Java Calling a super method which calls an overridden method

Question

public class SuperClass       public void method1                 System out println  superclass method1            this method2               public void method2                 System out println  superclass method2              public class SubClass extends SuperClass        Override     public void method1                 System out println  subclass method1            super method1                Override     public void method2                 System out println  subclass method2               public class Demo        public static void main String   args                 SubClass mSubClass   new SubClass            mSubClass method1              my expected output      subclass method1   superclass method1   superclass method2     actual output      subclass method1   superclass method1   subclass method2   I know technically I have overriden a public method  but I figured that because I was calling the super  any calls within the super would stay in the super  this isn t happening  Any ideas as to how I can make it happen

User · Answer

To summarize  this points to current object and the method invocation in java is polymorphic by nature  So  method selection for execution  totally depends upon object pointed by this  Therefore  invoking method method2   from parent class invokes method2   of child class  as the this points to object of child class  The definition of this doesn t changes  irrespective of whichever class it s used   PS  unlike methods  member variables of class are not polymorphic

User · Answer

I think of it this way   ----------------        super         ----------------   lt -----------------     ------------                               this         lt -                     ------------                             method1                                method2                              ------------                             method4                                method5                            ----------------                          We instantiate that class  not that one    Let me move that subclass a little to the left to reveal what s beneath     Man  I do love ASCII graphics   We are here                      ----------------                  super              v    ----------------   ------------                     this                     ------------                   method1     method1          method2     method2        ------------  method3                      method4                      method5                  ----------------   Then we call the method over here                           ----------------                              super                               ----------------       ------------          bar                    this              foo                ------------          method0        - gt     method1    --- gt      method1        lt ------------------------------         method2             method2                                             ------------          method3                                                                   method4                                                                   method5                                                               ----------------                                                                                                                                                                                                                                                                    which calls super  thus calling the super s method1   here  so that that method  the overidden one  is executed instead of the overriding one    Keep in mind that  in the inheritance hierarchy  since the instantiated class is the sub one  for methods called via super something   everything is the same except for one thing  two  actually    this  means  the only this we have   a pointer to the class we have instantiated  the subclass   even when java syntax allows us to omit  this   most of the time    super   though  is polymorphism-aware and always refers to the superclass of the class  instantiated or not  that we re actually executing code from   this  is about objects  and can t be used in a static context   super is about classes     In other words  quoting from the Java Language Specification      The form super Identifier refers to the field named Identifier of the   current object  but with the current object viewed as an instance of   the superclass of the current class       The form T super Identifier refers to the field named Identifier of   the lexically enclosing instance corresponding to T  but with that   instance viewed as an instance of the superclass of T    In layman s terms  this is basically an object   the   object  the very same object you can move around in variables   the instance of the instantiated class  a plain variable in the data domain  super is like a pointer to a borrowed block of code that you want to be executed  more like a mere function call  and it s relative to the class where it is called   Therefore if you use super from the superclass you get code from the superduper class  the grandparent  executed   while if you use this  or if it s used implicitly  from a superclass it keeps pointing to the subclass  because nobody has changed it - and nobody could

User · Answer

this  keyword refers to current class reference  That means  when it is used inside the method  the  current  class is still SubClass and so  the answer is explained

User · Answer

You re using the this keyword which actually refers to the  currently running instance of the object you re using   that is  you re invoking this method2    on your superclass  that is  it will call the method2   on the object you re using  which is the SubClass

User · Answer

The keyword super doesn t  stick   Every method call is handled individually  so even if you got to SuperClass method1   by calling super  that doesn t influence any other method call that you might make in the future   That means there is no direct way to call SuperClass method2   from SuperClass method1   without going though SubClass method2   unless you re working with an actual instance of SuperClass   You can t even achieve the desired effect using Reflection  see the documentation of java lang reflect Method invoke Object  Object         EDIT  There still seems to be some confusion  Let me try a different explanation   When you invoke foo    you actually invoke this foo    Java simply lets you omit the this  In the example in the question  the type of this is SubClass   So when Java executes the code in SuperClass method1    it eventually arrives at this method2     Using super doesn t change the instance pointed to by this  So the call goes to SubClass method2   since this is of type SubClass   Maybe it s easier to understand when you imagine that Java passes this as a hidden first parameter   public class SuperClass       public void method1 SuperClass this                System out println  superclass method1            this method2 this       lt --- this    mSubClass            public void method2 SuperClass this                System out println  superclass method2              public class SubClass extends SuperClass        Override     public void method1 SubClass this                System out println  subclass method1            super method1 this               Override     public void method2 SubClass this                System out println  subclass method2               public class Demo        public static void main String   args                 SubClass mSubClass   new SubClass            mSubClass method1 mSubClass             If you follow the call stack  you can see that this never changes  it s always the instance created in main

