Is there a way to override class variables in Java

Question

class Dad       protected static String me    dad        public void printMe                 System out println me            class Son extends Dad       protected static String me    son      public void doIt         new Son   printMe        The function doIt will print  dad   Is there a way to make it print  son

User · Answer

You cannot override variables in a class  You can override only methods  You should keep the variables private otherwise you can get a lot of problems

User · Answer

only by overriding printMe     class Son extends Dad        public void printMe                  System out println  son              the reference to me in the Dad printMe method implicitly points to the static field Dad me  so one way or another you re changing what printMe does in Son

User · Answer

Yes  But as the variable is concerned it is overwrite  Giving new value to variable  Giving new definition to the function is Override   Just don t declare the variable but initialize  change  in the constructor or static block  The value will get reflected when using in the blocks of parent class if the variable is static then change the value during initialization itself with static block  class Son extends Dad       static           me    quot son quot             or else change in constructor  You can also change the value later in any blocks  It will get reflected in super class

User · Answer

class Dad       protected static String me    dad        public void printMe                 System out println me            class Son extends Dad       protected static String  me   me    son      public void doIt         new Son   printMe            will print  son

User · Answer

You can create a getter and then override that getter   It s particularly useful if the variable you are overriding is a sub-class of itself   Imagine your super class has an Object member but in your sub-class this is now more defined to be an Integer   class Dad           private static final String me    dad            protected String getMe                 return me                     public void printMe                             System out println getMe                  class Son extends Dad           private static final String me    son             Override         protected String getMe                 return me               public void doIt             new Son   printMe      Prints  son

User · Answer

Variables don t take part in overrinding  Only methods do  A method call is resolved at runtime  that is  the decision to call a method is taken at runtime  but the variables are decided at compile time only  Hence that variable is called whose reference is used for calling and not of the runtime object   Take a look at following snippet   package com demo   class Bike     int max speed   90    public void disp speed         System out println  Inside bike          public class Honda bikes extends Bike     int max speed   150    public void disp speed         System out println  Inside Honda       public static void main String   args        Honda bikes obj1   new Honda bikes        Bike obj2   new Honda bikes        Bike obj3   new Bike         obj1 disp speed        obj2 disp speed        obj3 disp speed         System out println  Max Speed       obj1 max speed       System out println  Max Speed       obj2 max speed       System out println  Max Speed       obj3 max speed            When you run the code  console will show    Inside Honda Inside Honda Inside bike  Max Speed   150 Max Speed   90 Max Speed   90

User · Answer

In short  no  there is no way to override a class variable    You do not override class variables in Java you hide them  Overriding is for instance methods  Hiding is different from overriding    In the example you ve given  by declaring the class variable with the name  me  in class Son you hide the class variable it would have inherited from its superclass Dad with the same name  me   Hiding a variable in this way does not affect the value of the class variable  me  in the superclass Dad   For the second part of your question  of how to make it print  son   I d set the value via the constructor  Although the code below departs from your original question quite a lot  I would write it something like this   public class Person       private String name       public Person String name            this name   name             public void printName             System out println name             The JLS gives a lot more detail on hiding in section 8 3 - Field Declarations

User · Answer

Why would you want to override variables when you could easily reassign them in the subClasses   I follow this pattern to work around the language design  Assume a case where you have a weighty service class in your framework which needs be used in different flavours in multiple derived applications In that case   the best way to configure the super class logic is by reassigning its  defining  variables    public interface ExtensibleService  void init       public class WeightyLogicService implements ExtensibleService      private String directoryPath  c  hello        public void doLogic               never forget to call init   before invocation or build safeguards          init             some logic goes here          public void init             public class WeightyLogicService myAdaptation extends WeightyLogicService       Override    public void init        directoryPath  c  my hello

User · Answer

This looks like a design flaw   Remove the static keyword and set the variable for example in the constructor  This way Son just sets the variable to a different value in his constructor

User · Answer

Of course using private attributes  and getters and setters would be the recommended thing to do  but I tested the following  and it works     See the comment in the code  class Dad       protected static String me    dad        public void printMe                 System out println me            class Son extends Dad       protected static String me    son                Adding Method printMe   to this class  outputs son      even though Attribute me from class Dad can apparently not be overridden             public void printMe                 System out println me            class Tester       public static void main String   arg                new Son   printMe              Sooo     did I just redefine the rules of inheritance or did I put Oracle into a tricky situation   To me  protected static String me is clearly overridden  as you can see when you execute this program  Also  it does not make any sense to me why attributes should not be overridable

