Do subclasses inherit private fields

Question

This is an interview question      Does subclasses inherit private   fields    I answered  No   because we can t access them using the  normal OOP way   But the interviewer thinks that they are inherited  because we can access such fields indirectly or using reflection and they still exist in the object   After I came back  I found the following quote in the javadoc      Private Members in a Superclass      A   subclass does not inherit the private   members of its parent class    Do you know any arguments for the interviewer s opinion

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A subclass does not inherit the private members of its parent class  However  if the superclass has public or protected methods for accessing its private fields  these can also be used by the subclass

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I will demonstrate the concept with code  Subclasses ACTUALLY inherit the private variables of super class  The only problem is that they are not accessible to the  child objects unless you provide public getters and setters for the private variables  in the super class    Consider two class in package Dump  Child extends Parent    If I remember correctly  a child object in memory consists of two regions  One is the parent part only and the other is the child part only  A child can access the private  section in the code of its parent only via a public method in the parent     Think of it this way  Borat s father Boltok has a safe containing  100 000  He does not want to share his  private  variable safe  So  he does not provide a key for the safe  Borat inherits the safe  But  what good is it if he cannot even open it   If only his  dad had provided the key   Parent -   package Dump   public class Parent        private String reallyHidden      private String notReallyHidden       public String getNotReallyHidden             return notReallyHidden             public void setNotReallyHidden String notReallyHidden            this notReallyHidden   notReallyHidden            Parent   Child -   package Dump   public class Child extends Parent        private String childOnly       public String getChildOnly             return childOnly             public void setChildOnly String childOnly            this childOnly   childOnly             public static void main String    args            System out println  Testing               Child c1   new Child            c1 setChildOnly  childOnly            c1 setNotReallyHidden  notReallyHidden               Attempting to access parent s reallyHidden             c1 reallyHidden   Does not even compile         main     Child

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A subclass does not inherit the private members of its parent class  However  if the superclass has public or protected methods for accessing its private fields  these can also be used by the subclass reference  https   docs oracle com javase tutorial java IandI subclasses html    text Private 20Members 20in 20a 20Superclass be 20used 20by 20the 20subclass

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We can simply state that when a superclass is inherited  then the private members of superclass actually become private members of the subclass and cannot be further inherited or are inacessible to the objects of subclass

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Private members  state and behavior  are inherited  They  can  affect the behavior and size of the object which is instantiated by the class  Not to mention that they are very well visible to the subclasses via all the encaptulation-breaking mechanisms that are available  or can be assumed by their implementers   Although inheritance has a  defacto  definition  it definitely has no link to  visibility  aspects  which get assumed by the  no  answers   So  there is no need to be diplomatic  JLS is just wrong at this point   Any assumption that they are not  inherited  is unsafe and dangerous   So among two defacto  partially  conflicting definitions  which I will not repeat   the only one that should be followed is the one that is safer  or safe

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Padding bits Alignment and the inclusion of Object Class in the VTABLE is not considered  So the object of the subclass does have a place for the private members of the Super class  However  it cannot be accessed from the subclass s objects

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No  They don t inherit it    The fact some other class may use it indirectly says nothing about inheritance  but about encapsulation    For instance   class Some       private int count      public void increment            count            public String toString             return Integer toString  count            class UseIt        void useIt              Some s   new Some            s increment            s increment            s increment            int v   Integer parseInt  s toString                 hey  can you say you inherit it             You can also get the value of count inside UseIt via reflection  It doesn t means  you inherit it    UPDATE   Even though the value is there  it is not inherited by the subclass   For instance a subclass defined as   class SomeOther extends Some        private int count   1000       Override     public void increment              super increment            count    10000           class UseIt        public static void main  String     args              s   new SomeOther            s increment            s increment            s increment            v   Integer parseInt  s toString                 what is the value of v                        This is exactly the same situation as the first example  The attribute count is hidden and not inherited by the subclass at all  Still  as DigitalRoss points out  the value is there  but not by means on inheritance    Put it this way  If your father is wealthy and gives you a credit card  you can still buy thing with his money  but doesn t mean you have inherited all that money  does it    Other update  It is very interesting though  to know why the attribute is there   I frankly don t have the exact term to describe it  but it s the JVM and the way it works that loads also the  not inherited  parent definition    We could actually change the parent and the subclass will still work   For instance     A java class A      private int i     public String toString     return     i         B java class B extends A       Main java class Main      public static void main  String    args           System out println  new B   toString                 Compile all the files javac A java B java Main java    Run Main java Main    Outout is 0 as expected as B is using the A  toString  definition 0     Change A java class A      public String toString           return  Nothing here             Recompile ONLY A java javac A java java Main    B wasn t modified and yet it shows a different behaviour  this is not due to     inheritance but the way Java loads the class Output  Nothing here   I guess the exact term could be found here   The JavaTM Virtual Machine Specification

