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

Answer 1

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.

Answer 2

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.