How does the final keyword in Java work I can still modify an object

Question

In Java we use final keyword with variables to specify its values are not to be changed  But I see that you can change the value in the constructor   methods of the class  Again  if the variable is static then it is a compilation error   Here is the code    import java util ArrayList  import java util List   class Test     private final List foo     public Test             foo   new ArrayList          foo add  foo       Modification-1       public static void main String   args             Test t   new Test          t foo add  bar       Modification-2       System out println  print -     t foo           Above code works fine and no errors   Now change the variable as static   private static final List foo    Now it is a compilation error  How does this final really work

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Since the final variable is non-static  it can be initialized in constructor  But if you make it static it can not be initialized by constructor  because constructors are not static   Addition to list is not expected to stop by making list final  final just binds the reference to particular object  You are free to change the  state  of that object  but not the object itself

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I thought of writing an updated and in depth answer here   final keyword can be used in several places    classes    A final class means that no other class can extend that final class  When Java Run Time  JRE  knows an object reference is in type of a final class  say F   it knows that the value of that reference can only be in type of F    Ex   F myF  myF   new F         ok myF   someOther     someOther cannot be in type of a child class of F                      because F cannot be extended    So when it executes any method of that object  that method doesn t need to be resolved at run time using a virtual table  i e  run-time polymorphism cannot be applied  So the run time doesn t bother about that  Which means it saves processing time  which will improve performance    methods   A final method of any class means that any child class extending that class cannot override that final method s   So the run time behavior in this scenario is also quite same with the previous behavior I mentioned for classes    fields  local variables  method parameters   If one specified any kind of above as final  it means that the value is already finalized  so the value cannot be changed   Ex   For fields  local parameters  final FinalClass fc   someFC    need to assign straight away  otherwise compile error  final FinalClass fc    compile error  need assignment  initialization inside a constructor Ok  constructor can be called only once  final FinalClass fc   new FinalClass      ok fc   someOtherFC    compile error fc someMethod      no problem someOtherFC someMethod      no problem   For method parameters  void someMethod final String s       s   someOtherString    compile error     This simply means that value of the final reference value cannot be changed  i e  only one initialization is allowed  In this scenario  in run time  since JRE knows that values cannot be changed  it loads all these finalized values  of final references  into L1 cache  Because it doesn t need to load back again and again from main memory  Otherwise it loads to L2 cache and does time to time loading from main memory  So it is also a performance improvement   So in all above 3 scenarios  when we have not specified the final keyword in places we can use  we don t need to worry  compiler optimizations will do that for us  There are also lots of other things that compiler optimizations do for us

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The final keyword indicates that a variable may only be initialized once   In your code you are only performing one initialization of final so the terms are satisfied   This statement performs the lone initialization of foo   Note that final    immutable  it only means that the reference cannot change   foo   new ArrayList      When you declare foo as static final the variable must be initialized when the class is loaded and cannot rely on instantiation  aka call to constructor  to initialize foo since static fields must be available without an instance of a class   There is no guarantee that the constructor will have been called prior to using the static field   When you execute your method under the static final scenario the Test class is loaded prior to instantiating t at this time there is no instantiation of foo meaning it has not been initialized so foo is set to the default for all objects which is null   At this point I assume your code throws a NullPointerException when you attempt to add an item to the list

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If you make foo static  you must initialize it in the class constructor  or inline where you define it  like the following examples   Class constructor  not instance    private static final List foo   static      foo   new ArrayList        Inline   private static final List foo   new ArrayList      The problem here is not how the final modifier works  but rather how the static modifier works    The final modifier enforces an initialization of your reference by the time the call to your constructor completes  i e  you must initialize it in the constructor    When you initialize an attribute in-line  it gets initialized before the code you have defined for the constructor is run  so you get the following outcomes    if foo is static  foo   new ArrayList   will be executed before the static   constructor you have defined for your class is executed if foo is not static  foo   new ArrayList   will be executed before your constructor is run   When you do not initilize an attribute in-line  the final modifier enforces that you initialize it and that you must do so in the constructor  If you also have a static modifier  the constructor you will have to initialize the attribute in is the class  initialization block   static     The error you get in your code is from the fact that static   is run when the class is loaded  before the time you instantiate an object of that class  Thus  you will have not initialized foo when the class is created    Think of the static   block as a constructor for an object of type Class  This is where you must do the initialization of your static final class attributes  if not done inline    Side note   The final modifier assures const-ness only for primitive types and references    When you declare a final object  what you get is a final reference to that object  but the object itself is not constant    What you are really achieving when declaring a final attribute is that  once you declare an object for your specific purpose  like the final List that you have declared   that and only that object will be used for that purpose  you will not be able to change List foo to another List  but you can still alter your List by adding removing items  the List you are using will be the same  only with its contents altered

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Worth to mention some straightforward definitions   Classes Methods     You can declare some or all of a class methods as final  in order to indicate that the method cannot be overridden by subclasses    Variables     Once a final variable has been initialized  it always contains the same value     final basically avoid overwrite superscribe by anything  subclasses  variable  reassign    depending on the case

