Why are only final variables accessible in anonymous class

Question

a can only be final here   Why   How can I reassign a in onClick   method without keeping it as private member   private void f Button b  final int a       b addClickHandler new ClickHandler               Override         public void onClick ClickEvent event                int b   a 5                        How can I return the 5   a when it clicked  I mean   private void f Button b  final int a       b addClickHandler new ClickHandler               Override         public void onClick ClickEvent event                 int b   a 5               return b     but return type is void

User · Answer

Java anonymous class is very similar to Javascript closure, but Java implement that in different way. (check Andersen's answer)

So in order not to confuse the Java Developer with the strange behavior that might occur for those coming from Javascript background. I guess that's why they force us to use final, this is not the JVM limitation.

Let's look at the Javascript example below:

var add = (function () {
  var counter = 0;

  var func = function () {
    console.log("counter now = " + counter);
    counter += 1; 
  };

  counter = 100; // line 1, this one need to be final in Java

  return func;

})();


add(); // this will print out 100 in Javascript but 0 in Java

In Javascript, the counter value will be 100, because there is only one counter variable from the beginning to end.

But in Java, if there is no final, it will print out 0, because while the inner object is being created, the 0 value is copied to the inner class object's hidden properties. (there are two integer variable here, one in the local method, another one in inner class hidden properties)

So any changes after the inner object creation (like line 1), it will not affect the inner object. So it will make confusion between two different outcome and behaviour (between Java and Javascript).

I believe that's why, Java decide to force it to be final, so the data is 'consistent' from the beginning to end.

User · Answer

As noted in comments  some of this becomes irrelevant in Java 8  where final can be implicit  Only an effectively final variable can be used in an anonymous inner class or lambda expression though     It s basically due to the way Java manages closures   When you create an instance of an anonymous inner class  any variables which are used within that class have their values copied in via the autogenerated constructor  This avoids the compiler having to autogenerate various extra types to hold the logical state of the  local variables   as for example the C  compiler does     When C  captures a variable in an anonymous function  it really captures the variable - the closure can update the variable in a way which is seen by the main body of the method  and vice versa    As the value has been copied into the instance of the anonymous inner class  it would look odd if the variable could be modified by the rest of the method - you could have code which appeared to be working with an out-of-date variable  because that s effectively what would be happening    you d be working with a copy taken at a different time   Likewise if you could make changes within the anonymous inner class  developers might expect those changes to be visible within the body of the enclosing method   Making the variable final removes all these possibilities - as the value can t be changed  at all  you don t need to worry about whether such changes will be visible  The only ways to allow the method and the anonymous inner class see each other s changes is to use a mutable type of some description  This could be the enclosing class itself  an array  a mutable wrapper type    anything like that  Basically it s a bit like communicating between one method and another  changes made to the parameters of one method aren t seen by its caller  but changes made to the objects referred to by the parameters are seen   If you re interested in a more detailed comparison between Java and C  closures  I have an article which goes into it further  I wanted to focus on the Java side in this answer

User · Answer

Maybe this trick gives u an idea  Boolean var  new anonymousClass        private String myVar    String for example      Overriden public Boolean method int i               use myVar and i           public String setVar String var  myVar var  return this     Returns self instane   setVar  Hello   method 3

User · Answer

Well  in Java  a variable can be final not just as a parameter  but as a class-level field  like  public class Test    public final int a   3    or as a local variable  like  public static void main String   args     final int a   3    If you want to access and modify a variable from an anonymous class  you might want to make the variable a class-level variable in the enclosing class   public class Test    public int a   public void doSomething        Runnable runnable      new Runnable            public void run              System out println a        a   a 1                     You can t have a variable as final and give it a new value  final means just that  the value is unchangeable and final   And since it s final  Java can safely copy it to local anonymous classes  You re not getting some reference to the int  especially since you can t have references to primitives like int in Java  just references to Objects    It just copies over the value of a into an implicit int called a in your anonymous class

User · Answer

An anonymous class is an inner class and the strict rule applies to inner classes  JLS 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  Any local variable  used but not declared in an inner class must be definitely assigned before the body of the inner class    I haven t found a reason or an explanation on the jls or jvms yet  but we do know  that the compiler creates a separate class file for each inner class and it has to make sure  that the methods declared on this class file  on byte code level  at least have access to the values of local variables    Jon has the complete answer - I keep this one undeleted because one might interested in the JLS rule

User · Answer

When an anonymous inner class is defined within the body of a method  all variables declared final in the scope of that method are accessible from within the inner class  For scalar values  once it has been assigned  the value of the final variable cannot change  For object values  the reference cannot change  This allows the Java compiler to  capture  the value of the variable at run-time and store a copy as a field in the inner class  Once the outer method has terminated and its stack frame has been removed  the original variable is gone but the inner class s private copy persists in the class s own memory    http   en wikipedia org wiki Final  28Java 29