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Ok  this is a very interesting problem I researched a lot and came to a conclusion that private members of a superclass are indeed available  but not accessible  in the subclass s objects  To prove this  here is a sample code with a parent class and a child class and I am writing child class object to a txt file and reading a private member named  bhavesh  in the file  hence proving it is indeed available in the child class but not accessible due to the access modifier   import java io Serializable  public class ParentClass implements Serializable   public ParentClass         public int a 32131 b c   private int bhavesh 5555 rr weq refw        import java io    import java io Serializable  public class ChildClass extends ParentClass  public ChildClass     super       public static void main String   args    ChildClass childObj   new ChildClass    ObjectOutputStream oos  try           oos   new ObjectOutputStream new FileOutputStream  C   MyData1 txt             oos writeObject childObj     Writing child class object and not parent class object         System out println  Writing complete            catch  IOException e                    Open MyData1 txt and search for the private member named  bhavesh   Please let me know what you guys think

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For example  class Person       private String name       public String getName              return this name             Person String name            this name   name           public class Student extends Person        Student String name            super name                  public String getStudentName             return this getName       works             quot return this name  quot  doesn t work  and the error is  quot The field Person name is not visible quot             public class Main       public static void main String   args            Student s   new Student  quot Bill quot             String name   s getName       works             quot String name   s name  quot  doesn t work  and the error is  quot The field Person name is not visible quot           System out println name

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I believe  answer is totally dependent on the question  which has been asked  I mean  if question is      Can we directly access the private field of the super-class from   their sub-class     Then answer is No  if we go through the access specifier details  it is mentioned  private members are accessible only within the class itself   But  if question is     Can we access the private field of the super-class from   their sub-class     Which means  it doesn t matters  what you will do to access the private member  In that case  we can make public method in the super-class and you can access the private member  So  in this case you are creating one interface bridge to access the private member    Other OOPs language like C    have the friend function concept  by which we can access the private member of other class

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I would have to answer that private fields in Java are inherited  Allow me to demonstrate   public class Foo        private int x     This is the private field       public Foo             x   0     Sets int x to 0               The following methods are declared  final  so that they can t be overridden      public final void update     x         Increments x by 1      public final int getX     return x       Returns the x value       public class Bar extends Foo        public Bar              super       Because this extends a class with a constructor  it is required to run before anything else           update      Runs the inherited update   method twice         update            System out println getX        Prints the inherited  x  int              If you run in a program Bar bar   new Bar     then you will always see the number  2  in the output box  Because the integer  x  is encapsulated with the methods update   and getX    then it can be proven that the integer is inherited   The confusion is that because you can t directly access the integer  x   then people argue that it isn t inherited  However  every non-static thing in a class  be it field or method  is inherited

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It depends on your definition of  inherit   Does the subclass still have the fields in memory  Definitely  Can it access them directly  No  It s just subtleties of the definition  the point is to understand what s really happening

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A private class member or constructor is accessible only within the body of the top level class    7 6  that encloses the declaration of the member or constructor  It is not inherited by subclasses  https   docs oracle com javase specs jls se7 html jls-6 html jls-6 6

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Most of the confusion in the question answers here surrounds the definition of Inheritance   Obviously  as  DigitalRoss explains an OBJECT of a subclass must contain its superclass s private fields   As he states  having no access to a private member doesn t mean its not there   However   This is different than the notion of inheritance for a class   As is the case in the java world  where there is a question of semantics the arbiter is the Java Language Specification  currently 3rd edition    As the JLS states  https   docs oracle com javase specs jls se8 html jls-8 html jls-8 2         Members of a class that are declared   private are not inherited by   subclasses of that class  Only members   of a class that are declared protected   or public are inherited by subclasses   declared in a package other than the   one in which the class is declared    This addresses the exact question posed by the interviewer    do subCLASSES inherit private fields    emphasis added by me   The answer is No   They do not   OBJECTS of subclasses contain private fields of their superclasses   The subclass itself has NO NOTION of private fields of its superclass   Is it semantics of a pedantic nature   Yes   Is it a useful interview question  Probably not   But the JLS establishes the definition for the Java world  and it does so  in this case  unambiguously   EDITED  removed a parallel quote from Bjarne Stroustrup which due to the differences between java and c   probably only add to the confusion   I ll let my answer rest on the JLS

