Difference between final and effectively final

Question

I m playing with lambdas in Java 8 and I came across warning local variables referenced from a lambda expression must be final or effectively final  I know that when I use variables inside anonymous class they must be final in outer class  but still - what is the difference between final and effectively final

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Effectively final  is a variable which would not give compiler error if it were to be appended by  final   From a article by  Brian Goetz       Informally  a local variable is effectively final if its initial value is never changed -- in other words  declaring it final would not cause a compilation failure    lambda-state-final- Brian Goetz

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The Effectively final variable is a local variable that is    Not defined as final  Assigned to ONLY once    While a final variable is a variable that is    declared with a final keyword

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A variable is final or effectively final when it s initialized once and it s never mutated in its owner class  And we can t initialize it in loops or inner classes   Final   final int number  number   23    Effectively Final   int number  number   34       Note  Final and Effective Final are similar Their value don t change after assignment  but just that effective Final variables   are not declared with Keyword final

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public class LambdaScopeTest       public int x   0              class FirstLevel           public int x   1              void methodInFirstLevel int x                    The following statement causes the compiler to generate                the error  local variables referenced from a lambda expression                must be final or effectively final  in statement A                                x   99                            As others have said  a variable or parameter whose value is never changed after it is initialized is effectively final  In the above code  if you change the value of x in inner class FirstLevel then the compiler will give you the error message      Local variables referenced from a lambda expression must be final or effectively final

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However  starting in Java SE 8  a local class can access local variables and parameters of the  enclosing block that are final or effectively final    This didn t start on Java 8  I use this since long time  This code used  before java 8  to be legal   String str          lt -- not accesible from anonymous classes implementation final String strFin          lt -- accesible  button addActionListener new ActionListener          Override     public void actionPerformed ActionEvent e             String ann   str      lt ---- error  must be final  IDE s gives the hint            String ann   strFin      lt ---- legal           String str    legal statement on java 7                     Java 8 doesn t allow this  it thinks that I m trying to use the str declared before the anonymous impl                we are forced to use another name than str

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This variable below is final  so we can t change it s value once initialised  If we try to we ll get a compilation error     final int variable   123    But if we create a variable like this  we can change it s value     int variable   123  variable   456    But in Java 8  all variables are final by default  But the existence of the 2nd line in the code makes it non-final  So if we remove the 2nd line from the above code  our variable is now  effectively final      int variable   123    So   Any variable that is assigned once and only once  is  effectively final

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I find the simplest way to explain  effectively final  is to imagine adding the final modifier to a variable declaration  If  with this change  the program continues to behave in the same way  both at compile time and at run time  then that variable is effectively final

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Declaring a variable final or not declaring it final  but keeping it effectively final may result  depends on compiler  in different bytecode   Let s have a look on a small example       public static void main String   args            final boolean i   true       6     final by declaration         boolean j   true             7     effectively final          if  i                        9             System out println i     10                   if   i                       12             System out println i     13                   if  j                        15             System out println j     16                   if   j                       18             System out println j     19                   The corresponding bytecode of the main method  Java 8u161 on Windows 64 Bit      public static void main java lang String         Code         0  iconst 1        1  istore 1        2  iconst 1        3  istore 2        4  getstatic      16                    Field java lang System out Ljava io PrintStream         7  iconst 1        8  invokevirtual  22                    Method java io PrintStream println  Z V       11  iload 2       12  ifeq          22       15  getstatic      16                    Field java lang System out Ljava io PrintStream        18  iload 2       19  invokevirtual  22                    Method java io PrintStream println  Z V       22  iload 2       23  ifne          33       26  getstatic      16                    Field java lang System out Ljava io PrintStream        29  iload 2       30  invokevirtual  22                    Method java io PrintStream println  Z V       33  return   The corresponding line number table    LineNumberTable     line 6  0    line 7  2    line 10  4    line 15  11    line 16  15    line 18  22    line 19  26    line 21  33   As we see the source code at lines 12  13  14 doesn t appear in the byte code  That s because i is true and will not change it s state  Thus this code is unreachable  more in this answer   For the same reason the code at line 9 misses too  The state of i doesn t have to be evaluated since it is true for sure   On the other hand though the variable j is effectively final it s not processed in the same way  There are no such optimizations applied  The state of j is evaluated two times  The bytecode is the same regardless of j being effectively final

