Asked  7 Months ago    Answers:  5   Viewed   388 times

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;
}

 Answers

95

A final variable means that it can be instantiated only one time. in Java you can't reassign non-final local variables in lambda as well as in anonymous inner classes.

You can refactor your code with the old for-each loop:

private TimeZone extractCalendarTimeZoneComponent(Calendar cal,TimeZone calTz) {
    try {
        for(Component component : cal.getComponents().getComponents("VTIMEZONE")) {
        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;
}

Even if I don't get the sense of some pieces of this code:

  • you call a v.getTimeZoneId(); without using its return value
  • with the assignment calTz = TimeZone.getTimeZone(v.getTimeZoneId().getValue()); you don't modify the originally passed calTz and you don't use it in this method
  • You always return null, why don't you set void as return type?

Hope also these tips helps you to improve.

Tuesday, June 1, 2021
 
Eddas
answered 7 Months ago
69

An anonymous inner class (AIC) can be used to create a subclass of an abstract class or a concrete class. An AIC can also provide a concrete implementation of an interface, including the addition of state (fields). An instance of an AIC can be referred to using this in its method bodies, so further methods can be called on it, its state can be mutated over time, etc. None of these apply to lambdas.

I'd guess that the majority of uses of AICs were to provide stateless implementations of single functions and so can be replaced with lambda expressions, but there are other uses of AICs for which lambdas cannot be used. AICs are here to stay.

UPDATE

Another difference between AICs and lambda expressions is that AICs introduce a new scope. That is, names are resolved from the AIC's superclasses and interfaces and can shadow names that occur in the lexically enclosing environment. For lambdas, all names are resolved lexically.

Friday, June 4, 2021
 
Markol
answered 7 Months ago
90

It is related to multi-thread programming.

Local variables in Java have until now been immune to race conditions and visibility problems because they are accessible only to the thread executing the method in which they are declared. But a lambda can be passed from the thread that created it to a different thread, and that immunity would therefore be lost if the lambda, evaluated by the second thread, were given the ability to mutate local variables. - Source

Friday, July 23, 2021
 
ariel
answered 5 Months ago
21

We tend to forget that instanceCounter is actually this.instanceCounter, you are capturing this which is well, effectively final. As to why this is needed, the answer is obviously here

Thursday, July 29, 2021
 
muffe
answered 5 Months ago
84

You need to download the binaries which contain the Lambda expressions feature. Try downloading from here http://jdk8.java.net/lambda/. I remember reading in the mailing list that the lambda expression branch is being merged in the main JDK8 build, but not sure if its been done. But I use the build from the Lambda project page.

Wednesday, August 4, 2021
 
Ujjawal Khare
answered 4 Months ago
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