Explanation of “ClassCastException” in Java

Question

I read some articles written on \"ClassCastException\", but I couldn\'t get a good idea on that. Is there a good article or what would be a brief explanation?

Answer 1

It is an Exception which occurs if you attempt to downcast a class, but in fact the class is not of that type.

Consider this heirarchy:

Object -> Animal -> Dog

You might have a method called:

 public void manipulate(Object o) {
     Dog d = (Dog) o;
 }

If called with this code:

 Animal a = new Animal();
 manipulate(a);

It will compile just fine, but at runtime you will get a ClassCastException because o was in fact an Animal, not a Dog.

In later versions of Java you do get a compiler warning unless you do:

 Dog d;
 if(o instanceof Dog) {
     d = (Dog) o;
 } else {
     //what you need to do if not
 }