Downcasting in Java

Question

Upcasting is allowed in Java  however downcasting gives a compile error    The compile error can be removed by adding a cast but would anyway break at the runtime    In this case why Java allows downcasting if it cannot be executed at the runtime  Is there any practical use for this concept    public class demo     public static void main String a            B b    B  new A       compiles with the cast                               but runtime exception - java lang ClassCastException        class A     public void draw         System out println  1           public void draw1         System out println  2           class B extends A     public void draw         System out println  3          public void draw2         System out println  4

User · Answer

Downcast works in the case when we are dealing with an upcasted object. Upcasting:

int intValue = 10;
Object objValue = (Object) intvalue;

So now this objValue variable can always be downcasted to int because the object which was cast is an Integer,

int oldIntValue = (Integer) objValue;
// can be done

but because objValue is an Object it cannot be cast to String because int cannot be cast to String.

User · Answer

Downcasting is allowed when there is a possibility that it succeeds at run time   Object o   getSomeObject    String s    String  o     this is allowed because o could reference a String   In some cases this will not succeed   Object o   new Object    String s    String  o     this will fail at runtime  because o doesn t reference a String   When a cast  such as this last one  fails at runtime a ClassCastException will be thrown   In other cases it will work   Object o    a String   String s    String  o     this will work  since o references a String   Note that some casts will be disallowed at compile time  because they will never succeed at all   Integer i   getSomeInteger    String s    String  i     the compiler will not allow this  since i can never reference a String

User · Answer

To do downcasting in Java  and avoid run-time exceptions  take a reference of the following code   if  animal instanceof Dog      Dog dogObject    Dog  animal      Here  Animal is the parent class and Dog is the child class      instanceof is a keyword that is used for checking if a reference variable is containing a given type of object reference or not

User · Answer

We can all see that the code you provided won t work at run time  That s because we know that the expression new A   can never be an object of type B   But that s not how the compiler sees it  By the time the compiler is checking whether the cast is allowed  it just sees this   variable of type B    B expression of type A    And as others have demonstrated  that sort of cast is perfectly legal  The expression on the right could very well evaluate to an object of type B  The compiler sees that A and B have a subtype relation  so with the  expression  view of the code  the cast might work   The compiler does not consider the special case when it knows exactly what object type expression of type A will really have  It just sees the static type as A and considers the dynamic type could be A or any descendant of A  including B

User · Answer

Original Poster - see inline comments   public class demo        public static void main String a                   B b    B  new A       compiles with the cast  but runtime exception - java lang ClassCastException            - A subclass variable cannot hold a reference to a superclass  variable  so  the above statement will not work             For downcast  what you need is a superclass ref containing a subclass object          A superClassRef   new B     just for the sake of illustration         B subClassRef    B superClassRef     Valid downcast            class A        public void draw                  System out println  1               public void draw1                  System out println  2             class B extends A        public void draw                  System out println  3               public void draw2                  System out println  4

User · Answer

To do downcasting in Java  and avoid run-time exceptions  take a reference of the following code   if  animal instanceof Dog      Dog dogObject    Dog  animal      Here  Animal is the parent class and Dog is the child class      instanceof is a keyword that is used for checking if a reference variable is containing a given type of object reference or not

User · Answer

Downcasting is very useful in the following code snippet I use this all the time  Thus proving that downcasting is useful    private static String printAll LinkedList c        Object arr   c toArray        String list string         for int i 0 i lt c size   i                  String mn  String arr i           list string   mn             return list string      I store String in the Linked List   When I retrieve the elements of Linked List  Objects are returned  To access the elements as Strings or any other Class Objects   downcasting helps me    Java allows us to compile downcast code trusting us that we are doing the wrong thing   Still if humans make a mistake  it is caught at runtime

