Set object in lambda expression

Question

how could I set my object to another in lambda expression? I got error

variable used in lambda expression should be final or effectively final

with following code:

public MyObj myFun(MyObj o) {
   MyObj obj = null;

   myObjList.stream().filter(o -> ...).forEach(o -> {
      // I do some stuff here

      // I need here set obj to o, but I got error
      obj = o;
   });

   return myObj; // I need return found obj or null;
}

So you're trying to find the last object in the list that matches the condition? If so, this could be a duplicate of [this question](https://stackoverflow.com/q/21426843). — Dawood ibn Kareem, Oct 30 '17 at 19:13
no you are not allowed to do this in Java. state your actual end goal. — Jason Hu, Oct 30 '17 at 19:15
@DawoodibnKareem no, I am looking for some specific object in list, and next do some operations with that object, and on the end, set its to variable which I return, or variable stay null; — Denis Stephanov, Oct 30 '17 at 19:18
@DenisStephanov ok, but what should happen if there are multiple Elements that match your filter? Should it just use the first one? Or anyone? — Thomas Böhm, Oct 30 '17 at 19:19
@Aominè I think he wants `obj` to end up being the _last_ object that matches the condition, not the _first_. Unless, of course, there's only one such object; but he hasn't stated this. — Dawood ibn Kareem, Oct 30 '17 at 19:23
But how this solve my problem? I need do operations with founded object, also update some values in this object, and on the end set it into variable which I return on the on of function, but I can't due to error. — Denis Stephanov, Oct 30 '17 at 19:27
"Effectively final" mean that your code should work if you actually added the "final" keyword to the variable. Setting it violates this requirement. — Thorbjørn Ravn Andersen, Oct 30 '17 at 19:31
@Aominè Ooh, you're right. I missed that. So `findFirst` would be a really good solution then. Frankly though, if I were writing this, I'd use a good old for-each loop and an if statement. — Dawood ibn Kareem, Oct 30 '17 at 19:33
@ThorbjørnRavnAndersen for a variable to be effectively final it need not be declared final rather it needs not be reassigned after being assigned the first time. — Ousmane D., Oct 30 '17 at 19:36
@Aominè Would "should _still_ work" be better. The whole point of "effectively final" is that the variable is used as if it was declared final. — Thorbjørn Ravn Andersen, Oct 30 '17 at 19:48

score 5 · Accepted Answer · answered Oct 30 '17 at 19:26

5

You can't assign o to the outer variable. But you can return it as a result of you stream processing. Use map instead of forEach and return o from the mapper. Then do findAny or findFirst - this will give you an Optional<MyObj>. Finally do orElse(null) to return the found and processed object or null if none was found.

MyObj obj = myObjList
   .stream()
   .filter(o -> ...)
   .map(o -> {
       // I do some stuff here

       // I need here set obj to o, but I got error
       return o;
   })
   .findAny()
   .orElse(null);

answered Oct 30 '17 at 19:26

lexicore

42,748
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to OP - if the logic inside “map” is more than 2-3 lines I’d extract it into another method and call that from “map”. – Ousmane D. Oct 30 '17 at 19:28
No harm using findAny but that’s more used when performing Parallel processing. – Ousmane D. Oct 30 '17 at 19:31
Thans for answer, but in "some stuff" I also need update founded object, and after update return it. – Denis Stephanov Oct 30 '17 at 19:32
@Aominè What's "patel processing"? – lexicore Oct 30 '17 at 19:32
@DenisStephanov So what's the problem? Update it, return it. – lexicore Oct 30 '17 at 19:33
Oh yes, you right, sorry my bad. I'll check this as answer, thanks. – Denis Stephanov Oct 30 '17 at 19:34
1

Whatever updates you need to do to the object can be done _after_ you've finished trawling through the stream. – Dawood ibn Kareem Oct 30 '17 at 20:18

score 1 · Answer 2 · answered Oct 30 '17 at 19:30

Instead of using forEach instead consider using map(o->{...}).findFirst().get() (or similar depending on your logic if no item is relevant - your null case).

In general, you should let streams finish in a collector or a selector (findFirst/findAny etc.) because that will allow you to write simpler and clearer code

score 0 · Answer 3 · answered Oct 30 '17 at 19:28

0

Instead of streaming through it, you could simply iterate through it in a loop and then return the value when you find it.

The streamy sort of answer is probably to use something like findFirst.

answered Oct 30 '17 at 19:28

BillRobertson42

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Set object in lambda expression

3 Answers3

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