Why doesn t Java allow overriding of static methods

Question

Why is it not possible to override static methods   If possible  please use an example

User · Answer

What good will it do to override static methods. You cannot call static methods through an instance.

MyClass.static1()
MySubClass.static1()   // If you overrode, you have to call it through MySubClass anyway.

EDIT : It appears that through an unfortunate oversight in language design, you can call static methods through an instance. Generally nobody does that. My bad.

User · Answer

By overriding  you achieve dynamic polymorphism  When you say overriding static methods  the words you are trying to use are contradictory   Static says - compile time  overriding is used for dynamic polymorphism  Both are opposite in nature  and hence can t be used together   Dynamic polymorphic behavior comes when a programmer uses an object and accessing an instance method  JRE will map different instance methods of different classes based on what kind of object you are using   When you say overriding static methods  static methods we will access by using the class name  which will be linked at compile time  so there is no concept of linking methods at runtime with static methods  So the term  overriding  static methods itself doesn t make any meaning   Note  even if you access a class method with an object  still java compiler is intelligent enough to find it out  and will do static linking

User · Answer

Answer of this question is simple  the method or variable marked as static belongs to the class only  So that static method cannot be inherited in the sub class because they belong to the super class only

User · Answer

Easy solution  Use singleton instance  It will allow overrides and inheritance    In my system  I have SingletonsRegistry class  which returns instance for passed Class  If instance is not found  it is created   Haxe language class   package rflib common utils  import haxe ds ObjectMap     class SingletonsRegistry     public static var instances Map lt Class lt Dynamic gt   Dynamic gt      static function   init             StaticsInitializer addCallback SingletonsRegistry  function               instances   null                   public static function getInstance cls Class lt Dynamic gt    args Array lt Dynamic gt           if  instances    null          instances   untyped new ObjectMap lt Dynamic  Dynamic gt                      if   instances exists cls                if  args    null  args             instances set cls  Type createInstance cls  args               return instances get cls           public static function validate inst Dynamic  cls Class lt Dynamic gt           if  instances    null  return       var inst2   instances cls       if  inst2    null  amp  amp  inst    inst2  throw  Can  t create multiple instances of     Type getClassName cls      - it s singleton

User · Answer

In general it doesn t make sense to allow  overriding  of static methods as there would be no good way to determine which one to call at runtime  Taking the Employee example  if we call RegularEmployee getBonusMultiplier   - which method is supposed to be executed   In the case of Java  one could imagine a language definition where it is possible to  override  static methods as long as they are called through an object instance  However  all this would do is to re-implement regular class methods  adding redundancy to the language without really adding any benefit

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The following code shows that it is possible     class OverridenStaticMeth       static void printValue        System out println  Overriden Meth                   public class OverrideStaticMeth extends OverridenStaticMeth       static void printValue        System out println  Overriding Meth             public static void main String   args       OverridenStaticMeth osm   new OverrideStaticMeth       osm printValue        System out println  now  from main    printValue

User · Answer

Well    the answer is NO if you think from the perspective of how an overriden method should behave in Java  But  you don t get any compiler error if you try to override a static method  That means  if you try to override  Java doesn t stop you doing that  but you certainly don t get the same effect as you get for non-static methods  Overriding in Java simply means that the particular method would be called based on the run time type of the object and not on the compile time type of it  which is the case with overriden static methods   Okay    any guesses for the reason why do they behave strangely  Because they are class methods and hence access to them is always resolved during compile time only using the compile time type information  Accessing them using object references is just an extra liberty given by the designers of Java and we should certainly not think of stopping that practice only when they restrict it  -   Example  let s try to see what happens if we try overriding a static method -  class SuperClass             public static void staticMethod         System out println  SuperClass  inside staticMethod                   public class SubClass extends SuperClass                overriding the static method public static void staticMethod         System out println  SubClass  inside staticMethod                 public static void main String   args                      SuperClass superClassWithSuperCons   new SuperClass        SuperClass superClassWithSubCons   new SubClass        SubClass subClassWithSubCons   new SubClass         superClassWithSuperCons staticMethod        superClassWithSubCons staticMethod        subClassWithSubCons staticMethod                     Output - SuperClass  inside staticMethod  SuperClass  inside staticMethod  SubClass  inside staticMethod  Notice the second line of the output  Had the staticMethod been overriden this line should have been identical to the third line as we re invoking the  staticMethod    on an object of Runtime Type as  SubClass  and not as  SuperClass   This confirms that the static methods are always resolved using their compile time type information only

