What is the difference between up-casting and down-casting with respect to class variable

Question

What is the difference between up-casting and down-casting with respect to class variable   For example in the following program class Animal contains only one method but Dog class              contains two methods  then how we cast the Dog variable to the Animal Variable   If casting is done then how can we call the Dog s another method with Animal s variable   class Animal         public void callme                 System out println  In callme of Animal              class Dog extends Animal         public void callme                 System out println  In callme of Dog               public void callme2                 System out println  In callme2 of Dog             public class UseAnimlas        public static void main  String    args                 Dog d   new Dog                  Animal a    Animal d          d callme            a callme              Dog  a  callme2

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We can create object to Downcasting  In this type also    calling the base class methods   Animal a new Dog    a callme      Dog a  callme2

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Parent  Car Child  Figo Car c1   new Figo           Upcasting - Method  Object c1 will refer to Methods of Class  Figo - Method must be overridden  because class  Figo  is specified with  new   Instance Variable  Object c1 will refer to instance variable of Declaration Class   Car       When Declaration class is parent and object is created of child then implicit casting happens which is  Upcasting               Downcasting - Figo f1    Figo  c1     Method  Object f1 will refer to Method of Class  figo  as initial object c1 is created with class  Figo    but once down casting is done  methods which are only present in class  Figo  can also be referred by variable f1  Instance Variable  Object f1 will not refer to instance variable of Declaration class of object c1  declaration class for c1 is CAR  but with down casting it will refer to instance variables of class Figo              Use  When Object is of Child Class and declaration class is Parent and Child class wants to access Instance variable of it s own class and not of parent class then it can be done with  Downcasting

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Down-casting and up-casting was as follows    Upcasting  When we want to cast a Sub class to Super class  we use Upcasting or widening   It happens automatically  no need to do anything explicitly     Downcasting   When we want to cast a Super class to Sub class  we use Downcasting or narrowing   and Downcasting is not directly possible in Java  explicitly we have to do     Dog d   new Dog    Animal a    Animal  d    Explicitly you have done upcasting  Actually no need  we can directly type cast like Animal a   d  compiler now treat Dog as Animal but still it is Dog even after upcasting d callme    a callme       It calls Dog s method even though we use Animal reference    Dog  a  callme2       Downcasting  Compiler does know Animal it is  In order to use Dog methods  we have to do typecast explicitly     Internally if it is not a Dog object it throws ClassCastException

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I know this question asked quite long time ago but for the new users of this question  Please read this article where contains complete description on upcasting  downcasting and use of  instanceof operator    There s no need to upcast manually  it happens on its own   Mammal m    Mammal new Cat    equals to Mammal m   new Cat    But downcasting must always be done manually   Cat c1   new Cat          Animal a   c1         automatic upcasting to Animal Cat c2    Cat  a       manual downcasting back to a Cat    Why is that so  that upcasting is automatical  but downcasting must be manual  Well  you see  upcasting can never fail  But if you have a group of different Animals and want to downcast them all to a Cat  then there s a chance  that some of these Animals are actually Dogs  and process fails  by throwing ClassCastException  This is where is should introduce an useful feature called  instanceof   which tests if an object is instance of some Class    Cat c1   new Cat                 Animal a   c1          upcasting to Animal     if a instanceof Cat      testing if the Animal is a Cat         System out println  It s a Cat  Now i can safely downcast it to a Cat  without a fear of failure                     Cat c2    Cat a          For more information please read this article

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upcasting means casting the object to a supertype  while downcasting means casting to a subtype   In java  upcasting is not necessary as it s done automatically  And it s usually referred as implicit casting  You can specify it to make it clear to others   Thus  writing  Animal a    Animal d    or  Animal a   d    leads to exactly the same point and in both cases will be executed the callme   from Dog   Downcasting is instead necessary because you defined a as object of Animal  Currently you know it s a Dog  but java has no guarantees it s  Actually at runtime it could be different and java will throw a ClassCastException  would that happen  Of course it s not the case of your very sample example  If you wouldn t cast a to Animal  java couldn t even compile the application because Animal doesn t have method callme2     In your example you cannot reach the code of callme   of Animal from UseAnimlas  because Dog overwrite it  unless the method would be as follow   class Dog extends Animal         public void callme                 super callme            System out println  In callme of Dog

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Maybe this table helps  Calling the callme   method of class Parent or class Child  As a principle      UPCASTING --  Hiding      DOWNCASTING --  Revealing

