Cannot refer to a non-final variable inside an inner class defined in a different method

Question

Edited  I need to change the values of several variables as they run several times thorugh a timer  I need to keep updating the values with every iteration through the timer  I cannot set the values to final as that will prevent me from updating the values however I am getting the error I describe in the initial question below  I had previously written what is below   I am getting the error  quot cannot refer to a non-final variable inside an inner class defined in a different method quot   This is happening for the double called price and the Price called priceObject  Do you know why I get this problem  I do not understand why I need to have a final declaration  Also if you can see what it is I am trying to do  what do I have to do to get around this problem   public static void main String args           int period   2000      int delay   2000       double lastPrice   0      Price priceObject   new Price        double price   0       Timer timer   new Timer         timer scheduleAtFixedRate new TimerTask             public void run                 price   priceObject getNextPrice lastPrice               System out println                lastPrice   price                   delay  period

User · Answer

To avoid strange side-effects with closures in java variables referenced by an anonymous delegate must be marked as final, so to refer to lastPrice and price within the timer task they need to be marked as final.

This obviously won't work for you because you wish to change them, in this case you should look at encapsulating them within a class.

public class Foo {
    private PriceObject priceObject;
    private double lastPrice;
    private double price;

    public Foo(PriceObject priceObject) {
        this.priceObject = priceObject;
    }

    public void tick() {
        price = priceObject.getNextPrice(lastPrice);
        lastPrice = price;
    }
}

now just create a new Foo as final and call .tick from the timer.

public static void main(String args[]){
    int period = 2000;
    int delay = 2000;

    Price priceObject = new Price();
    final Foo foo = new Foo(priceObject);

    Timer timer = new Timer();
    timer.scheduleAtFixedRate(new TimerTask() {
        public void run() {
            foo.tick();
        }
    }, delay, period);
}

User · Answer

With anonymous classes  you are actually declaring a  nameless  nested class  For nested classes  the compiler generates a new standalone public class with a constructor that will take all the variables it uses as arguments  for  named  nested classes  this is always an instance of the original enclosing class   This is done because the runtime environment has no notion of nested classes  so there needs to be a  automatic  conversion from a nested to a standalone class    Take this code for example   public class EnclosingClass       public void someMethod             String shared    hello            new Thread                 public void run                        this is not valid  won t compile                 System out println shared      this instance expects shared to point to the reference where the String object  hello  lives in heap                         start                change the reference  shared  points to  with a new value         shared    other hello            System out println shared             That won t work  because this is what the compiler does under the hood   public void someMethod         String shared    hello        new EnclosingClass 1 shared  start            change the reference  shared  points to  with a new value     shared    other hello        System out println shared       The original anonymous class is replaced by some standalone class that the compiler generates  code is not exact  but should give you a good idea    public class EnclosingClass 1 extends Thread       String shared      public EnclosingClass 1 String shared            this shared   shared             public void run             System out println shared             As you can see  the standalone class holds a reference to the shared object  remember that everything in java is pass-by-value  so even if the reference variable  shared  in EnclosingClass gets changed  the instance it points to is not modified  and all other reference variables pointing to it  like the one in the anonymous class  Enclosing 1   will not be aware of this  This is the main reason the compiler forces you to declare this  shared  variables as final  so that this type of behavior won t make it into your already running code    Now  this is what happens when you use an instance variable inside an anonymous class  this is what you should do to solve your problem  move your logic to an  instance  method or a constructor of a class    public class EnclosingClass       String shared    hello       public void someMethod             new Thread                 public void run                     System out println shared      this is perfectly valid                         start                change the reference  shared  points to  with a new value         shared    other hello            System out println shared             This compiles fine  because the compiler will modify the code  so that the new generated class Enclosing 1 will hold a reference to the instance of EnclosingClass where it was instantiated  this is only a representation  but should get you going    public void someMethod         new EnclosingClass 1 this  start            change the reference  shared  points to  with a new value     shared    other hello        System out println shared      public class EnclosingClass 1 extends Thread       EnclosingClass enclosing      public EnclosingClass 1 EnclosingClass enclosing            this enclosing   enclosing             public void run             System out println enclosing shared             Like this  when the reference variable  shared  in EnclosingClass gets reassigned  and this happens before the call to Thread run    you ll see  other hello  printed twice  because now EnclosingClass 1 enclosing variable will keep a reference to the object of the class where it was declared  so changes to any attribute on that object will be visible to instances of EnclosingClass 1   For more information on the subject  you can see this excelent blog post  not written by me   http   kevinboone net java inner html

User · Answer

Good explanations for why you can t do what you re trying to do already provided  As a solution  maybe consider   public class foo       static class priceInfo               public double lastPrice   0          public double price   0          public Price priceObject   new Price                public static void main   String args                    int period   2000          int delay   2000           final priceInfo pi   new priceInfo             Timer timer   new Timer              timer scheduleAtFixedRate   new TimerTask                          public void run                                  pi price   pi priceObject getNextPrice   pi lastPrice                    System out println                     pi lastPrice   pi price                            delay  period              Seems like probably you could do a better design than that  but the idea is that you could group the updated variables inside a class reference that doesn t change