User · Answer

Yes  Practically Java allows overriding static method  and No theoretically if you Override a static method in Java then it will compile and run smoothly but it will lose Polymorphism which is the basic property of Java  You will Read Everywhere that it is not possible to try yourself compiling and running  you will get your answer  e g  If you Have Class Animal and a static method eat   and you Override that static method in its Subclass lets called it Dog  Then when wherever you Assign a Dog object to an Animal Reference and call eat   according to Java Dog s eat   should have been called but in static Overriding Animals  eat   will Be Called   class Animal       public static void eat             System out println  Animal Eating             class Dog extends Animal      public static void eat             System out println  Dog Eating             class Test       public static void main String args             Animal obj  new Dog     Dog object in animal        obj eat      should call dog s eat but it didn t           Output Animal Eating   According to Polymorphism Principle of Java  the Output Should be Dog Eating  But the result was different because to support Polymorphism Java uses Late Binding that means methods are called only at the run-time but not in the case of static methods  In static methods compiler calls methods at the compile time rather than the run-time  so we get methods according to the reference and not according to the object a reference a containing that s why You can say Practically it supports static overring but theoretically  it doesn t

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Method overriding is made possible by dynamic dispatching  meaning that the declared type of an object doesn t determine its behavior  but rather its runtime type   Animal lassie   new Dog    lassie speak       outputs  woof   Animal kermit   new Frog    kermit speak       outputs  ribbit     Even though both lassie and kermit are declared as objects of type Animal  their behavior  method  speak    varies because dynamic dispatching will only bind the method call  speak   to an implementation at run time - not at compile time   Now  here s where the static keyword starts to make sense  the word  static  is an antonym for  dynamic   So the reason why you can t override static methods is because there is no dynamic dispatching on static members - because static literally means  not dynamic   If they dispatched dynamically  and thus could be overriden  the static keyword just wouldn t make sense anymore

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Actually we were wrong  Despite Java doesn t allow you to override static methods by default  if you look thoroughly through documentation of Class and Method classes in Java  you can still find a way to emulate static methods overriding by following workaround   import java lang reflect InvocationTargetException  import java math BigDecimal   class RegularEmployee        private BigDecimal salary   BigDecimal ONE       public void setSalary BigDecimal salary            this salary   salary            public static BigDecimal getBonusMultiplier             return new BigDecimal   02              public BigDecimal calculateBonus             return salary multiply this getBonusMultiplier               public BigDecimal calculateOverridenBonus             try                  System out println this getClass   getDeclaredMethod                  getBonusMultiplier   toString                 try                   return salary multiply  BigDecimal  this getClass                        getDeclaredMethod  getBonusMultiplier   invoke this                  catch  IllegalAccessException e                    e printStackTrace                  catch  IllegalArgumentException e                    e printStackTrace                  catch  InvocationTargetException e                    e printStackTrace                            catch  NoSuchMethodException e                e printStackTrace              catch  SecurityException e                e printStackTrace                      return null                   presumably lots of other code        final class SpecialEmployee extends RegularEmployee        public static BigDecimal getBonusMultiplier             return new BigDecimal   03             public class StaticTestCoolMain        static public void main String   args            RegularEmployee Alan   new RegularEmployee            System out println Alan calculateBonus             System out println Alan calculateOverridenBonus             SpecialEmployee Bob   new SpecialEmployee            System out println Bob calculateBonus             System out println Bob calculateOverridenBonus               Resulting output   0 02 0 02 0 02 0 03   what we were trying to achieve     Even if we declare third variable Carl as RegularEmployee and assign to it instance of SpecialEmployee  we will still have call of RegularEmployee method in first case and call of SpecialEmployee method in second case  RegularEmployee Carl   new SpecialEmployee     System out println Carl calculateBonus     System out println Carl calculateOverridenBonus       just look at output console   0 02 0 03