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final is a variable declare with key word final   example     final double pi   3 14     it remains final through out the program    effectively final   any local variable or parameter that is assigned a value only once right now or updated only once   It may not remain effectively final through out the program  so this means that effectively final variable might loose its  effectively final property after immediately the time it gets assigned updated at least one more assignment  example   class EffectivelyFinal        public static void main String   args            calculate 124 53              public static void calculate  int operand1  int operand2           int rem   0        operand1  operand2 and rem are effectively final here      rem   operand1 2     rem lost its effectively final property here because it gets its second assignment                            operand1  operand2 are still effectively final here          class operators               void setNum                    operand1     operand2 2      operand1 lost its effectively final property here because it gets its second assignment                            int add                    return rem   operand2      does not compile because rem is not effectively final                           int multiply                    return rem   operand1      does not compile because both rem and operand1 are not effectively final

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When a lambda expression uses an assigned local variable from its enclosing space there is an important restriction  A lambda expression may only use local variable whose value doesn t change  That restriction is referred as  variable capture  which is described as  lambda expression capture values  not variables   The local variables that a lambda expression may use are known as  effectively final    An effectively final variable is one whose value does not change after it is first assigned  There is no need to explicitly declare such a variable as final  although doing so would not be an error  Let s see it with an example  we have a local variable i which is initialized with the value 7  with in the lambda expression we are trying to change that value by assigning a new value to i  This will result in compiler error -  Local variable i defined in an enclosing scope must be final or effectively final    FunctionalInterface interface IFuncInt       int func int num1  int num2       public String toString       public class LambdaVarDemo        public static void main String   args                        int i   7          IFuncInt funcInt    num1  num2  - gt                i   num1   num2              return i

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starting in Java SE 8  a local class can access local variables and parameters of the enclosing block that are final or effectively final  A variable or parameter whose value is never changed after it is initialized is effectively final    For example  suppose that the variable numberLength is not declared final  and you add the marked assignment statement in the PhoneNumber constructor   public class OutterClass        int numberLength      lt    not  final     class PhoneNumber        PhoneNumber String phoneNumber            numberLength   7        lt    assignment to numberLength         String currentNumber   phoneNumber replaceAll              regularExpression               if  currentNumber length      numberLength              formattedPhoneNumber   currentNumber          else             formattedPhoneNumber   null                               Because of this assignment statement  the variable numberLength is not effectively final anymore  As a result  the Java compiler generates an error message similar to  local variables referenced from an inner class must be final or effectively final  where the inner class PhoneNumber tries to access the numberLength variable   http   codeinventions blogspot in 2014 07 difference-between-final-and html  http   docs oracle com javase tutorial java javaOO localclasses html

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If you could add the final modifier to a local variable  it was   effectively final    Lambda expressions can access    static variables   instance variables  effectively final method parameters  and  effectively final local variables    Source  OCP  Oracle Certified Professional Java SE 8 Programmer II Study Guide  Jeanne Boyarsky  Scott Selikoff  Additionally       An effectively final variable is a variable whose value is never   changed  but it isn   t declared with the final keyword    Source  Starting Out with Java  From Control Structures through Objects  6th Edition   Tony Gaddis  Furthermore  don t forget the meaning of final that  it is initialized exactly once before it is used for the first time

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According to the docs      A variable or parameter whose value is never changed after it is initialized is effectively final    Basically  if the compiler finds a variable does not appear in assignments outside of its initialization  then the variable is considered effectively final   For example  consider some class   public class Foo        public void baz int bar               While the next line is commented  bar is effectively final            and while it is uncommented  the assignment means it is not            effectively final              bar   2

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Effective final topic is described in JLS 4 12 4 and the last paragraph consists a clear explanation      If a variable is effectively final  adding the final modifier to its declaration will not introduce any compile-time errors  Conversely  a local variable or parameter that is declared final in a valid program becomes effectively final if the final modifier is removed

[java] Difference between final and effectively final

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