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);

}

enum `INT` is not allowed in java so I am trying to do it other way. But as you can see it is error prone if some once passes `int.class` it will raise an `ClassCastException`. And compiler does restrict `INT` but this way it does not. — Amit Deshpande, Oct 21 '12 at 06:05

Rohit Jain · Answer 1 · 2012-10-21T15:20:54.770

3

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.

edited Oct 21 '12 at 15:20

answered Oct 21 '12 at 05:37

Rohit Jain

209,639
45
409
525

Found the cause added it as answer. – Amit Deshpande Oct 21 '12 at 06:18
1

@AmitD.. Yeah I just got that link also. But as you posted it. So no need to post. :) – Rohit Jain Oct 21 '12 at 06:19
I think little room for improvement in your last statement. It's a fact that all the classes in Java extend `Object`. There is no instanceof check against `Object`. – Bhesh Gurung Oct 21 '12 at 06:47
"So, the value of int.class is int." This doesn't make any sense at all. `int` is a type, not a value. The value of `int.class` is a `Class` object that represents the type `int`. – newacct Oct 21 '12 at 10:31

score 3 · Answer 2 · answered Oct 21 '12 at 06:13

3

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;

answered Oct 21 '12 at 06:13

Amit Deshpande

19,001
4
46
72

And the autoboxing causes obj to be an instance of `Integer`. – Bhesh Gurung Oct 21 '12 at 06:19

score 0 · Answer 3 · edited May 23 '17 at 12:00

Integer is a class wrapper for int values, while int is a primitive type, not a class. Don't confuse between primitive types and classes, for example, in code when you must use classes:

List<int> lstInteger; //won't compile!
List<Integer> lstInteger; //compiles fine

More info:

Think of int type as a primitive C/C++ int type. Knowing this, the int is not a class, thus not having any attributes, any methods, just holding a integer value. On the other hand, java.lang.Integer is a class wrapper for int primitive type. This is in order to use the int in cases where only object instances could be used. A good example of this is the generic system in Java that works only for classes (as proposed in your code example).

For deeper info in this, you can read from the Oracle Java Tutorial: Primitive Data Types.

Both of these links does not specify that `cast` should fail ? — Amit Deshpande, Oct 21 '12 at 05:47

Bhesh Gurung · Answer 4 · 2012-10-21T06:35:16.623

0

I think it's because of the fact that int.class is not same as Integer.class.

Update

We can look at the cast method, it parameter is an Object so it's obvious that 1000 get autoboxed, thus the actual argument to the method is an instance of Integer:

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

edited Oct 21 '12 at 06:35

answered Oct 21 '12 at 05:46

Bhesh Gurung

50,430
22
93
142

I know that but why `cast` operation fails as finally its a class object ? – Amit Deshpande Oct 21 '12 at 05:47
There's no such class int, it's a primitive type. – Mordechai Oct 21 '12 at 05:48
1

@AmitD: Cause they are not same class and they don't share parent-child relationship. – Bhesh Gurung Oct 21 '12 at 05:50
It looks like you're confusing Java with other OOP languages. In Java, `int` is just a primitive type, not a class. `int.anyMethod` will be a compiler error. – Luiggi Mendoza Oct 21 '12 at 05:56
@LuiggiMendoza: You should give it a try on your IDE. – Bhesh Gurung Oct 21 '12 at 05:58

score 0 · Answer 5 · answered Oct 21 '12 at 05:56

0

A class cast exception is thrown by Java when you try to cast an Object of one data type to another. Here int is not an Object, it's a primitive which gets instantiated by some native code at runtime.

answered Oct 21 '12 at 05:56

Arham

2,072
2
18
30

Yogendra Singh · Answer 6 · 2012-10-21T06:35:03.907

I think this is what happening:

When you call return toClass.cast(1000);, the literal 1000 is getting promoted to Integer. Promotion rules are mentioned here : Java Language Specification.

Now when, cast method is called on intClass, its checks if argument(1000) is of same instance i.e. int.class and it fails because its not.

On the other hand, when you cast to Integer.class, it is successful as instance type does match.

So the ClassCastException is coming because of Promotion of literal 1000 to Integer.

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

6 Answers6

Linked

Related