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I like and double Jay s comment  https   stackoverflow com a 2223803 1517187   I agree that this is the bad design of Java  Many other languages support overriding static methods  as we see in previous comments  I feel Jay has also come to Java from Delphi like me  Delphi  Object Pascal  was one of the languages implementing OOP before Java and one of the first languages used for commercial application development  It is obvious that many people had experience with that language since it was in the past the only language to write commercial GUI products  And - yes  we could in Delphi override static methods  Actually  static methods in Delphi are called  quot class methods quot   while Delphi had the different concept of  quot Delphi static methods quot  which were methods with early binding  To override methods you had to use late binding  declare  quot virtual quot  directive  So it was very convenient and intuitive and I would expect this in Java

User · Answer

Here is a simple explanation  A static method is associated with a class while an instance method is associated with a particular object  Overrides allow calling the different implementation of the overridden methods associated with the particular object  So it is counter-intuitive to override static method which is not even associated with objects but the class itself in the first place  So static methods cannot be overridden based on what object is calling it  it will always be associated with the class where it was created

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A Static method  variable  block or nested class belongs to the entire class rather than an object   A Method in Java is used to expose the behaviour of an Object   Class  Here  as the method is static  i e  static method is used to represent the behaviour of a class only   changing  overriding the behaviour of entire class will violate the phenomenon of one of the fundamental pillar of Object oriented programming i e  high cohesion   remember a constructor is a special kind of method in Java    High Cohesion - One class should have only one role  For example  A car class should produce only car objects and not bike  trucks  planes etc  But the Car class may have some features behaviour  that belongs to itself only   Therefore  while designing the java programming language  The language designers thought to allow developers to keep some behaviours of a class to itself only by making a method static in nature      The below piece code tries to override the static method  but will not encounter any compilation error   public class Vehicle   static int VIN   public static int getVehileNumber         return VIN      class Car extends Vehicle   static int carNumber   public static int getVehileNumber         return carNumber       This is because  here we are not overriding a method but we are just re-declaring it  Java allows re-declaration of a method  static non-static    Removing the static keyword from getVehileNumber   method of Car class will result into compilation error  Since  we are trying to change the functionality of static method which belongs to Vehicle class only   Also  If the getVehileNumber   is declared as final then the code will not compile  Since the final keyword restricts the programmer from re-declaring the method   public static final int getVehileNumber     return VIN          Overall  this is upto software designers for where to use the static methods  I personally prefer to use static methods to perform some actions without creating any instance of a class  Secondly  to hide the behaviour of a class from outside world

User · Answer

Static methods are treated as global by the JVM  there are not bound to an object instance at all   It could conceptually be possible if you could call static methods from class objects  like in languages like Smalltalk  but it s not the case in Java    EDIT  You can overload static method  that s ok  But you can not override a static method  because class are no first-class object  You can use reflection to get the class of an object at run-time  but the object that you get does not parallel the class hierarchy    class MyClass         class MySubClass extends MyClass          MyClass obj1   new MyClass    MySubClass obj2   new MySubClass     ob2 instanceof MyClass -- gt  true  Class clazz1   obj1 getClass    Class clazz2   obj2 getClass     clazz2 instanceof clazz1 -- gt  false   You can reflect over the classes  but it stops there  You don t invoke a static method by using clazz1 staticMethod    but using MyClass staticMethod    A static method is not bound to an object and there is hence no notion of this nor super in a static method  A static method is a global function  as a consequence there is also no notion of polymorphism and  therefore  method overriding makes no sense    But this could be possible if MyClass was an object at run-time on which you invoke a method  as in Smalltalk  or maybe JRuby as one comment suggest  but I know nothing of JRuby    Oh yeah    one more thing  You can invoke a static method through an object obj1 staticMethod   but that really syntactic sugar for MyClass staticMethod   and should be avoided  It usually raises a warning in modern IDE  I don t know why they ever allowed this shortcut