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It would seem that a subclass does inherit the private fields in that these very fields are utilized in the inner workings of the subclass  philosophically speaking   A subclass  in its constructor  calls the superclass constructor  The superclass private fields are obviously inherited by the subclass calling the superclass constructor if the superclass constructor has initialized these fields in its constructor  That s just an example  But of course without accessor methods the subclass cannot access the superclass private fields  it s like not being able to pop the back panel of an iPhone to take the battery out to reset the phone    but the battery is still there    PS One of the many definitions of inheritance that I have come across   Inheritance -- a programming technique that allows a derived class to extend the functionality of a base class  inheriting all of its STATE  emphasis is mine  and behaviour    The private fields  even if not accessible by the subclass  are the inherited state of the superclass

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No  Private fields are not inherited    and that s why Protected was invented  It is by design  I guess this justified the existence of protected modifier     Now coming to the contexts  What you mean by inherited -- if it is there in the object created from derived class  yes  it is   If you mean can it be useful to derived class  Well  no   Now  when you come to functional programming the private field of super class is not inherited in a meaningful way for the subclass  For the subclass  a private field of super class is same as a private field of any other class   Functionally  it s not inherited  But ideally  it is     OK  just looked into Java tutorial they quote this      Private Members in a Superclass      A subclass does not inherit the private members of its parent class  However  if the superclass has public or protected methods for accessing its private fields  these can also be used by the subclass     refer  http   download oracle com javase tutorial java IandI subclasses html  I agree  that the field is there  But  subclass does not get any privilege on that private field  To a subclass  the private field is same as any private field of any other class   I believe it s purely matter of point-of-view  You may mould the argument either side  It s better justify both way    nbsp

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Yes  It s important to realize that while there are two classes  there is only one object   So  yes  of course it inherited the private fields  They are  presumably  essential for proper object functionality  and while an object of the parent class is not an object of the derived class  an instance of the derived class is mostly definitely an instance of the parent class  It could t very well be that without all of the fields   No  you can t directly access them  Yes  they are inherited  They have to be   It s a good question     Update   Err   No   Well  I guess we all learned something  Since the JLS originated the exact  not inherited  wording  it is correct to answer  no    Since the subclass can t access or modify the private fields  then  in other words  they are not inherited  But there really is just one object  it really does contain the private fields  and so if someone takes the JLS and tutorial wording the wrong way  it will be quite difficult to understand OOP  Java objects  and what is really happening   Update to update   The controversy here involves a fundamental ambiguity  what exactly is being discussed  The object   Or are we talking in some sense about the class itself   A lot of latitude is allowed when describing the class as opposed to the object  So the subclass does not inherit private fields  but an object that is an instance of the subclass certainly does contain the private fields

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No  private fields are not inherited  The only reason is that subclass can not access them directly

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Well  my answer to interviewer s question is - Private members are not inherited in sub-classes but they are accessible to subclass or subclass s object only via public getter or setter methods or any such appropriate methods of original class  The normal practice is to keep the members private and access them using getter and setter methods which are public  So whats the point in only inheriting getter and setter methods when the private member they deal with are not available to the object  Here   inherited  simply means it is available directly in the sub-class to play around by newly introduced methods in sub-class    Save the below file as ParentClass java and try it yourself -   public class ParentClass     private int x     public int getX         return x         public void setX int x        this x   x         class SubClass extends ParentClass     private int y     public int getY         return y         public void setY int y        this y   y         public void setXofParent int x        setX x           class Main     public static void main String   args        SubClass s   new SubClass        s setX 10       s setY 12       System out println  X is    s getX         System out println  Y is    s getY         s setXofParent 13       System out println  Now X is    s getX            Output  X is  10 Y is  12 Now X is  13   If we try to use private variable x of ParentClass in SubClass s method then it is not directly accessible for any modifications  means not inherited   But x can be modified in SubClass via setX   method of original class as done in setXofParent   method OR it can be modified using ChildClass object using setX   method or setXofParent   method which ultimately calls setX    So here setX   and getX   are kind of gates to the private member x of a ParentClass   Another simple example is Clock superclass has hours and mins as private members and appropriate getter and setter methods as public  Then comes DigitalClock as a sub-class of Clock  Here if the DigitalClock s object doesn t contain hours and mins members then things are screwed up

[java] Do subclasses inherit private fields?

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