Variable used in lambda expression should be final or effectively final

java lambda
user3610470 picture user3610470 · Jan 18, 2016 · Viewed 194.6k times · Source

Variable used in lambda expression should be final or effectively final

When I try to use calTz it is showing this error.

private TimeZone extractCalendarTimeZoneComponent(Calendar cal, TimeZone calTz) {
    try {
        cal.getComponents().getComponents("VTIMEZONE").forEach(component -> {
            VTimeZone v = (VTimeZone) component;
            v.getTimeZoneId();
            if (calTz == null) {
                calTz = TimeZone.getTimeZone(v.getTimeZoneId().getValue());
            }
        });
    } catch (Exception e) {
        log.warn("Unable to determine ical timezone", e);
    }
    return null;
}

Answer

Dioxin picture Dioxin · May 15, 2018

Although other answers prove the requirement, they don't explain why the requirement exists.

The JLS mentions why in §15.27.2:

The restriction to effectively final variables prohibits access to dynamically-changing local variables, whose capture would likely introduce concurrency problems.

To lower risk of bugs, they decided to ensure captured variables are never mutated.

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