User · Answer

In this case why Java allows downcasting if it cannot be executed at the runtime    I believe this is because there is no way for the compiler to know at compile-time if the cast will succeed or not  For your example  it s simple to see that the cast will fail  but there are other times where it is not so clear    For instance  imagine that types B  C  and D all extend type A  and then a method public A getSomeA   returns an instance of either B  C or D depending on a randomly generated number  The compiler cannot know which exact run-time type will be returned by this method  so if you later cast the results to B  there is no way to know if the cast will succeed  or fail   Therefore the compiler has to assume casts will succeed

User · Answer

Using your example  you could do   public void doit A a        if a instanceof B               needs to cast to B to access draw2 which isn t present in A            note that this is probably not a good OO-design  but that would            be out-of-scope for this discussion              B a  draw2              a draw

User · Answer

Using your example  you could do   public void doit A a        if a instanceof B               needs to cast to B to access draw2 which isn t present in A            note that this is probably not a good OO-design  but that would            be out-of-scope for this discussion              B a  draw2              a draw

User · Answer

Using your example  you could do   public void doit A a        if a instanceof B               needs to cast to B to access draw2 which isn t present in A            note that this is probably not a good OO-design  but that would            be out-of-scope for this discussion              B a  draw2              a draw

User · Answer

Consider the below example  public class ClastingDemo            param args     public static void main String   args        AOne obj   new Bone          Bone  obj  method2         class AOne   public void method1         System out println  this is superclass           class Bone extends AOne    public void method2         System out println  this is subclass          here we create the object of subclass Bone and assigned it to super class AOne reference and now superclass reference does not know  about the method method2 in the subclass i e Bone  during compile time therefore we need to downcast this reference of superclass to subclass reference so as the resultant reference can know about the presence of methods in the subclass i e Bone

User · Answer

I believe this applies to all statically typed languages   String s    some string   Object o   s     ok String x   o     gives compile-time error  o is not neccessarily a string String x    String o     ok compile-time  but might give a runtime exception if o is not infact a String   The typecast effectively says  assume this is a reference to the cast class and use it as such   Now  lets say o is really an Integer  assuming this is a String makes no sense and will give unexpected results  thus there needs to be a runtime check and an exception to notify the runtime environment that something is wrong   In practical use  you can write code working on a more general class  but cast it to a subclass if you know what subclass it is and need to treat it as such   A typical example is overriding Object equals     Assume we have a class for Car    Override boolean equals Object o        if   o instanceof Car   return false      Car other    Car o         compare this to other and return

User · Answer

I believe this applies to all statically typed languages   String s    some string   Object o   s     ok String x   o     gives compile-time error  o is not neccessarily a string String x    String o     ok compile-time  but might give a runtime exception if o is not infact a String   The typecast effectively says  assume this is a reference to the cast class and use it as such   Now  lets say o is really an Integer  assuming this is a String makes no sense and will give unexpected results  thus there needs to be a runtime check and an exception to notify the runtime environment that something is wrong   In practical use  you can write code working on a more general class  but cast it to a subclass if you know what subclass it is and need to treat it as such   A typical example is overriding Object equals     Assume we have a class for Car    Override boolean equals Object o        if   o instanceof Car   return false      Car other    Car o         compare this to other and return

User · Answer

In this case why Java allows downcasting if it cannot be executed at the runtime    I believe this is because there is no way for the compiler to know at compile-time if the cast will succeed or not  For your example  it s simple to see that the cast will fail  but there are other times where it is not so clear    For instance  imagine that types B  C  and D all extend type A  and then a method public A getSomeA   returns an instance of either B  C or D depending on a randomly generated number  The compiler cannot know which exact run-time type will be returned by this method  so if you later cast the results to B  there is no way to know if the cast will succeed  or fail   Therefore the compiler has to assume casts will succeed

User · Answer

In this case why Java allows downcasting if it cannot be executed at the runtime    I believe this is because there is no way for the compiler to know at compile-time if the cast will succeed or not  For your example  it s simple to see that the cast will fail  but there are other times where it is not so clear    For instance  imagine that types B  C  and D all extend type A  and then a method public A getSomeA   returns an instance of either B  C or D depending on a randomly generated number  The compiler cannot know which exact run-time type will be returned by this method  so if you later cast the results to B  there is no way to know if the cast will succeed  or fail   Therefore the compiler has to assume casts will succeed

