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https://issues.apache.org/jira/browse/GROOVY-8826?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16643108#comment-16643108
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Daniel Sun commented on GROOVY-8826:
------------------------------------
[~blackdrag] Thanks for your explanation in details. You are always my good
mentor. The issue is more complex than I thought...
Could you tell me why the following code get two {{2}}? In other words, why
will the capatured {{i}} be changed even if the {{i}} is captured by closure.
As far as I know, closure will store capatured variables in its instance
fields. If we assign the value to field, even if i changed, field will hold the
original value(i.e. 1). I do not understand why field's value will change while
{{i}} changes(See the second following code snippet). Thanks in advance!
{code:java}
import java.util.ArrayList;
import java.util.Arrays;
public class Test {
public static void main(String... args) {
ArrayList<Runnable> closures = new ArrayList<>();
for (int i: new int[] {1, 2})
closures.add(() -> System.out.println(i));
for (Runnable closure: closures)
closure.run();
}
}
{code}
{code:java}
Integer i = 1
Integer f = i // if we assign the value to f, even if i changed, f will hold
the original value, i.e. 1
i = 2
println f // gets 1
{code}
> Captured loop variables have surprising and Java-incompatible behavior
> ----------------------------------------------------------------------
>
> Key: GROOVY-8826
> URL: https://issues.apache.org/jira/browse/GROOVY-8826
> Project: Groovy
> Issue Type: Bug
> Affects Versions: 3.0.0-alpha-3
> Reporter: Роман Донченко
> Priority: Major
>
> Consider this source:
> {code:java}
> import java.util.ArrayList;
> import java.util.Arrays;
> public class Test {
> public static void main(String... args) {
> ArrayList<Runnable> closures = new ArrayList<>();
> for (int i: new int[] {1, 2})
> closures.add(() -> System.out.println(i));
> for (Runnable closure: closures)
> closure.run();
> }
> }
> {code}
> If it's compiled as Java, the output is:
> {code}
> 1
> 2
> {code}
> But if it's compiled as Groovy, the output is:
> {code}
> 2
> 2
> {code}
> In other words, in Java each iteration gets a unique instance of the {{i}}
> variable, which is captured in the corresponding closure, while in Groovy,
> only one {{i}} variable exists and is captured into both closures. IMO, the
> Groovy behavior is significantly less useful, and it would be nice if in 3.0
> it was changed to match Java.
> Groovy also exhibits the same behavior with Groovy closures and
> {{for(...in...)}} loops. I just used Java-compatible syntax for this example
> in order to contrast Groovy and Java.
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