ClassCastException casting Integer to Double

Question

ArrayList marks   new ArrayList    Double sum   0 0  sum     Double marks get i      Everytime I try to run my program  I get a ClassCastException that states  java lang Integer cannot be cast to java lang Double

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We can cast an int to a double but we can t do the same with the wrapper classes Integer and Double    int     a   1   Integer b   1       inboxing  requires Java 1 5    double  c    double  a       OK  Double  d    Double  b       No way    This shows the compile time error that corresponds to your runtime exception

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I think the main problem is that you are casting using wrapper class  seems that they are incompatible types    But another issue is that  i  is an int so you are casting the final result and you should cast i as well  Also try using the keyword  double  to cast and not  Double  wrapper class   You can check here     Java Docs Cast Exception Stack Overflow Thread about ClassCastException   Hope this helps  I found the thread useful but I think this helps further clarify it

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Well the code you ve shown doesn t actually include adding any Integers to the ArrayList - but if you do know that you ve got integers  you can use   sum    double    Integer  marks get i   intValue      That will convert it to an int  which can then be converted to double  You can t just cast directly between the boxed classes   Note that if you can possibly use generics for your ArrayList  your code will be clearer

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Integer x 10  Double y   x doubleValue

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The code posted in the question is obviously not a a complete example  it s not adding anything to the arraylist  it s not defining i anywhere    First as others have said you need to understand the difference between primitive types and the class types that box them  E g  Integer boxes int  Double boxes double  Long boxes long and so-on  Java automatically boxes and unboxes in various scenarios  it used to be you had to box and unbox manually with library calls but that was deemed an ugly PITA     http   docs oracle com javase tutorial java data autoboxing html  You can mostly cast from one primitive type to another  the exception being boolean  but you can t do the same for boxed types  To convert one boxed type to another is a bit more complex  Especially if you don t know the box type in advance  Usually it will involve converting via one or more primitive types   So the answer to your question depends on what is in your arraylist  if it s just objects of type Integer you can do   sum     double  int marks get i      The cast to int will behind the scenes first cast the result of marks get to Integer  then it will unbox that integer  We then use another cast to convert the primitive int to a primitive double  Finally the result will be autoboxed back into a Double when it is assigned to the sum variable   asside  it would probablly make more sense for sum to be of type double rather than Double in most cases    If your arraylist contains a mixture of types but they all implement the Number interface  Integer  Short  Long  Float and Double all do but Character and Boolean do not  then you can do   sum     Number marks get i   doubleValue      If there are other types in the mix too then you might need to consider using the instanceof operator to identify them and take appropriate action

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sum   Double parseDouble    marks get i

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Changing an integer to a double  int abc 12    setting up integer  abc   System out println  double abc      The code will output integer  abc  as a double  which means that it will display as  12 0   Notice how there is a decimal place  indicating that this precision digit has been stored   Same with  double if you want to change it back   double number 13 94   System out println  int number      This code will print on one line   number  as an integer  The output will be  13   Notice that the value has not been rounded up  the data has actually been omitted

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specify your marks    List lt Double gt  marks   new ArrayList lt Double gt       This is called generics

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This means that your ArrayList has integers in some elements  The casting should work unless there s an integer in one of your elements    One way to make sure that your arraylist has no integers is by declaring it as a Doubles array       ArrayList lt Double gt  marks   new ArrayList lt Double gt

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2 things to understand here -  1  If you are casting Primitive interger to Primitive double   It works  e g  It works fine   int pri 12  System out println  double pri    2  if you try to Cast Integer object to Double object or vice - versa   It fails   Integer a   1  Double b    double  a     WRONG  Fails with class cast excptn   Solution -  Soln 1  Integer i   1  Double b   new Double i   soln 2  Double d   2 0  Integer x   d intValue

[java] ClassCastException, casting Integer to Double

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