17 at 21:58

Dawood ibn Kareem

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answered Oct 29 '17 at 21:56

Jason Hu

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Probably not. There's an overhead in building a stream. Much better to do it the old-fashioned (pre Java 8) way. – Dawood ibn Kareem Oct 29 '17 at 21:57
5

@DawoodibnKareem i assume OP wants something in functional style. judging from his scores, he should know how to do it in the old-fashioned way. – Jason Hu Oct 29 '17 at 21:59
1

Maybe, but this is clearly a case where the "functional style" is the wrong solution. – Dawood ibn Kareem Oct 29 '17 at 22:00
Thank you very much. I'll do the old fashioned way then. – Punter Vicky Oct 29 '17 at 22:01
@DawoodibnKareem how can you be so affirmative about it? This solution is perfectly fine. In fact, this is even clearly a case where the functional solution is cleaner and simpler than the old-style one. If it's about the overhead, how is that a problem? You should never use streams if you don't accept their overhead. – Didier L Oct 29 '17 at 23:35
2

@DidierL I entirely agree with you. real life software practice puts micro performance like this to the very back of the priority queue. there are lots of arguments to object that claim. – Jason Hu Oct 29 '17 at 23:37
OK, "wrong" was the wrong word for me to use. How about "not as good"? I would _never_ favour a solution that builds a stream over a solution that doesn't; and I certainly don't find this "cleaner and simpler" than the old-fashioned solution, once you write the lambda out in full. – Dawood ibn Kareem Oct 29 '17 at 23:39
2

@DawoodibnKareem Considering everything implemented with streams can be rewritten without, by your logic you should never use streams then. – Didier L Oct 29 '17 at 23:41

score 11 · Accepted Answer · answered Oct 29 '17 at 22:02

11

The forEach method in a Collection expects a Consumer which means a function that takes a value, but doesn't return anything. That's why you can't use return true; but a return; works fine.

I you want to break out of the loop when your condition is met, it's better to use a simple for(...) loop. I assumed that the type of obj is Object:

for (Object obj : someObjects) {
  if (some_condition_met) {
    return true;
  }
}

return false;

answered Oct 29 '17 at 22:02

pablochan

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Thanks , how does return work when the return is void , I assume? Do return just exit the loop? – Punter Vicky Oct 29 '17 at 22:03
No. In your example a `return;` will simply exit the lambda and proceed to the next element in the collection. The loop will not be ended. – pablochan Oct 29 '17 at 22:06
Thank you! So I guess it is same as continue. – Punter Vicky Oct 29 '17 at 22:07
Yes, it's effectively the same as a `continue` in a "standard" loop. – pablochan Oct 29 '17 at 22:08

Ousmane D. · Answer 3 · 2017-10-29T22:21:29.803

4

forEach accepts a Consumer therefore you cannot pass in a behaviour that does not return void. you need to do something like:

return someObjects.stream().anyMatch(e -> condition);

edited Oct 29 '17 at 22:21

answered Oct 29 '17 at 21:54

Ousmane D.

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Return a value from Java 8 forEach loop

3 Answers3