User · Answer

By overriding we can create a polymorphic nature depending on the object type  Static method has no relation with object  So java can not support static method overriding

User · Answer

overriding is reserved for instance members to support polymorphic behaviour  static class members do not belong to a particular instance  instead  static members belong to the class and as a result overriding is not supported because subclasses only inherit protected and public instance members and not static members  You may want to define an inerface and research factory and or strategy design patterns to evaluate an alternate approach

User · Answer

In Java  and many OOP languages  but I cannot speak for all  and some do not have static at all  all methods have a fixed signature - the parameters and types  In a virtual method  the first parameter is implied  a reference to the object itself and when called from within the object  the compiler automatically adds this   There is no difference for static methods - they still have a fixed signature  However  by declaring the method static you have explicitly stated that the compiler must not include the implied object parameter at the beginning of that signature  Therefore  any other code that calls this must must not attempt to put a reference to an object on the stack  If it did do that  then the method execution would not work since the parameters would be in the wrong place - shifted by one - on the stack   Because of this difference between the two  virtual methods always have a reference to the context object  i e  this  so then it is possible to reference anything within the heap that belong to that instance of the object  But with static methods  since there is no reference passed  that method cannot access any object variables and methods since the context is not known   If you wish that Java would change the definition so that a object context is passed in for every method  static or virtual  then you would in essence have only virtual methods    As someone asked in a comment to the op - what is your reason and purpose for wanting this feature   I do not know Ruby much  as this was mentioned by the OP  I did some research  I see that in Ruby classes are really a special kind of object and one can create  even dynamically  new methods  Classes are full class objects in Ruby  they are not in Java  This is just something you will have to accept when working with Java  or C    These are not dynamic languages  though C  is adding some forms of dynamic  In reality  Ruby does not have  static  methods as far as I could find - in that case these are methods on the singleton class object  You can then override this singleton with a new class and the methods in the previous class object will call those defined in the new class  correct    So if you called a method in the context of the original class it still would only execute the original statics  but calling a method in the derived class  would call methods either from the parent or sub-class  Interesting and I can see some value in that  It takes a different thought pattern   Since you are working in Java  you will need to adjust to that way of doing things  Why they did this  Well  probably to improve performance at the time based on the technology and understanding that was available  Computer languages are constantly evolving  Go back far enough and there is no such thing as OOP  In the future  there will be other new ideas   EDIT  One other comment  Now that I see the differences and as I Java C  developer myself  I can understand why the answers you get from Java developers may be confusing if you are coming from a language like Ruby  Java static methods are not the same as Ruby class methods  Java developers will have a hard time understanding this  as will conversely those who work mostly with a language like Ruby Smalltalk  I can see how this would also be greatly confusing by the fact that Java also uses  class method  as another way to talk about static methods but this same term is used differently by Ruby  Java does not have Ruby style class methods  sorry   Ruby does not have Java style static methods which are really just old procedural style functions  as found in C   By the way - thanks for the question  I learned something new for me today about class methods  Ruby style

