Variable used in lambda expression should be final or effectively final

Question

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 - gt                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

User · Answer

In your example  you can replace the forEach with lamdba with a simple for loop and modify any variable freely  Or  probably  refactor your code so that you don t need to modify any variables  However  I ll explain for completeness what does the error mean and how to work around it   Java 8 Language Specification    15 27 2      Any local variable  formal parameter  or exception parameter used but not declared in a lambda expression must either be declared final or be effectively final    4 12 4   or a compile-time error occurs where the use is attempted    Basically you cannot modify a local variable  calTz in this case  from within a lambda  or a local anonymous class   To achieve that in Java  you have to use a mutable object and modify it  via a final variable  from the lambda  One example of a mutable object here would be an array of one element   private TimeZone extractCalendarTimeZoneComponent Calendar cal  TimeZone calTz        TimeZone   result     null        try           cal getComponents   getComponents  VTIMEZONE   forEach component - gt                                result 0                                         catch  Exception e            log warn  Unable to determine ical timezone   e             return result 0

User · Answer

Java 8 has a new concept called    Effectively final    variable  It means that a non-final local variable whose value never changes after initialization is called    Effectively Final       This concept was introduced because prior to Java 8   we could not use a non-final local variable in an anonymous class  If you wanna have access to a local variable in anonymous class  you have to make it final   When lambda was introduced  this restriction was eased  Hence to the need to make local variable final if it   s not changed once it is initialized as lambda in itself is nothing but an anonymous class    Java 8 realized the pain of declaring local variable final every time a developer used lambda  introduced this concept  and made it unnecessary to make local variables final  So if you see the rule for anonymous classes has not changed  it   s just you don   t have to write the final keyword every time when using lambdas   I found a good explanation here

User · Answer

if it is not necessary to modify the variable than a general workaround for this kind of problem would be to extract the part of code which use lambda and use final keyword on method-parameter

User · Answer

From a lambda  you can t get a reference to anything that isn t final  You need to declare a final wrapper from outside the lamda to hold your variable   I ve added the final  reference  object as this wrapper   private TimeZone extractCalendarTimeZoneComponent Calendar cal TimeZone calTz        final AtomicReference lt TimeZone gt  reference   new AtomicReference lt  gt          try          cal getComponents   getComponents  VTIMEZONE   forEach component- gt           VTimeZone v    VTimeZone  component             v getTimeZoneId               if reference get    null                   reference set TimeZone getTimeZone v getTimeZoneId   getValue                                        catch  Exception e              log warn  Unable to determine ical timezone   e             return reference get

User · Answer

A final variable means that it can be instantiated only one time  in Java you can t use non-final 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

User · Answer

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

User · Answer

A variable used in lambda expression should be a final or effectively final  but you can assign a value to a final one element array   private TimeZone extractCalendarTimeZoneComponent Calendar cal  TimeZone calTz        try           TimeZone calTzLocal     new TimeZone 1           calTzLocal 0    calTz          cal getComponents   get  VTIMEZONE   forEach component - gt                TimeZone v   component              v getTimeZoneId                if  calTzLocal 0     null                    calTzLocal 0    TimeZone getTimeZone v getTimeZoneId   getValue                                     catch  Exception e            log warn  Unable to determine ical timezone   e             return null

[java] Variable used in lambda expression should be final or effectively final

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