Casting objects in Java

Question

I m confused about what it means to cast objects in Java   Say you have     Superclass variable   new Subclass object     Superclass variable  method      What is happening here  Does the variable type change  or is it the object within the variable that changes  Very confused

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Superclass variable   new subclass object    This just creates an object of type subclass  but assigns it to the type superclass  All the subclasses  data is created etc  but the variable cannot access the subclasses data functions  In other words  you cannot call any methods or access data specific to the subclass  you can only access the superclasses stuff   However  you can cast Superclassvariable to the Subclass and use its methods data

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Have a look at this sample   public class A       statements    public class B extends A     public void foo          A a new B       To execute   foo     method     B a  foo

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Sometimes you will like to receive as argument a Parent reference and inside you probably want to do something specific of a child   abstract class Animal   public abstract void move      class Shark extends Animal   public void move      swim        public void swim      public void bite       class Dog extends Animal   public void move      run        public void run      public void bark             void somethingSpecific Animal animal       Here you don t know and may don t care which animal enters  animal move       You can call parent methods but you can t call bark or bite   if animal instanceof Shark     Shark shark    Shark animal    shark bite       Now you can call bite        doSomethingSharky animal      You cannot call this method           In above s method you can pass either Shark or Dog  but what if you have something like this   void doSomethingSharky Shark shark      Here you cannot receive an Animal reference     That method can only be called by passing shark references So if you have an Animal  and it is deeply a Shark  you can call it like this   Animal animal    doSomethingSharky  Shark  animal    Bottom line  you can use Parent references and it is usually better when you don t care about the implementation of the parent and use casting to use the Child as an specific object  it will be exactly the same object  but your reference know it  if you don t cast it  your reference will point to the same object but cannot be sure what kind of Animal would it be  therefore will only allow you to call known methods

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The example you are referring to is called Upcasting in java   It creates a subclass object with a super class variable pointing to it   The variable does not change  it is still the variable of the super class but it is pointing to the object of subclass   For example lets say you have two classes Machine and Camera   Camera is a subclass of Machine  class Machine       public void start             System out println  Machine Started             class Camera extends Machine       public void start                 System out println  Camera Started                   public void snap             System out println  Photo taken              Machine machine1   new Camera    machine1 start      If you execute the above statements it will create an instance of Camera class with a reference of Machine class pointing to it So  now the output will be  Camera Started  The variable is still a reference of Machine class  If you attempt machine1 snap    the code will not compile  The takeaway here is all Cameras are Machines since Camera is a subclass of Machine but all Machines are not Cameras  So you can create an object of subclass and point it to a super class refrence but you cannot ask the super class reference to do all the functions of a subclass object  In our example machine1 snap   wont compile   The superclass reference has access to only the functions known to the superclass  In our example machine1 start     You can not ask a machine reference to take a snap

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Lets say you have Class A as superclass and Class B subclass of A   public class A        public void printFromA            System out println  Inside A            public class B extends A        public void printFromB            System out println  Inside B              public class MainClass        public static void main String   args            A a   new B            a printFromA      this can be called without typecasting            B a  printFromB      the method printFromB needs to be typecast

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Say you have a superclass Fruit and the subclass Banana and you have a method addBananaToBasket    The method will not accept grapes for example so you want to make sure that you re adding a banana to the basket   So   Fruit myFruit   new Banana      Banana myFruit  addBananaToBasket      This is called casting

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In this example your superclass variable is telling the subclass object to implement the method of the superclass  This is the case of the java object type casting  Here the method   function is originally the method of the superclass but the superclass variable cannot access the other methods of the subclass object that are not present in the superclass

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Casting is necessary to tell that you are calling a child and not a parent method  So it s ever downward  However if the method is already defined in the parent class and overriden in the child class  you don t any cast  Here an example   class Parent     void method    System out print  this is the parent         class Child extends Parent      override    void method    System out print  this is the child            Parent o   new Child    o method      Child o  method        The two method call will both print    this is the child

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For example you have Animal superclass and Cat subclass Say your subclass has speak    method   class Animal     public void walk             class Cat extends Animal      Override   public void walk            public void speak            public void main String args          Animal a new Cat          a speak    Compile Error        If you use speak method for  a  reference variable you should downcast  Like this         Cat a  speak

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in some cases we can   t provide guarantee for the type of elements or objects present inside our collection or wise   at the time of retrieval compulsory we should perform type casting otherwise we will get compile time error   Arrays are always type safe that is we can provide the guarantee for the type of elements present inside array  to achieve type safety we have to use typecasting

[java] Casting objects in Java

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