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Upcasting and downcasting are important part of Java  which allow us to build complicated programs using simple syntax  and gives us great advantages  like Polymorphism or grouping different objects  Java permits an object of a subclass type to be treated as an object of any superclass type  This is called upcasting  Upcasting is done automatically  while downcasting must be manually done by the programmer  and i m going to give my best to explain why is that so  Upcasting and downcasting are NOT like casting primitives from one to other  and i believe that s what causes a lot of confusion  when programmer starts to learn casting objects  Polymorphism  All methods in java are virtual by default  That means that any method can be overridden when used in inheritance  unless that method is declared as final or static  You can see the example below how getType    works according to the object Dog Pet Police Dog  type  Assume you have three dogs  Dog - This is the super Class   Pet Dog    - Pet Dog extends Dog   Police Dog - Police Dog extends Pet Dog  public class Dog      public String getType            System out println  quot NormalDog quot          return  quot NormalDog quot                   Pet Dog has an extra method dogName          public class PetDog extends Dog      public String getType            System out println  quot PetDog quot          return  quot PetDog quot           public String dogName            System out println  quot I don t have Name    quot          return  quot NO Name quot                   Police Dog has an extra method secretId       public class PoliceDog extends PetDog    public String secretId         System out println  quot ID quot        return  quot ID quot        public String getType           System out println  quot I am a Police Dog quot         return  quot Police Dog quot           Polymorphism   All methods in java are virtual by default  That means that any method can be overridden when used in inheritance  unless that method is declared as final or static  Explanation Belongs to Virtual Tables Concept   Virtual Table   Dispatch Table   An object s dispatch table will contain the addresses of the object s dynamically bound methods  Method calls are performed by fetching the method s address from the object s dispatch table  The dispatch table is the same for all objects belonging to the same class  and is therefore typically shared between them   public static void main  String   args                       Creating the different objects with super class Reference                Dog obj1   new Dog                                Object of Pet Dog is created with Dog Reference since                               Upcasting is done automatically for us we don t have to worry about it                                    Dog obj2   new PetDog                                Object of Police Dog is created with Dog Reference since                               Upcasting is done automatically for us we don t have to worry                      about it here even though we are extending PoliceDog with PetDog                since PetDog is extending Dog Java automatically upcast for us                      Dog obj3   new PoliceDog          obj1 getType     Prints Normal Dog   obj2 getType     Prints Pet Dog  obj3 getType     Prints Police Dog Downcasting need to be done by the programmer manually When you try to invoke the secretID    method on obj3 which is PoliceDog object but referenced to Dog which is a super class in the hierarchy it throws error since obj3 don t have access to secretId   method In order to invoke that method you need to Downcast that obj3 manually to PoliceDog      PoliceDog obj3  secretID     which prints ID In the similar way to invoke the dogName   method in PetDog class you need to downcast obj2 to PetDog since obj2 is referenced to Dog and don t have access to dogName    method      PetDog obj2  dogName     Why is that so  that upcasting is automatical  but downcasting must be manual  Well  you see  upcasting can never fail  But if you have a group of different Dogs and want to downcast them all to a to their types  then there s a chance  that some of these Dogs are actually of different types i e   PetDog  PoliceDog  and process fails  by throwing ClassCastException  This is the reason you need to downcast your objects manually if you have referenced your objects to the super class type   Note  Here by referencing means you are not changing the memory address of your ojects when you downcast it it still remains same you are just grouping them to particular type in this case Dog

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Upcasting is casting to a supertype  while downcasting is casting to a subtype  Upcasting is always allowed  but downcasting involves a type check and can throw a ClassCastException   In your case  a cast from a Dog to an Animal is an upcast  because a Dog is-a Animal  In general  you can upcast whenever there is an is-a relationship between two classes   Downcasting would be something like this   Animal animal   new Dog    Dog castedDog    Dog  animal    Basically what you re doing is telling the compiler that you know what the runtime type of the object really is  The compiler will allow the conversion  but will still insert a runtime sanity check to make sure that the conversion makes sense  In this case  the cast is possible because at runtime animal is actually a Dog even though the static type of animal is Animal   However  if you were to do this   Animal animal   new Animal    Dog notADog    Dog  animal    You d get a ClassCastException  The reason why is because animal s runtime type is Animal  and so when you tell the runtime to perform the cast it sees that animal isn t really a Dog and so throws a ClassCastException   To call a superclass s method you can do super method   or by performing the upcast   To call a subclass s method you have to do a downcast  As shown above  you normally risk a ClassCastException by doing this  however  you can use the instanceof operator to check the runtime type of the object before performing the cast  which allows you to prevent ClassCastExceptions   Animal animal   getAnimal       Maybe a Dog  Maybe a Cat  Maybe an Animal  if  animal instanceof Dog           Guaranteed to succeed  barring classloader shenanigans     Dog castedDog    Dog  animal

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Better try this method for upcasting  it s easy to understand      upcasting problem    class Animal        public void callme                 System out println  In callme of Animal             class Dog extends Animal         public void callme                 System out println  In callme of Dog               public void callme2                 System out println  In callme2 of Dog             public class Useanimlas        public static void main  String    args                 Animal animal   new Animal             Dog dog   new Dog            Animal ref          ref   animal          ref callme            ref   dog          ref callme

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1 - Upcasting  Doing an upcasting  you define a tag of some type  that points to an object of a subtype  Type and subtype may be called class and subclass  if you feel more comfortable      Animal animalCat   new Cat     What means that such tag  animalCat  will have the functionality  the methods  of type Animal only  because we ve declared it as type Animal  not as type Cat  We are allowed to do that in a  quot natural implicit automatic quot  way  at compile-time or at a run-time  mainly because Cat inherits some of its functionality from Animal  for example  move     At least  cat is an animal  isn t it   2 - Downcasting  But  what would happen if we need to get the functionality of Cat  from our type Animal tag   As we have created the animalCat tag pointing to a Cat object  we need a way to call the Cat object methods  from our animalCat tag in a some smart pretty way  Such procedure is what we call Downcasting  and we can do it only at the run-time  Time for some code  public class Animal       public String move             return  quot Going to somewhere quot            public class Cat extends Animal      public String makeNoise             return  quot Meow  quot               public class Test        public static void main String   args                   1 - Upcasting            Type     tag        object         Animal animalCat   new Cat          Some animal movement         System out println animalCat move               prints  quot Going to somewhere quot                 2 - Downcasting          Now you wanna make some Animal noise            First of all  type Animal hasn t any makeNoise   functionality            But Cat can do it   I wanna be an Animal Cat now                                         Downcast  tag      Cat s method         String animalNoise      Cat  animalCat   makeNoise                     System out println animalNoise             Prints  quot Meow  quot   as cats usually done                 3 - An Animal may be a Cat  but a Dog or a Rhinoceros too            All of them have their own noises and own functionalities            Uncomment below and read the error in the console                     Type     tag        object           Cat catAnimal   new Animal

[java] What is the difference between up-casting and down-casting with respect to class variable

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