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quot A final variable can only be assigned once quot   Reflection  -  quot wowo wait  hold my beer quot    Freeze of final fields happen in two scenarios   End of constructor  When reflection sets the field s value   as many times as it wants to    Let s break the law public class HoldMyBeer        final int notSoFinal           public HoldMyBeer                notSoFinal   1             static void holdIt HoldMyBeer beer  int yetAnotherFinalValue  throws Exception              Class lt HoldMyBeer gt  cl   HoldMyBeer class         Field field   cl getDeclaredField  quot notSoFinal quot           field setAccessible true          field set beer  yetAnotherFinalValue              public static void main String   args  throws Exception               HoldMyBeer beer   new HoldMyBeer           System out println beer notSoFinal          holdIt beer  50          System out println beer notSoFinal          holdIt beer  100          System out println beer notSoFinal          holdIt beer  666          System out println beer notSoFinal          holdIt beer  8888          System out println beer notSoFinal                Output  1 50 100 666 8888  The  quot final quot  field has been assigned 5 different  quot final quot  values  note the quotes   And it could keep being assigned different values over and over    Why  Because reflection is like Chuck Norris  and if it wants to change the value of an initialized final field  it does  Some say he himself is the one that pushes the new values into the stack   Code     7  astore 1   11  aload 1   12  getfield                   18  aload 1   19  bipush        50          wait what   27  aload 1   28  getfield                   34  aload 1   35  bipush        100         come on      43  aload 1   44  getfield                   50  aload 1   51  sipush        666           you were supposed to be final      60  aload 1   61  getfield                   67  aload 1   68  sipush        8888       ok i m out whatever dude   77  aload 1   78  getfield

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Final keyword has a numerous way to use    A final class cannot be subclassed  A final method cannot be overridden by subclasses A final variable can only be initialized once   Other usage    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   A static class variable will exist from the start of the JVM  and should be initialized in the class  The  error message won t appear if you do this

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First of all  the place in your code where you are initializing  i e  assigning for the first time  foo is here   foo   new ArrayList      foo is an object  with type List  so it is a reference type  not a value type  like int   As such  it holds a reference to a memory location  e g  0xA7D2A834  where your List elements are stored  Lines like this  foo add  foo       Modification-1   do not change the value of foo  which  again  is just a reference to a memory location   Instead  they just add elements into that referenced memory location  To violate the final keyword  you would have to try to re-assign foo as follows again   foo   new ArrayList      That would give you a compilation error     Now  with that out of the way  think about what happens when you add the static keyword    When you do NOT have the static keyword  each object that instantiates the class has its own copy of foo  Therefore  the constructor assigns a value to a blank  fresh copy of the foo variable  which is perfectly fine   However  when you DO have the static keyword  only one foo exists in memory that is associated with the class  If you were to create two or more objects  the constructor would be attempting to re-assign that one foo each time  violating the final keyword

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This is a favorite interview question  With this questions  the interviewer tries to find out how well you understand the behavior of objects with respect to constructors  methods  class variables  static variables  and instance variables   import java util ArrayList  import java util List   class Test       private final List foo       public Test             foo   new ArrayList            foo add  foo       Modification-1            public void setFoo List foo             this foo   foo  Results in compile time error            In the above case  we have defined a constructor for  Test  and gave it a  setFoo  method    About constructor   Constructor can be invoked only one time per object creation by using the new keyword  You cannot invoke constructor multiple times  because constructor are not designed to do so    About method  A method can be invoked as many times as you want  Even never  and the compiler knows it    Scenario 1  private final List foo      1   foo is an instance variable  When we create Test class object then the instance variable foo  will be copied inside the object of Test class  If we assign foo inside the constructor  then the compiler knows that the constructor will be invoked only once  so there is no problem assigning it inside the constructor   If we assign foo inside a method  the compiler knows that a method can be called multiple times  which means the value will have to be changed multiple times  which is not allowed for a final variable  So the compiler decides constructor is good choice  You can assign a value to a final variable only one time   Scenario 2  private static final List foo   new ArrayList      foo is now a static variable  When we create an instance of Test class  foo will not be copied to the object because foo is static  Now foo is not an independent property of each object  This is a property of Test class  But foo can be seen by multiple objects and if every object which is created by using the new keyword which will ultimately invoke the Test constructor which changes the value at the time of multiple object creation  Remember static foo is not copied in every object  but is shared between multiple objects     Scenario 3   t foo add  bar       Modification-2   Above Modification-2 is from your question  In the above case  you are not changing the first referenced object  but you are adding content inside foo which is allowed  Compiler complains if you try to assign a new ArrayList   to the foo reference variable  Rule If you have initialized a final variable  then you cannot change it to refer to a different object   In this case ArrayList   final classes cannot be subclassed final methods cannot be overridden   This method is in superclass  final methods can override   Read this in grammatical way  This method is in a subclass