User · Answer

The main concern is whether a variable inside the anonymous class instance can be resolved at run-time  It is not a must to make a variable final as long as it is guaranteed that the variable is inside the run-time scope  For example  please see the two variables  statusMessage and  statusTextView inside updateStatus   method   public class WorkerService extends Service    Worker  worker  ExecutorService  executorService  ScheduledExecutorService  scheduledStopService   TextView  statusTextView     Override public void onCreate          worker   new Worker this        worker monitorGpsInBackground            To get a thread pool service containing merely one thread      executorService   Executors newSingleThreadExecutor            schedule something to run in the future      scheduledStopService   Executors newSingleThreadScheduledExecutor        Override public int onStartCommand Intent intent  int flags  int startId         ServiceRunnable runnable   new ServiceRunnable this  startId        executorService execute runnable           the return value tells what the OS should        do if this service is killed for resource reasons        1  START STICKY  the OS restarts the service when resources become        available by passing a null intent to onStartCommand        2  START REDELIVER INTENT  the OS restarts the service when resources        become available by passing the last intent that was passed to the        service before it was killed to onStartCommand        3  START NOT STICKY  just wait for next call to startService  no        auto-restart     return Service START NOT STICKY      Override public void onDestroy          worker stopGpsMonitoring        Override public IBinder onBind Intent intent        return null     class ServiceRunnable implements Runnable        WorkerService  theService      int  startId      String  statusMessage       public ServiceRunnable WorkerService theService  int startId             theService   theService           startId   startId              Override     public void run               statusTextView   MyActivity getActivityStatusView                get most recently available location as a latitude              longtitude         Location location    worker getLocation            updateStatus  Starting                convert lat lng to a human-readable address         String address    worker reverseGeocode location           updateStatus  Reverse geocoding                Write the location and address out to a file          worker save location  address   ResponsiveUx out            updateStatus  Done             DelayedStopRequest stopRequest   new DelayedStopRequest  theService   startId               schedule a stopRequest after 10 seconds          theService  scheduledStopService schedule stopRequest  10  TimeUnit SECONDS              void updateStatus String message             statusMessage   message           if   statusTextView    null                 statusTextView post new Runnable                       Override                 public void run                          statusTextView setText  statusMessage

User · Answer

Just an another explanation  Consider this example below  public class Outer       public static void main String   args            Outer o   new Outer             o m1                     o null              public void m1               int x   10           class Inner               Thread t   new Thread new Runnable                     public void run                         for int i 0 i lt 10 i                              try                               Thread sleep 2000                                                         catch InterruptedException e                                  handle InterruptedException e                                                     System out println  Thread t running                                                                                                                new Inner   t start             System out println  m1 Completes              Here Output will be  m1 Completes   Thread t running  Thread t running  Thread t running                    Now method m1   completes and we assign reference variable o to null   Now Outer Class Object is eligible for GC but Inner Class Object is still exist who has  Has-A  relationship with Thread object which is running  Without existing Outer class object there is no chance of existing m1   method and without existing m1   method there is no chance of existing its local variable but if Inner Class Object uses the local variable of m1   method then everything is self explanatory   To solve this we have to create a copy of local variable and then have to copy then into the heap with Inner class object  what java does for only final variable because they are not actually variable they are like constants Everything happens at compile time only not at runtime

User · Answer

One solution I have noticed isn t mentioned  unless I missed it  if I did please correct me   is the use of a class variable  Ran into this issue attempting to run a new thread within a method  new Thread    Do Something      Calling doSomething   from the following will work  You do not necessarily have to declare it final  just need to change the scope of the variable so it is not collected before the innerclass  This is unless of course your process is huge and changing the scope might create some sort of conflict  I didn t want to make my variable final as it was in no way a final constant   public class Test        protected String var1      protected String var2       public void doSomething                 new Thread                         public void run                                 System out println  In Thread variable 1      var1                   System out println  In Thread variable 2      var2                           start

User · Answer

you can just declare the variable outside the outer class  After this  you will be able to edit the variable from within the inner class  I sometimes face similar problems while coding in android so I declare the variable as global and it works for me

User · Answer

You can only access final variables from the containing class when using an anonymous class   Therefore you need to declare the variables being used final  which is not an option for you since you are changing lastPrice and price   or don t use an anonymous class   So your options are to create an actual inner class  in which you can pass in the variables and use them in a normal fashion  or   There is a quick  and in my opinion ugly  hack for your lastPrice and price variable which is to declare it like so  final double lastPrice 1   final double price 1     and in your anonymous class you can set the value like this  price 0    priceObject getNextPrice lastPrice 0    System out println    lastPrice 0    price 0

User · Answer

Because it s confusing if the variable isn t final  as the changes to it won t be picked up in the anonymous class   Just make the variables  price  and  lastPrice  final   -- Edit  Oops  and you ll also need to not assign to them  obviously  in your function  You ll need new local variables  Anyway  I suspect someone has given you a better answer by now

