final keyword in method parameters

Question

I often encounter methods which look like the following   public void foo final String a  final int   b  final Object1 c       What happens if this method is called without passing it final parameters  i e  an Object1 that is later changed  so is not declared as final  can be passed to this method just fine

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Java is only pass-by-value   or better - pass-reference-by-value   So the passed argument and the argument within the method are two different handlers pointing to the same object  value     Therefore if you change the state of the object  it is reflected to every other variable that s referencing it  But if you re-assign a new object  value  to the argument  then other variables pointing to this object  value  do not get re-assigned

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Java always makes a copy of parameters before sending them to methods  This means the final doesn t mean any difference for the calling code  This only means that inside the method the variables can not be reassigned  Note that if you have a final object  you can still change the attributes of the object  This is because objects in Java really are pointers to objects  And only the pointer is copied  and will be final in your method   not the actual object

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final keyword in the method input parameter is not needed  Java creates a copy of the reference to the object  so putting final on it doesn t make the object final but just the reference  which doesn t make sense

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Consider this implementation of foo     public void foo final String a        SwingUtilities invokeLater new Runnable             public void run                 System out print a                          Because the Runnable instance would outlive the method  this wouldn t compile without the final keyword -- final tells the compiler that it s safe to take a copy of the reference  to refer to it later   Thus  it s the reference that s considered final  not the value  In other words  As a caller  you can t mess anything up

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The final keyword on a method parameter means absolutely nothing to the caller  It also means absolutely nothing to the running program  since its presence or absence doesn t change the bytecode  It only ensures that the compiler will complain if the parameter variable is reassigned within the method  That s all  But that s enough   Some programmers  like me  think that s a very good thing and use final on almost every parameter  It makes it easier to understand a long or complex method  though one could argue that long and complex methods should be refactored   It also shines a spotlight on method parameters that aren t marked with final

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If you declare any parameter as final  you cannot change the value of it   class Bike11         int cube final int n              n n 2   can t be changed as n is final           n n n                 public static void main String args                Bike11 b new Bike11              b cube 5                    Output  Compile Time Error  For more details  please visit my blog  http   javabyroopam blogspot com

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stuXnet  I could make the exact opposite argument  If you pass an object to a function  and you change the properties of the passed object then the caller of the function will see the changed value in its variable  This implies a pass by reference system  not pass by value   What is confusing is the definition of pass by value or pass by reference in a system where the use of pointers is completely hidden to the end user    Java is definitely NOT pass by value  as to be such  would mean one could mutate the passed object and the original would be unaffected    Notice you cannot mutate primitives you can only assign them to variables  So testing a Pass by reference or by value by using primitives is not a test     What you cannot do in Java that can be done in other languages is to reassign the caller s variables to a new value because there are no pointers in Java  so this makes it confusing

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There is a circumstance where you re required to declare it final --otherwise it will result in compile error--  namely passing them through into anonymous classes  Basic example   public FileFilter createFileExtensionFilter final String extension        FileFilter fileFilter   new FileFilter             public boolean accept File pathname                return pathname getName   endsWith extension                            What would happen when it s allowed to change extension here         extension    foo        return fileFilter      Removing the final modifier would result in compile error  because it isn t guaranteed anymore that the value is a runtime constant  Changing the value from outside the anonymous class would namely cause the anonymous class instance to behave different after the moment of creation

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Strings are immutable  so actully you can t change the String afterwards  you can only make the variable that held the String object point to a different String object    However  that is not the reason why you can bind any variable to a final parameter  All the compiler checks is that the parameter is not reassigned within the method  This is good for documentation purposes  arguably good style  and may even help optimize the byte code for speed  although this seems not to do very much in practice     But even if you do reassign a parameter within a method  the caller doesn t notice that  because java does all parameter passing by value  After the sequence    a   someObject      process a     the fields of a may have changed  but a is still the same object it was before  In pass-by-reference languages this may not be true

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final means you can t change the value of that variable once it was assigned   Meanwhile  the use of final for the arguments in those methods means it won t allow the programmer to change their value during the execution of the method  This only means that inside the method the final variables can not be reassigned

[java] final keyword in method parameters

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