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The short answer is  it is entirely possible  but Java doesn t do it   Here is some code which illustrates the current state of affairs in Java   File Base java   package sp trial  public class Base     static void printValue         System out println    Called static Base method           void nonStatPrintValue         System out println    Called non-static Base method           void nonLocalIndirectStatMethod         System out println    Non-static calls overridden    static         System out print            this printValue            File Child java   package sp trial  public class Child extends Base     static void printValue         System out println    Called static Child method           void nonStatPrintValue         System out println    Called non-static Child method           void localIndirectStatMethod         System out println    Non-static calls own static         System out print            printValue          public static void main String   args        System out println  Object  static type Base  runtime type Child         Base base   new Child        base printValue        base nonStatPrintValue        System out println  Object  static type Child  runtime type Child         Child child   new Child        child printValue        child nonStatPrintValue        System out println  Class  Child static call         Child printValue        System out println  Class  Base static call         Base printValue        System out println  Object  static runtime type Child -- call static from non-static method of Child         child localIndirectStatMethod        System out println  Object  static runtime type Child -- call static from non-static method of Base         child nonLocalIndirectStatMethod            If you run this  I did it on a Mac  from Eclipse  using Java 1 6  you get   Object  static type Base  runtime type Child    Called static Base method    Called non-static Child method  Object  static type Child  runtime type Child    Called static Child method    Called non-static Child method  Class  Child static call    Called static Child method  Class  Base static call    Called static Base method  Object  static runtime type Child -- call static from non-static method of Child    Non-static calls own static      Called static Child method  Object  static runtime type Child -- call static from non-static method of Base    Non-static calls overridden    static      Called static Base method    Here  the only cases which might be a surprise  and which the question is about  appear to be the first case    The run-time type is not used to determine which static methods are called  even when called with an object instance  obj staticMethod       and the last case    When calling a static method from within an object method of a class  the static method chosen is the one accessible from the class itself and not from the class defining the run-time type of the object    Calling with an object instance  The static call is resolved at compile-time  whereas a non-static method call is resolved at run-time   Notice that although static methods are inherited  from parent  they are not overridden  by child    This could be a surprise if you expected otherwise   Calling from within an object method  Object method calls are resolved using the run-time type  but static  class  method calls are resolved using the compile-time  declared  type   Changing the rules  To change these rules  so that the last call in the example called Child printValue    static calls would have to be provided with a type at run-time  rather than the compiler resolving the call at compile-time with the declared class of the object  or context   Static calls could then use the  dynamic  type hierarchy to resolve the call  just as object method calls do today   This would easily be doable  if we changed Java  -O   and is not at all unreasonable  however  it has some interesting considerations   The main consideration is that we need to decide which static method calls should do this   At the moment  Java has this  quirk  in the language whereby obj staticMethod   calls are replaced by ObjectClass staticMethod   calls  normally with a warning    Note  ObjectClass is the compile-time type of obj   These would be good candidates for overriding in this way  taking the run-time type of obj   If we did it would make method bodies harder to read  static calls in a parent class could potentially be dynamically  re-routed    To avoid this we would have to call the static method with a class name -- and this makes the calls more obviously resolved with the compile-time type hierarchy  as now    The other ways of invoking a static method are more tricky  this staticMethod   should mean the same as obj staticMethod    taking the run-time type of this   However  this might cause some headaches with existing programs  which call  apparently local  static methods without decoration  which is arguably equivalent to this method      So what about unadorned calls staticMethod     I suggest they do the same as today  and use the local class context to decide what to do   Otherwise great confusion would ensue   Of course it means that method   would mean this method   if method was a non-static method  and ThisClass method   if method were a static method   This is another source of confusion   Other considerations  If we changed this behaviour  and made static calls potentially dynamically non-local   we would probably want to revisit the meaning of final  private and protected as qualifiers on static methods of a class   We would then all have to get used to the fact that private static and public final methods are not overridden  and can therefore be safely resolved at compile-time  and are  safe  to read as local references

User · Answer

Overriding in Java simply means that the particular method would be called based on the runtime type  of the object and not on the compile-time type of it  which is the case with overridden static methods   As static methods are class methods they are not instance methods so they have nothing to do with the fact which reference is pointing to which Object or instance  because due to the nature of static method it belongs to a specific class  You can redeclare it in the subclass but that subclass won t know anything about the parent class  static methods because  as I said  it is specific to only that class in which it has been declared  Accessing them using object references is just an extra liberty given by the designers of Java and we should certainly not think of stopping that practice only when they restrict it more details and example http   faisalbhagat blogspot com 2014 09 method-overriding-and-method-hiding html