User · Answer

Downcasting is allowed when there is a possibility that it succeeds at run time   Object o   getSomeObject    String s    String  o     this is allowed because o could reference a String   In some cases this will not succeed   Object o   new Object    String s    String  o     this will fail at runtime  because o doesn t reference a String   When a cast  such as this last one  fails at runtime a ClassCastException will be thrown   In other cases it will work   Object o    a String   String s    String  o     this will work  since o references a String   Note that some casts will be disallowed at compile time  because they will never succeed at all   Integer i   getSomeInteger    String s    String  i     the compiler will not allow this  since i can never reference a String

User · Answer

We can all see that the code you provided won t work at run time  That s because we know that the expression new A   can never be an object of type B   But that s not how the compiler sees it  By the time the compiler is checking whether the cast is allowed  it just sees this   variable of type B    B expression of type A    And as others have demonstrated  that sort of cast is perfectly legal  The expression on the right could very well evaluate to an object of type B  The compiler sees that A and B have a subtype relation  so with the  expression  view of the code  the cast might work   The compiler does not consider the special case when it knows exactly what object type expression of type A will really have  It just sees the static type as A and considers the dynamic type could be A or any descendant of A  including B

User · Answer

We can all see that the code you provided won t work at run time  That s because we know that the expression new A   can never be an object of type B   But that s not how the compiler sees it  By the time the compiler is checking whether the cast is allowed  it just sees this   variable of type B    B expression of type A    And as others have demonstrated  that sort of cast is perfectly legal  The expression on the right could very well evaluate to an object of type B  The compiler sees that A and B have a subtype relation  so with the  expression  view of the code  the cast might work   The compiler does not consider the special case when it knows exactly what object type expression of type A will really have  It just sees the static type as A and considers the dynamic type could be A or any descendant of A  including B

User · Answer

I believe this applies to all statically typed languages   String s    some string   Object o   s     ok String x   o     gives compile-time error  o is not neccessarily a string String x    String o     ok compile-time  but might give a runtime exception if o is not infact a String   The typecast effectively says  assume this is a reference to the cast class and use it as such   Now  lets say o is really an Integer  assuming this is a String makes no sense and will give unexpected results  thus there needs to be a runtime check and an exception to notify the runtime environment that something is wrong   In practical use  you can write code working on a more general class  but cast it to a subclass if you know what subclass it is and need to treat it as such   A typical example is overriding Object equals     Assume we have a class for Car    Override boolean equals Object o        if   o instanceof Car   return false      Car other    Car o         compare this to other and return

User · Answer

Downcast works in the case when we are dealing with an upcasted object. Upcasting:

int intValue = 10;
Object objValue = (Object) intvalue;

So now this objValue variable can always be downcasted to int because the object which was cast is an Integer,

int oldIntValue = (Integer) objValue;
// can be done

but because objValue is an Object it cannot be cast to String because int cannot be cast to String.

User · Answer

Using your example  you could do   public void doit A a        if a instanceof B               needs to cast to B to access draw2 which isn t present in A            note that this is probably not a good OO-design  but that would            be out-of-scope for this discussion              B a  draw2              a draw

User · Answer

I believe this applies to all statically typed languages   String s    some string   Object o   s     ok String x   o     gives compile-time error  o is not neccessarily a string String x    String o     ok compile-time  but might give a runtime exception if o is not infact a String   The typecast effectively says  assume this is a reference to the cast class and use it as such   Now  lets say o is really an Integer  assuming this is a String makes no sense and will give unexpected results  thus there needs to be a runtime check and an exception to notify the runtime environment that something is wrong   In practical use  you can write code working on a more general class  but cast it to a subclass if you know what subclass it is and need to treat it as such   A typical example is overriding Object equals     Assume we have a class for Car    Override boolean equals Object o        if   o instanceof Car   return false      Car other    Car o         compare this to other and return