User · Answer

https   docs oracle com javase tutorial java IandI hidevariables html  It s called Hiding Fields  From the link above     Within a class  a field that has the same name as a field in the superclass hides the superclass s field  even if their types are different  Within the subclass  the field in the superclass cannot be referenced by its simple name  Instead  the field must be accessed through super  which is covered in the next section  Generally speaking  we don t recommend hiding fields as it makes code difficult to read

User · Answer

It indeed prints  dad   since the field is not overridden but hidden  There are three approaches to make it print  son    Approach 1  override printMe  class Dad       protected static String me    dad        public void printMe                 System out println me            class Son extends Dad       protected static String me    son         override     public void printMe                 System out println me            public void doIt         new Son   printMe        Approach 2  don t hide the field and initialize it in the constructor  class Dad       protected static String me    dad        public void printMe                 System out println me            class Son extends Dad       public Son                 me    son            public void doIt         new Son   printMe        Approach 3  use the static value to initialize a field in the constructor  class Dad       private static String meInit    Dad        protected String me       public Dad                 me   meInit             public void printMe                 System out println me            class Son extends Dad       private static String meInit    son        public Son                 me   meInit            public void doIt         new Son   printMe

User · Answer

Just Call super variable in sub class constructor  public abstract class Beverage    int cost    int getCost          return cost          public class Coffee extends Beverage     int cost   10  Coffee        super cost   cost          public class Driver    public static void main String   args         Beverage coffee   new Coffee         System out println coffee getCost             Output is 10

User · Answer

If you are going to override it I don t see a valid reason to keep this static  I would suggest the use of abstraction  see example code            public interface Person           public abstract String getName             this will be different for each person  so no need to make it concrete         public abstract void setName String name           Now we can add the Dad   public class Dad implements Person        private String name       public Dad String name            setName name               Override     public final String getName         return name              Override     public final void setName String name            this name   name            the son   public class Son implements Person        private String name       public Son String name            setName name               Override     public final String getName             return name              Override     public final void setName String name            this name   name            and Dad met a nice lady   public class StepMom implements Person        private String name       public StepMom String name            setName name               Override     public final String getName             return name              Override     public final void setName String name            this name   name            Looks like we have a family  lets tell the world their names   public class ConsoleGUI        public static void main String   args            List lt Person gt  family   new ArrayList lt Person gt             family add new Son  Tommy             family add new StepMom  Nancy             family add new Dad  Dad             for  Person person   family                  using the getName vs printName lets the caller  in this case the               ConsoleGUI determine versus being forced to output through the console               System out print person getName                       System err print person getName                       JOptionPane showMessageDialog null  person getName                  System out Output   Tommy Nancy Dad  System err is the same as above just has red font  JOption Output  Tommy then  Nancy then       Dad

User · Answer

Though it is true that class variables may only be hidden in subclasses  and not overridden  it is still possible to do what you want without overriding printMe    in subclasses  and reflection is your friend  In the code below I omit exception handling for clarity  Please note that declaring me as protected does not seem to have much sense in this context  as it is going to be hidden in subclasses     class Dad         static String me    dad        public void printMe                    java lang reflect Field field   this getClass    getDeclaredField   me            System out println  field get  null

User · Answer

No  Class variables Also applicable to instance variables  don t exhibit overriding feature in Java as class variables are invoked on the basis of the type of calling object  Added one more class Human  in the hierarchy to make it more clear  So now we have   Son extends Dad extends Human  In the below code  we try to iterate over an array of Human  Dad and Son objects  but it prints Human Class   s values in all cases as the type of calling object was Human       class Human       static String me    human        public void printMe                 System out println me           class Dad extends Human       static String me    dad       class Son extends Dad       static String me    son       public class ClassVariables       public static void main String   abc              Human   humans   new Human 3           humans 0    new Human            humans 1    new Dad            humans 2    new Son            for Human human  humans                  System out println human me             prints human for all objects                     Will print   human human human      So no overriding of Class variables    If we want to access the class variable of actual object from a reference variable of its parent class  we need to explicitly tell this to compiler by casting parent reference  Human object  to its type       System out println   Dad humans 1   me             prints dad      System out println   Son humans 2   me             prints son   Will print   dad son   On how part of this question - As already suggested override the printMe   method in Son class  then on calling  Son   printMe      Dad s Class variable  me  will be hidden because the nearest declaration from Son class printme   method  of the  me  in Son class  will get the precedence

User · Answer

Yes  just override the printMe   method   class Son extends Dad           public static final String me    son             Override         public void printMe                     System out println me

[java] Is there a way to override class variables in Java?

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