Why do I get class cast exception for int.class cast

Question

In below program I fail to understand why there is ClassCastException for cast from int.class

Update: I should specify I know what primitive types are. What I don't understand is why int.class is provided with broken implementation?

  public static void main(String[] args) {
    System.out.println(DataType.INT.getValue(Integer.class));
    System.out.println(DataType.INT.getValue(int.class));//Class cast exception here
}

enum DataType {
    INT {
        @Override
        public <T> T getValue(Class<T> toClass) {
            return toClass.cast(1000);//ClassCastException here for int.class
        }
    };
    public abstract <T> T getValue(Class<T> toClass);

}

Answer 1

This happens because cast() operation of Class uses isInstance() method which returns false for primitive classes.

If this Class object represents a primitive type, this method returns false.

Code for cast() method is below

public T cast(Object obj) {
if (obj != null && !isInstance(obj))//It fails here since isInstance returns false
    throw new ClassCastException();
return (T) obj;

Answer 2

Ok, after going through some links, and trying out some code, I found out that: -

• int.class == Integer.TYPE == int
• int.class != Integer.class

So, the value of int.class is Class object representing the type int.

So, when you invoke your DataType.INT with int.class, the toClass containts int, on which you cannot invoke cast. May be because it does not extend from Object class. Because, cast method internally uses isinstance to check whether the invoking type is an Object type or not.

public T cast(Object obj) {
if (obj != null && !isInstance(obj))
    throw new ClassCastException();
return (T) obj;
}

So, if the type that invokes cast is not an instance of Object, which of course primitive types are not, it will return false, and hence a ClassCastException.