User · Answer

If you don t want superClass method1 to call subClass method2  make method2 private so it cannot be overridden   Here s a suggestion   public class SuperClass      public void method1         System out println  superclass method1        this internalMethod2           public void method2             this method can be overridden           It can still be invoked by a childclass using super     internalMethod2           private void internalMethod2             this one cannot   Call this one if you want to be sure to use        this implementation      System out println  superclass method2            public class SubClass extends SuperClass       Override   public void method1         System out println  subclass method1        super method1            Override   public void method2         System out println  subclass method2            If it didn t work this way  polymorphism would be impossible  or at least not even half as useful

User · Answer

You can only access overridden methods in the overriding methods  or in other methods of the overriding class    So  either don t override method2   or call super method2   inside the overridden version

User · Answer

Further more extended the output of the raised question  this will give more insight on the access specifier and override behavior              package overridefunction              public class SuperClass                                    public void method1                                         System out println  quot superclass method1 quot                        this method2                        this method3                        this method4                        this method5                                      public void method2                                         System out println  quot superclass method2 quot                                      private void method3                                         System out println  quot superclass method3 quot                                      protected void method4                                         System out println  quot superclass method4 quot                                      void method5                                         System out println  quot superclass method5 quot                                                 package overridefunction              public class SubClass extends SuperClass                                Override                 public void method1                                         System out println  quot subclass method1 quot                        super method1                                       Override                 public void method2                                         System out println  quot subclass method2 quot                                          Override                 private void method3                                         System out println  quot subclass method3 quot                                       Override                 protected void method4                                         System out println  quot subclass method4 quot                                       Override                 void method5                                         System out println  quot subclass method5 quot                                                 package overridefunction              public class Demo                                public static void main String   args                                         SubClass mSubClass   new SubClass                        mSubClass method1                                                 subclass method1             superclass method1             subclass method2             superclass method3             subclass method4             subclass method5

User · Answer

this always refers to currently executing object   To further illustrate the point here is a simple sketch    ----------------     Subclass         ----------------      method1            method2                              ------------        Superclass        ------------        method1            method2           ------------     ----------------    If you have an instance of the outer box  a Subclass object  wherever you happen to venture inside the box  even into the Superclass  area   it is still the instance of the outer box   What s more  in this program there is only one object that gets created out of the three classes  so this can only ever refer to one thing and it is     as shown in the Netbeans  Heap Walker

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class SuperClass       public void method1                 System out println  superclass method1            SuperClass se new SuperClass            se method2               public void method2                 System out println  superclass method2              class SubClass extends SuperClass        Override     public void method1                 System out println  subclass method1            super method1                Override     public void method2                 System out println  subclass method2              calling  SubClass mSubClass   new SubClass    mSubClass method1      outputs     subclass method1 superclass method1 superclass method2

User · Answer

I don t believe you can do it directly  One workaround would be to have a private internal implementation of method2 in the superclass  and call that  For example   public class SuperClass       public void method1                 System out println  superclass method1            this internalMethod2               public void method2                 this internalMethod2               private void internalMethod2                 System out println  superclass method2

User · Answer

Since the only way to avoid a method to get overriden is to use the keyword super  I ve thought to move up the method2   from SuperClass to another new Base class and then call it from SuperClass   class Base        public void method2                 System out println  superclass method2             class SuperClass extends Base       public void method1                 System out println  superclass method1            super method2             class SubClass extends SuperClass        Override     public void method1                 System out println  subclass method1            super method1                Override     public void method2                 System out println  subclass method2             public class Demo        public static void main String   args                 SubClass mSubClass   new SubClass            mSubClass method1              Output   subclass method1 superclass method1 superclass method2

User · Answer

During my research for a similar case  I have been ending up by checking the stack trace in the subclass method to find out from where the call is coming from  There are probably smarter ways to do so  but it works out for me and it s a dynamic approach   public void method2            Exception ex new Exception            StackTraceElement   ste ex getStackTrace            if ste 1  getClassName   equals this getClass   getSuperclass   getName                  super method2                      else                subclass method2 code               I think the question to have a solution for the case is reasonable  There are of course ways to solve the issue with different method names or even different parameter types  like already mentioned in the thread  but in my case I dindn t like to confuse by different method names

[java] Java: Calling a super method which calls an overridden method

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