User · Answer

To solve the problem above  different languages make different decisions   for Java  the solution is as what we see in this article   for C   the solution is allow side-effects and capture by reference is the only option   for C  11  the solution is to allow the programmer make the decision  They can choose to capture by value or by reference  If capturing by value  no side-effects would occur because the variable referenced is actually different  If capture by reference  side-effects may occur but the programmer should realize it

User · Answer

what worked for me is just define the variable outside this function of your   Just before main function declare i e   Double price  public static void main String   args    -------- --------

User · Answer

Can you make lastPrice  priceObject  and price fields of the anonymous inner class

User · Answer

You cannot refer to non-final variables because Java Language Specification says so  From 8 1 3   Any local variable  formal method parameter or exception handler parameter used but not declared in an inner class must be declared final   Whole paragraph  I can see only part of your code - according to me scheduling modification of local variables is a strange idea  Local variables cease to exist when you leave the function  Maybe static fields of a class would be better

User · Answer

I just wrote something to handle something along the authors intention   I found the best thing to do was to let the constructor take all the objects and then in your implemented method use that constructor objects    However  if you are writing a generic interface class  then you have to pass an Object  or better a list of Objects  This could be done by Object   or even better  Object     because it is easier to call   See my example piece just below   List lt String gt  lst   new ArrayList lt String gt     lst add  1    lst add  2             SomeAbstractClass p   new SomeAbstractClass  lst   another parameter   20  true                     public void perform                                         ArrayList lt String gt  lst    ArrayList lt String gt  getArgs   0                                      public abstract class SomeAbstractClass          private Object   args       public SomeAbstractClass Object     args            this args   args                              public abstract void perform                 public Object   getArgs             return args             Please see this post about Java closures that supports this out of the box  http   mseifed blogspot se 2012 09 closure-implementation-for-java-5-6-and html  Version 1 supports passing of non-final closures with autocasting  https   github com MSeifeddo Closure-implementation-for-Java-5-6-and-7 blob master org mo closure v1 Closure java      SortedSet lt String gt  sortedNames   new TreeSet lt String gt            NOTE  Instead of enforcing final  we pass it through the constructor     eachLine randomFile0  new V1 lt String gt  sortedNames            public void call String line                SortedSet lt String gt  sortedNames   castFirst        Read contructor arg zero  and auto cast it             sortedNames add extractName line

User · Answer

use ClassName this variableName to reference the non-final variable

User · Answer

Declare the variable as a static and reference it in the required method using className variable

User · Answer

If the variable required to be final  cannot be then you can assign the value of the variable to another variable and make THAT final so you can use it instead

User · Answer

When I stumble upon this issue  I just pass the objects to the inner class through the constructor  If I need to pass primitives or immutable objects  as in this case   a wrapper class is needed   Edit  Actually  I don t use an anonymous class at all  but a proper subclass   public class PriceData           private double lastPrice   0          private double price   0           public void setlastPrice double lastPrice                this lastPrice   lastPrice                     public double getLastPrice                 return lastPrice                     public void setPrice double price                this price   price                     public double getPrice                 return price                       public class PriceTimerTask extends TimerTask           private PriceData priceData          private Price priceObject           public PriceTimerTask PriceData priceData  Price priceObject                this priceData   priceData              this priceObject   priceObject                     public void run                 priceData setPrice priceObject getNextPrice lastPrice                System out println                priceData setLastPrice priceData getPrice                           public static void main String args               int period   2000          int delay   2000           PriceData priceData   new PriceData            Price priceObject   new Price             Timer timer   new Timer             timer scheduleAtFixedRate new PriceTimerTask priceData  priceObject   delay  period

User · Answer

If you want to change a value in a method call within an anonymous class  that  value  is actually a Future  So  if you use Guava  you can write      final SettableFuture lt Integer gt  myvalue   SettableFuture lt Integer gt  create        someclass run new Runnable         public void run                        myvalue set value                          return myvalue get

User · Answer

Java doesn t support true closures  even though using an anonymous class like you are using here  new TimerTask            looks like a kind of closure   edit - See the comments below - the following is not a correct explanation  as KeeperOfTheSoul points out   This is why it doesn t work   The variables lastPrice and price are local variables in the main   method  The object that you create with the anonymous class might last until after the main   method returns   When the main   method returns  local variables  such as lastPrice and price  will be cleaned up from the stack  so they won t exist anymore after main   returns   But the anonymous class object references these variables  Things would go horribly wrong if the anonymous class object tries to access the variables after they have been cleaned up   By making lastPrice and price final  they are not really variables anymore  but constants  The compiler can then just replace the use of lastPrice and price in the anonymous class with the values of the constants  at compile time  of course   and you won t have the problem with accessing non-existent variables anymore   Other programming languages that do support closures do it by treating those variables specially - by making sure they don t get destroyed when the method ends  so that the closure can still access the variables    Ankur  You could do this   public static void main String args          int period   2000      int delay   2000       Timer timer   new Timer         timer scheduleAtFixedRate new TimerTask                Variables as member variables instead of local variables in main           private double lastPrice   0          private Price priceObject   new Price            private double price   0           public void run                 price   priceObject getNextPrice lastPrice               System out println                lastPrice   price                   delay  period

[java] Cannot refer to a non-final variable inside an inner class defined in a different method

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