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The final keyword can be interpreted in two different ways depending on what it s used on   Value types  For ints  doubles etc  it will ensure that the value cannot change   Reference types  For references to objects  final ensures that the reference will never change  meaning that it will always refer to the same object  It makes no guarantees whatsoever about the values inside the object being referred to staying the same   As such  final List lt Whatever gt  foo  ensures that foo always refers to the same list  but the contents of said list may change over time

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You are always allowed to initialize a final variable   The compiler makes sure that you can do it only once    Note that calling methods on an object stored in a final variable has nothing to do with the semantics of final  In other words  final is only about the reference itself  and not about the contents of the referenced object   Java has no concept of object immutability  this is achieved by carefully designing the object  and is a far-from-trivial endeavor

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Read all the answers    There is another user case where final keyword can be used i e  in a method argument   public void showCaseFinalArgumentVariable final int someFinalInt       someFinalInt   9     won t compile as the argument is final      Can be used for variable which should not be changed

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This is a very good interview question  Sometimes they might even ask you what is the difference between a final object and immutable object   1  When someone mentions a final object  it means that the reference cannot be changed  but its state instance variables  can be changed     2  An immutable object is one whose state can not be changed  but its reference can be changed   Ex       String x   new String  abc         x    BCG     ref variable x can be changed to point a different string  but value of  abc  cannot be changed    3  Instance variables non static fields  are initialized when a constructor is called  So you can initialize values to you variables inside a constructor   4   But i see that you can change the value in the constructor methods of the class   -- You cannot change it inside a method   5  A static variable is initialized during class loading  So you cannot initialize inside a constructor  it has to be done even before it  So you need to assign values to a static variable during declaration itself

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Above all are correct  Further if you do not want others to create sub classes from your class  then declare your class as final  Then it becomes the leaf level of your class tree hierarchy that no one can extend it further  It is a good practice to avoid huge hierarchy of classes

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When you make it static final it should be initialized in a static initialization block      private static final List foo       static           foo   new ArrayList               public Test                 foo   new ArrayList            foo add  foo       Modification-1

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final is a reserved keyword in Java to restrict the user and it can be applied to member variables  methods  class and local variables  Final variables are often declared with the static keyword in Java and are treated as constants  For example   public static final String hello    Hello     When we use the final keyword with a variable declaration  the value stored inside that variable cannot be changed latter    For example   public class ClassDemo     private final int var1   3    public ClassDemo                     Note  A class declared as final cannot be extended or inherited  i e  there cannot be a subclass of the super class   It is also good to note that methods declared as final cannot be overridden by subclasses    Benefits of using the final keyword are addressed in this thread

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Following are different contexts where final is used   Final variables A final variable can only be assigned once  If the variable is a reference  this means that the variable cannot be re-bound to reference another object   class Main      public static void main String args           final int i   20        i   30    Compiler Error cannot assign a value to final variable i twice          final variable can be assigned value later  not compulsory to assigned a value when declared   but only once   Final classes A final class cannot be extended  inherited   final class Base     class Derived extends Base       Compiler Error cannot inherit from final Base  public class Main      public static void main String args               Final methods A final method cannot be overridden by subclasses     Error in following program as we are trying to override a final method  class Base     public final void show            System out println  Base  show   called                 class Derived extends Base       public void show        Compiler Error  show   in Derived cannot override        System out println  Derived  show   called                 public class Main       public static void main String   args            Base b   new Derived             b show

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The final keyword in java is used to restrict the user  The java final keyword can be used in many context  Final can be    variable   method   class     The final keyword can be applied with the variables  a final variable that has no value  is called blank final variable or uninitialized final variable  It can be initialized in the constructor only  The blank final variable can be static also which will be initialized in the static block only    Java final variable   If you make any variable as final  you cannot change the value of final variable It will be constant    Example of final variable  There is a final variable speedlimit  we are going to change the value of this variable  but It can t be changed because final variable once assigned a value can never be changed   class Bike9        final int speedlimit 90   final variable       void run              speedlimit 400      this will make error              public static void main String args           Bike9 obj new  Bike9          obj run                 end of class     Java final class   If you make any class as final  you cannot extend it   Example of final class  final class Bike      class Honda1 extends Bike       cannot inherit from final Bike this will make error   void run          System out println  running safely with 100kmph              public static void main String args             Honda1 honda  new Honda            honda run                        Java final method   If you make any method as final  you cannot override it   Example of final method  run   in Honda cannot override run   in Bike   class Bike      final void run   System out println  running            class Honda extends Bike       void run   System out println  running safely with 100kmph           public static void main String args          Honda honda  new Honda         honda run                   shared from  http   www javatpoint com final-keyword

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Suppose you have two moneyboxes  red and white  You assign these moneyboxes only two children and they are not allowed interchange their boxes  So You have red or white moneyboxes final  you cannot modify the box but you can put money on your box Nobody cares  Modification-2

[java] How does the "final" keyword in Java work? (I can still modify an object.)

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