explicit casting from super class to subclass

Question

public class Animal       public void eat         public class Dog extends Animal       public void eat           public void main String   args            Animal animal   new Animal            Dog dog    Dog  animal            The assignment Dog dog    Dog  animal  does not generate a compilation error  but at runtime it generates a ClassCastException  Why can t the compiler detect this error

User · Answer

As explained  it is not possible  If you want to use a method of the subclass  evaluate the possibility to add the method to the superclass  may be empty  and call from the subclasses getting the behaviour you want  subclass  thanks to polymorphism  So when you call d method   the call will succeed withoug casting  but in case the object will be not a dog  there will not be a problem

User · Answer

The code generates a compilation error because your instance type is an Animal   Animal animal new Animal      Downcasting is not allowed in Java for several reasons  See here for details

User · Answer

To develop the answer of  Caumons      Imagine one father class has many children and there is a need to add a common   field into that class  If you consider the mentioned approach  you should   go to each children class one by one and refactor their constructors for the new field    therefore that solution is not a promising solution in this scenario   Now take a look at this solution      A father can receive an self object from each children  Here is a father   class    public class Father        protected String fatherField       public Father Father a           fatherField   a fatherField               Second constructor     public Father String fatherField           this fatherField   fatherField                    Other constructors   Getters and Setters for the Fields        Here is our child class that should implement one of its father   constructor  in this case the aforementioned constructor     public class Child extends Father        protected String childField       public Child Father father  String childField             super father           this childField   childField                    Other constructors   Getters and Setters for the Fields       Override     public String toString             return String format  Father Field is   s nChild Field is   s   fatherField  childField             Now we test out application   public class Test       public static void main String   args            Father fatherObj   new Father  Father String            Child child   new Child fatherObj   Child String            System out println child             And here is the result        Father Field is  Father String       Child Field is  Child String   Now you can easily add new fields to father class without being worried of your children codes to break

User · Answer

Because theoretically Animal animal can be a dog   Animal animal   new Dog      Generally  downcasting is not a good idea  You should avoid it  If you use it  you better include a check    if  animal instanceof Dog        Dog dog    Dog  animal

User · Answer

Elaborating the answer given by Michael Berry   Dog d    Dog Animal    Compiles but fails at runtime  Here you are saying to the compiler  Trust me  I know d is really referring to a Dog object  although it s not   Remember compiler is forced to trust us when we do a downcast       The compiler only knows about the declared reference type  The JVM at runtime knows what the object really is     So when the JVM at the runtime figures out that the Dog d is actually referring to an Animal and not a Dog object it says   Hey    you lied to the compiler and throws a big fat ClassCastException   So if you are downcasting you should use instanceof test to avoid screwing up   if  animal instanceof Dog        Dog dog    Dog  animal     Now a question comes to our mind  Why the hell compiler is allowing the downcast when eventually it is going to throw a java lang ClassCastException   The answer is that all the compiler can do is verify that the two types are in the same inheritance tree  so depending on whatever code might have  come before the downcast  it s possible that animal is of type dog       The compiler must allow things that might possible work at runtime    Consider the following code snipet   public static void main String   args            Dog d   getMeAnAnimal      ERROR  Type mismatch  cannot convert Animal to Dog     Dog d    Dog getMeAnAnimal       Downcast works fine  No ClassCastException        d eat        private static Animal getMeAnAnimal         Animal animal   new Dog        return animal      However  if the compiler is sure that the cast would not possible work  compilation will fail  I E  If you try to cast objects in different inheritance hierarchies   String s    String d     ERROR   cannot cast for Dog to String     Unlike downcasting  upcasting works implicitly because when you upcast you are implicitly restricting the number of method you can invoke     as opposite to downcasting  which implies that later on  you might want to invoke a more specific method    Dog d   new Dog     Animal animal1   d        Works fine with no explicit cast  Animal animal2    Animal  d     Works fine with n explicit cast  Both of the above upcast will work fine without any exception because a Dog IS-A Animal  anithing an Animal can do  a dog can do  But it s not true vica-versa

User · Answer

In order to avoid this kind of ClassCastException  if you have   class A class B extends A   You can define a constructor in B that takes an object of A  This way we can do the  cast  e g    public B A a        super a arg1  a arg2     arg1 and arg2 must be  at least  protected in class A        If B class has more attributes  then you would initilize them here

User · Answer

By using a cast you re essentially telling the compiler  trust me  I m a professional  I know what I m doing and I know that although you can t guarantee it  I m telling you that this animal variable is definitely going to be a dog    Since the animal isn t actually a dog  it s an animal  you could do Animal animal   new Dog    and it d be a dog  the VM throws an exception at runtime because you ve violated that trust  you told the compiler everything would be ok and it s not    The compiler is a bit smarter than just blindly accepting everything  if you try and cast objects in different inheritence hierarchies  cast a Dog to a String for example  then the compiler will throw it back at you because it knows that could never possibly work   Because you re essentially just stopping the compiler from complaining  every time you cast it s important to check that you won t cause a ClassCastException by using instanceof in an if statement  or something to that effect

[java] explicit casting from super class to subclass

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