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's really pretty simple: if you are trying to typecast an object of class A into an object of class B, and they aren't compatible, you get a class cast exception.

Let's think of a collection of classes.

class A {...}
class B extends A {...}
class C extends A {...}

1. You can cast any of these things to Object, because all Java classes inherit from Object.
2. You can cast either B or C to A, because they're both "kinds of" A
3. You can cast a reference to an A object to B only if the real object is a B.
4. You can't cast a B to a C even though they're both A's.

Answer 2

Straight from the API Specifications for the ClassCastException:

Thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance.

So, for example, when one tries to cast an Integer to a String, String is not an subclass of Integer, so a ClassCastException will be thrown.

Object i = Integer.valueOf(42);
String s = (String)i;            // ClassCastException thrown here.

Answer 3

You can better understand ClassCastException and casting once you realize that the JVM cannot guess the unknown. If B is an instance of A it has more class members and methods on the heap than A. The JVM cannot guess how to cast A to B since the mapping target is larger, and the JVM will not know how to fill the additional members.

But if A was an instance of B, it would be possible, because A is a reference to a complete instance of B, so the mapping will be one-to-one.

Answer 4

If you want to sort objects but if class didn't implement Comparable or Comparator, then you will get ClassCastException For example

class Animal{
   int age;
   String type;

   public Animal(int age, String type){
      this.age = age;
      this.type = type;
   }
}
public class MainCls{
   public static void main(String[] args){
       Animal[] arr = {new Animal(2, "Her"), new Animal(3,"Car")};
       Arrays.sort(arr);
   }
}

Above main method will throw below runtime class cast exception

Exception in thread "main" java.lang.ClassCastException: com.default.Animal cannot be cast to java.lang.Comparable

Answer 5

A very good example that I can give you for classcastException in Java is while using "Collection"

List list = new ArrayList();
list.add("Java");
list.add(new Integer(5));

for(Object obj:list) {
    String str = (String)obj;
}

This above code will give you ClassCastException on runtime. Because you are trying to cast Integer to String, that will throw the exception.

Answer 6

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
 }