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
– francium, Commented Oct 29, 2017 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
– cpp beginner, Commented Oct 29, 2017 at 21:56
Thanks @cppbeginner. So when I add a return (without return value) , it just exits the loop? — Punter Vicky
– Punter Vicky, Commented Oct 29, 2017 at 21:57

Dawood ibn Kareem · Accepted Answer · 2017-10-29 21:58:29Z

17

I guess you want to do this:

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

edited Oct 29, 2017 at 21:58

Dawood ibn Kareem

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answered Oct 29, 2017 at 21:56

Jason Hu

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8 Comments

Dawood ibn Kareem Over a year ago

Probably not. There's an overhead in building a stream. Much better to do it the old-fashioned (pre Java 8) way.

Jason Hu Over a year ago

@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.

Dawood ibn Kareem Over a year ago

Maybe, but this is clearly a case where the "functional style" is the wrong solution.

Jason Hu Over a year ago

@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.

Didier L Over a year ago

@DawoodibnKareem Considering everything implemented with streams can be rewritten without, by your logic you should never use streams then.

|

pablochan · Accepted Answer · 2017-10-29 22:02:26Z

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, 2017 at 22:02

pablochan

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4 Comments

Punter Vicky Over a year ago

Thanks , how does return work when the return is void , I assume? Do return just exit the loop?

pablochan Over a year ago

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.

Punter Vicky Over a year ago

Thank you! So I guess it is same as continue.

pablochan Over a year ago

Yes, it's effectively the same as a continue in a "standard" loop.

Ousmane D. · Accepted Answer · 2017-10-29 22:21:29Z

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, 2017 at 22:21

answered Oct 29, 2017 at 21:54

Ousmane D.

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