casting Object array to Integer array error

Question

What s wrong with the following code   Object   a   new Object 1   Integer b 1  a 0  b  Integer   c    Integer    a    The code has the following error at the last line       Exception in thread  main  java lang ClassCastException     Ljava lang Object  cannot be cast to  Ljava lang Integer

User · Answer

Or do the following          Integer   integerArray   new Integer integerList size       integerList toArray integerArray      return integerArray

User · Answer

java lang ClassCastException   Ljava lang Object  cannot be cast to  Ljava lang Integer    you try to cast an Array of Object to cast into Array of Integer  You cant do it  This type of downcast is not permitted   You can make an array of Integer  and after that copy every value of the first array into second array

User · Answer

Ross  you can use Arrays copyof   or Arrays copyOfRange   too   Integer   integerArray   Arrays copyOf a  a length  Integer   class   Integer   integerArray   Arrays copyOfRange a  0  a length  Integer   class     Here the reason to hitting an ClassCastException is you can t treat an array  of Integer as an array of Object  Integer   is a subtype of Object   but Object   is not a Integer     And the following also will not give an ClassCastException   Object   a   new Integer 1   Integer b 1  a 0  b  Integer   c    Integer    a

User · Answer

You can t cast an Object array to an Integer array   You have to loop through all elements of a and cast each one individually   Object   a   new Object 1   Integer b 1  a 0  b  Integer   c   new Integer a length   for int i   0  i  lt  a length  i          c i     Integer  a i       Edit   I believe the rationale behind this restriction is that when casting  the JVM wants to ensure type-safety at runtime  Since an array of Objects can be anything besides Integers  the JVM would have to do what the above code is doing anyway  look at each element individually    The language designers decided they didn t want the JVM to do that  I m not sure why  but I m sure it s a good reason    However  you can cast a subtype array to a supertype array  e g  Integer   to Object

User · Answer

When casting is done in Java  Java compiler as well as Java run-time check whether the casting is possible or not and throws errors in case not  When casting of Object types is involved  the instanceof test should pass in order for the assignment to go through   In your example it results     Object   a   new Object 1       boolean isIntegerArr    a instanceof Integer   If you do a sysout of the above line  it would return false    So trying an instance of check before casting would help  So  to fix the error  you can either add  instanceof  check OR use following line of code   Arrays asList a   toArray c    Please do note that the above code would fail  if the Object array contains any entry that is other than Integer

[java] casting Object array to Integer array error

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