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Personally I think this is a flaw in the design of Java  Yes  yes  I understand that non-static methods are attached to an instance while static methods are attached to a class  etc etc  Still  consider the following code   public class RegularEmployee       private BigDecimal salary       public void setSalary BigDecimal salary            this salary   salary             public static BigDecimal getBonusMultiplier             return new BigDecimal   02               public BigDecimal calculateBonus             return salary multiply getBonusMultiplier                       presumably lots of other code           public class SpecialEmployee extends RegularEmployee       public static BigDecimal getBonusMultiplier             return new BigDecimal   03              This code will not work as you might expect  Namely  SpecialEmployee s get a 2  bonus just like regular employees  But if you remove the  static s  then SpecialEmployee s get a 3  bonus    Admittedly  this example is poor coding style in that in real life you would likely want the bonus multiplier to be in a database somewhere rather than hard-coded  But that s just because I didn t want to bog down the example with a lot of code irrelevant to the point    It seems quite plausible to me that you might want to make getBonusMultiplier static  Perhaps you want to be able to display the bonus multiplier for all the categories of employees  without needing to have an instance of an employee in each category  What would be the point of searching for such example instances  What if we are creating a new category of employee and don t have any employees assigned to it yet  This is quite logically a static function   But it doesn t work   And yes  yes  I can think of any number of ways to rewrite the above code to make it work  My point is not that it creates an unsolvable problem  but that it creates a trap for the unwary programmer  because the language does not behave as I think a reasonable person would expect   Perhaps if I tried to write a compiler for an OOP language  I would quickly see why implementing it so that static functions can be overriden would be difficult or impossible    Or perhaps there is some good reason why Java behaves this way  Can anyone point out an advantage to this behavior  some category of problem that is made easier by this  I mean  don t just point me to the Java language spec and say  see  this is documented how it behaves   I know that  But is there a good reason why it SHOULD behave this way   Besides the obvious  making it work right was too hard       Update   VicKirk  If you mean that this is  bad design  because it doesn t fit how Java handles statics  my reply is   Well  duh  of course   As I said in my original post  it doesn t work  But if you mean that it is bad design in the sense that there would be something fundamentally wrong with a language where this worked  i e  where statics could be overridden just like virtual functions  that this would somehow introduce an ambiguity or it would be impossible to implement efficiently or some such  I reply   Why  What s wrong with the concept    I think the example I give is a very natural thing to want to do  I have a class that has a function that does not depend on any instance data  and which I might very reasonably want to call independent of an instance  as well as wanting to call from within an instance method  Why should this not work  I ve run into this situation a fair number of times over the years  In practice I get around it by making the function virtual  and then creating a static method whose only purpose in life is to be a static method that passes the call on to the virtual method with a dummy instance  That seems like a very roundabout way to get there

User · Answer

Now seeing above answers everyone knows that we can t override static methods  but one should not misunderstood about the concept of accessing static methods from subclass      We can access static methods of super class with subclass reference if this static method has not been hidden by new static method defined in sub class    For Example  see below code -  public class StaticMethodsHiding       public static void main String   args            SubClass hello              class SuperClass       static void hello            System out println  SuperClass saying Hello              class SubClass extends SuperClass          static void hello            System out println  SubClass Hello                 Output -  SuperClass saying Hello           See  Java oracle docs  and search for What You Can Do in a Subclass for details about hiding of static methods in sub class       Thanks

User · Answer

Overriding depends on having an instance of a class  The point of polymorphism is that you can subclass a class and the objects implementing those subclasses will have different behaviors for the same methods defined in the superclass  and overridden in the subclasses   A static method is not associated with any instance of a class so the concept is not applicable   There were two considerations driving Java s design that impacted this  One was a concern with performance  there had been a lot of criticism of Smalltalk about it being too slow  garbage collection and polymorphic calls being part of that  and Java s creators were determined to avoid that  Another was the decision that the target audience for Java was C   developers  Making static methods work the way they do had the benefit of familiarity for C   programmers and was also very fast  because there s no need to wait until runtime to figure out which method to call

[java] Why doesn't Java allow overriding of static methods?

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