User · Answer

Consider the below example  public class ClastingDemo            param args     public static void main String   args        AOne obj   new Bone          Bone  obj  method2         class AOne   public void method1         System out println  this is superclass           class Bone extends AOne    public void method2         System out println  this is subclass          here we create the object of subclass Bone and assigned it to super class AOne reference and now superclass reference does not know  about the method method2 in the subclass i e Bone  during compile time therefore we need to downcast this reference of superclass to subclass reference so as the resultant reference can know about the presence of methods in the subclass i e Bone

User · Answer

In this case why Java allows downcasting if it cannot be executed at the runtime    I believe this is because there is no way for the compiler to know at compile-time if the cast will succeed or not  For your example  it s simple to see that the cast will fail  but there are other times where it is not so clear    For instance  imagine that types B  C  and D all extend type A  and then a method public A getSomeA   returns an instance of either B  C or D depending on a randomly generated number  The compiler cannot know which exact run-time type will be returned by this method  so if you later cast the results to B  there is no way to know if the cast will succeed  or fail   Therefore the compiler has to assume casts will succeed

User · Answer

Downcasting transformation of objects is not possible  Only   DownCasting1  downCasting1    DownCasting1   DownCasting2 downCasting1     is posible  class DownCasting0       public int qwe             System out println  DownCasting0            return -0           class DownCasting1 extends DownCasting0       public int qwe1             System out println  DownCasting1            return -1           class DownCasting2 extends DownCasting1       public int qwe2             System out println  DownCasting2            return -2           public class DownCasting        public static void main String   args             try               DownCasting0 downCasting0   new DownCasting0                DownCasting1 downCasting1   new DownCasting1                DownCasting2 downCasting2   new DownCasting2                 DownCasting0 a1    DownCasting0  downCasting2              a1 qwe      good              System out println downCasting0 instanceof  DownCasting2      false             System out println downCasting1 instanceof  DownCasting2      false             System out println downCasting0 instanceof  DownCasting1      false              DownCasting2  downCasting1   DownCasting2 downCasting1        good             DownCasting1   downCasting1    DownCasting1  downCasting1     good             DownCasting2 a3    DownCasting2  downCasting0     java lang ClassCastException              if downCasting0 instanceof  DownCasting2     false                 DownCasting2 a2    DownCasting2  downCasting0                  a2 qwe      error                            byte b1   127              short b2  32 767              int b3   2 147 483 647              long  b4   9 223 372 036 854 775 807    int large number max 2 147 483 647             long b4   9 223 372 036 854 775 807L              float  b5   3 4e 038    double default             float b5   3 4e 038F    Sufficient for storing 6 to 7 decimal digits             double b6   1 7e 038              double b7   1 7e 038D    Sufficient for storing 15 decimal digits              long c1   b3              int c2    int b4                 int       4 bytes     Stores whole numbers from -2 147 483 648 to 2 147 483 647               float     4 bytes     Stores fractional numbers from 3 4e-038 to 3 4e 038  Sufficient for storing 6 to 7 decimal digits             float c3   b3    logic error             double c4   b4    logic error             catch  Throwable e                e printStackTrace

User · Answer

Original Poster - see inline comments   public class demo        public static void main String a                   B b    B  new A       compiles with the cast  but runtime exception - java lang ClassCastException            - A subclass variable cannot hold a reference to a superclass  variable  so  the above statement will not work             For downcast  what you need is a superclass ref containing a subclass object          A superClassRef   new B     just for the sake of illustration         B subClassRef    B superClassRef     Valid downcast            class A        public void draw                  System out println  1               public void draw1                  System out println  2             class B extends A        public void draw                  System out println  3               public void draw2                  System out println  4