User · Answer

Try this code   Create Array List and put value inside that and return it    private ArrayList f Button b  final int a        final ArrayList al   new ArrayList        b addClickHandler new ClickHandler                Override         public void onClick ClickEvent event                 int b   a 5               al add b                         return al

User · Answer

To understand the rationale for this restriction  consider the following program   public class Program        interface Interface           public void printInteger              static Interface interfaceInstance   null       static void initialize int val            class Impl implements Interface                Override             public void printInteger                     System out println val                                   interfaceInstance   new Impl               public static void main String   args            initialize 12345           interfaceInstance printInteger              The interfaceInstance remains in memory after the initialize method returns  but the parameter val does not   The JVM can   t access a local variable outside its scope  so Java makes the subsequent call to printInteger work by copying the value of val to an implicit field of the same name within interfaceInstance   The interfaceInstance is said to have captured the value of the local parameter   If the parameter weren   t final  or effectively final  its value could change  becoming out of sync with the captured value  potentially causing unintuitive behavior

User · Answer

There is a trick that allows anonymous class to update data in the outer scope   private void f Button b  final int a        final int   res   new int 1       b addClickHandler new ClickHandler              Override         public void onClick ClickEvent event                res 0    a   5                            But at this point handler is most likely not executed yet         How should we now res 0  is ready      However  this trick is not very good due to the synchronization issues  If handler is invoked later  you need to 1  synchronize access to res if handler was invoked from the different thread 2  need to have some sort of flag or indication that res was updated  This trick works OK  though  if anonymous class is invoked in the same thread immediately  Like           final int   res   new int 1   Runnable r   new Runnable     public void run     res 0    123       r run    System out println res 0

User · Answer

Java final variable inside an inner class About  inner class can use only  reference from outer class final local variables from out of scope which are a reference type  e g  Object     value primitive   e g  int     type can be wrapped by a final reference type  IntelliJ IDEA can help you covert it to one element array  When a non static nested  inner class  is generated by compiler - a new class -  lt OuterClass gt   lt InnerClass gt  class is created and bound parameters are passed into constructor Local variable on stack   It is similar to closure final variable is a variable which can not be reassign  final reference variable still can be changed by modifying a state If it was be possible it would be weird because as a programmer you could make like this   Not possible  private void foo          MyClass myClass   new MyClass      Case 1  address 1     int a   5    Case 2  a   5      Button button   new Button           just as an example     button addClickHandler new ClickHandler                       Override         public void onClick ClickEvent event                                myClass something       lt - what is the address               int b   a     lt - 5 or 10                  illusion that next changes are visible for Outer class             myClass   new MyClass                a   15                                        myClass   new MyClass      Case 1  address 2     int a   10    Case 2  a   10

User · Answer

private void f Button b  final int a           b addClickHandler new ClickHandler               Override         public void onClick ClickEvent event                a 0    a 0    5

User · Answer

As Jon has the implementation details answer an other possible answer would be that the JVM doesn t want to handle write in record that have ended his activation   Consider the use case where your lambdas instead of being apply  is stored in some place and run later   I remember that in Smalltalk you would get an illegal store raised when you do such modification

User · Answer

Methods within an anonomyous inner class may be invoked well after the thread that spawned it has terminated  In your example  the inner class will be invoked on the event dispatch thread and not in the same thread as that which created it  Hence  the scope of the variables will be different  So to protect such variable assignment scope issues you must declare them final

User · Answer

The reason why the access has been restricted only to the local final variables is that if all the local variables would be made accessible then they would first required to be copied to a separate section where inner classes can have access to them and maintaining multiple copies of mutable local variables may lead to inconsistent data  Whereas final variables are immutable and hence any number of copies to them will not have any impact on the consistency of data

User · Answer

You can create a class level variable to get returned value  I mean  class A       int k   0      private void f Button b  int a           b addClickHandler new ClickHandler              Override         public void onClick ClickEvent event                k   a   5                        now you can get value of K and use it where you want   Answer of your why is     A local inner class instance is tied to Main class and can access the final local variables of its containing method  When the instance uses a final local of its containing method  the variable retains the value it held at the time of the instance s creation  even if the variable has gone out of scope  this is effectively Java s crude  limited version of closures    Because a local inner class is neither the member of a class or package  it is not declared with an access level   Be clear  however  that its own members have access levels like in a normal class

[java] Why are only final variables accessible in anonymous class?

Examples related to java

Examples related to event-handling

Examples related to anonymous-class