CompletableFuture.runAsync(() ->... with a non-final variable

Question

I have this code that works fine:

String name = "Oscar";
CompletableFuture.runAsync(() -> doX(name));

Now I need to add some logic to the name variable:

String name = "Oscar";
if (x){
  name = "Tiger";
}
CompletableFuture.runAsync(() -> doX(name));

But now the compiler complains about Variable used in lambda expression should be final or effectively final

I understand from posts like this one that the name must be final or effectively final, but I wonder if there is a way to write the code differently in order to enable the logic on the name variable

If the code is indeed exactly as you wrote it, you could write `String name = x ? "Tiger" : "Oscar";` - this will make for a single initializer. But if the logic is more complicated, this may not work. — RealSkeptic, Feb 06 '19 at 10:52
Or possibly copy it into another variable to make it final as well. — Naman, Feb 06 '19 at 10:58
or worst,should not used, Use `String[] name`, and in lambda user `name[0]`. — dkb, Feb 06 '19 at 10:59
`CompletableFuture.completedFuture(name) .thenApplyAsync(this::doX)` use `ClassName::doX` if `doX` is `static`, use `thenAcceptAsync` if `doX` is `void`. — Holger, Feb 08 '19 at 19:59

score 2 · Accepted Answer · answered Feb 06 '19 at 11:20

2

You can use Conditional Operator, something like this:

        boolean condition = true;
        String name = condition ? "Tiger" : "Oscar";
        CompletableFuture.runAsync(() -> System.out.println(name));

Or using if statements:

 boolean condition = true;
    final String name;
    if(condition) {
        name = "Tiger";
    }else {
        name = "Oscar";
    }
    CompletableFuture.runAsync(() -> System.out.println(name));

answered Feb 06 '19 at 11:20

HRgiger

2,750
26
37

You can not change the value of a final variable. name = "Tiger" will not work. – vavasthi Feb 06 '19 at 13:49
@VinayAvasthi since he did not assign any value to it, it will work. you can assign a value to a final variable only once, which is what he did (if, else) – Uri Loya Jan 22 '20 at 09:43

score 0 · Answer 2 · answered Feb 06 '19 at 10:59

0

The lambda expression needs a final variable. If the initialization code is complicated enough then define a new final variable.

final String fName = name;
CompletableFuture.runAsync(() -> doX(fName));

answered Feb 06 '19 at 10:59

vavasthi

922
5
14

score 0 · Answer 3 · answered Feb 06 '19 at 11:42

Try to decouple resolver logic from the main flow:

public static void somewhere() {
    // Variable even may be explicitly final
    final String name = resolve(true);
    CompletableFuture.runAsync(() -> doX(name));
}

public String resolve(boolean condition) {
    if (condition)
        return "NameX";

    return "NameY";

    // Or:
    // return condition ? "NameX" : "NameY";
}

The advantage of the approach that you may add more complex conditions and can change this logic later without touching the original flow.

CompletableFuture.runAsync(() ->... with a non-final variable

3 Answers3