User · Answer

Downcasting transformation of objects is not possible  Only   DownCasting1  downCasting1    DownCasting1   DownCasting2 downCasting1     is posible  class DownCasting0       public int qwe             System out println  DownCasting0            return -0           class DownCasting1 extends DownCasting0       public int qwe1             System out println  DownCasting1            return -1           class DownCasting2 extends DownCasting1       public int qwe2             System out println  DownCasting2            return -2           public class DownCasting        public static void main String   args             try               DownCasting0 downCasting0   new DownCasting0                DownCasting1 downCasting1   new DownCasting1                DownCasting2 downCasting2   new DownCasting2                 DownCasting0 a1    DownCasting0  downCasting2              a1 qwe      good              System out println downCasting0 instanceof  DownCasting2      false             System out println downCasting1 instanceof  DownCasting2      false             System out println downCasting0 instanceof  DownCasting1      false              DownCasting2  downCasting1   DownCasting2 downCasting1        good             DownCasting1   downCasting1    DownCasting1  downCasting1     good             DownCasting2 a3    DownCasting2  downCasting0     java lang ClassCastException              if downCasting0 instanceof  DownCasting2     false                 DownCasting2 a2    DownCasting2  downCasting0                  a2 qwe      error                            byte b1   127              short b2  32 767              int b3   2 147 483 647              long  b4   9 223 372 036 854 775 807    int large number max 2 147 483 647             long b4   9 223 372 036 854 775 807L              float  b5   3 4e 038    double default             float b5   3 4e 038F    Sufficient for storing 6 to 7 decimal digits             double b6   1 7e 038              double b7   1 7e 038D    Sufficient for storing 15 decimal digits              long c1   b3              int c2    int b4                 int       4 bytes     Stores whole numbers from -2 147 483 648 to 2 147 483 647               float     4 bytes     Stores fractional numbers from 3 4e-038 to 3 4e 038  Sufficient for storing 6 to 7 decimal digits             float c3   b3    logic error             double c4   b4    logic error             catch  Throwable e                e printStackTrace

User · Answer

We can all see that the code you provided won t work at run time  That s because we know that the expression new A   can never be an object of type B   But that s not how the compiler sees it  By the time the compiler is checking whether the cast is allowed  it just sees this   variable of type B    B expression of type A    And as others have demonstrated  that sort of cast is perfectly legal  The expression on the right could very well evaluate to an object of type B  The compiler sees that A and B have a subtype relation  so with the  expression  view of the code  the cast might work   The compiler does not consider the special case when it knows exactly what object type expression of type A will really have  It just sees the static type as A and considers the dynamic type could be A or any descendant of A  including B

User · Answer

Downcasting is very useful in the following code snippet I use this all the time  Thus proving that downcasting is useful    private static String printAll LinkedList c        Object arr   c toArray        String list string         for int i 0 i lt c size   i                  String mn  String arr i           list string   mn             return list string      I store String in the Linked List   When I retrieve the elements of Linked List  Objects are returned  To access the elements as Strings or any other Class Objects   downcasting helps me    Java allows us to compile downcast code trusting us that we are doing the wrong thing   Still if humans make a mistake  it is caught at runtime

User · Answer

Downcasting is allowed when there is a possibility that it succeeds at run time   Object o   getSomeObject    String s    String  o     this is allowed because o could reference a String   In some cases this will not succeed   Object o   new Object    String s    String  o     this will fail at runtime  because o doesn t reference a String   When a cast  such as this last one  fails at runtime a ClassCastException will be thrown   In other cases it will work   Object o    a String   String s    String  o     this will work  since o references a String   Note that some casts will be disallowed at compile time  because they will never succeed at all   Integer i   getSomeInteger    String s    String  i     the compiler will not allow this  since i can never reference a String

User · Answer

Downcasting is allowed when there is a possibility that it succeeds at run time   Object o   getSomeObject    String s    String  o     this is allowed because o could reference a String   In some cases this will not succeed   Object o   new Object    String s    String  o     this will fail at runtime  because o doesn t reference a String   When a cast  such as this last one  fails at runtime a ClassCastException will be thrown   In other cases it will work   Object o    a String   String s    String  o     this will work  since o references a String   Note that some casts will be disallowed at compile time  because they will never succeed at all   Integer i   getSomeInteger    String s    String  i     the compiler will not allow this  since i can never reference a String

[java] Downcasting in Java

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Examples related to casting