casting Object array to Integer array error

Question

What's wrong with the following code?

Object[] a = new Object[1];
Integer b=1;
a[0]=b;
Integer[] c = (Integer[]) a;

The code has the following error at the last line :

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;

score 99 · Accepted Answer · edited Jan 27 '17 at 23:27

99

Ross, you can use Arrays.copyof() or Arrays.copyOfRange() too.

Integer[] integerArray = Arrays.copyOf(a, a.length, Integer[].class);
Integer[] integerArray = Arrays.copyOfRange(a, 0, a.length, Integer[].class);

Here the reason to hitting an ClassCastException is you can't treat an array of Integer as an array of Object. Integer[] is a subtype of Object[] but Object[] is not a Integer[].

And the following also will not give an ClassCastException.

Object[] a = new Integer[1];
Integer b=1;
a[0]=b;
Integer[] c = (Integer[]) a;

edited Jan 27 '17 at 23:27

Sotirios Delimanolis

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answered Nov 21 '11 at 23:59

namalfernandolk

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Why does `Arrays.copyOf` never throw a ClassCastException? – Kvass Aug 13 '14 at 04:55
1

Because in there I give the Integer[].class as the parameter. I suppose internally, each object will convert to Integer and added to the Integer array – namalfernandolk Aug 23 '14 at 02:08
Yeah well typically one encounters this when working with generics, where this answer is useless. – Tomáš Zato - Reinstate Monica Feb 19 '19 at 13:04

score 25 · Answer 2 · edited Aug 14 '16 at 23:24

25

You can't cast an Object array to an Integer array. You have to loop through all elements of a and cast each one individually.

Object[] a = new Object[1];
Integer b=1;
a[0]=b;
Integer[] c = new Integer[a.length];
for(int i = 0; i < a.length; i++)
{
    c[i] = (Integer) a[i];
}

Edit: I believe the rationale behind this restriction is that when casting, the JVM wants to ensure type-safety at runtime. Since an array of Objects can be anything besides Integers, the JVM would have to do what the above code is doing anyway (look at each element individually). The language designers decided they didn't want the JVM to do that (I'm not sure why, but I'm sure it's a good reason).

However, you can cast a subtype array to a supertype array (e.g. Integer[] to Object[])!

edited Aug 14 '16 at 23:24

Vladimir Vagaytsev

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answered Jul 12 '09 at 03:17

Sean

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The fact that arrays are covariant means that the JVM already has to check for type safety when it performs assignments - but not when it just reads an element. – Jon Skeet Jul 12 '09 at 06:27
4

The reason is pretty simple. If you were allowed to cast an Object[] to Integer[], then the JVM could never be sure of the actual type of the objects in the array, since old references to the array as an Object[] might still exist. It would have to typecheck every access of an object in an array, every time, since it would never know for sure what it was. And if the type check failed, it would throw an exception somewhere completely different from where the cause was. – Captain Ford Sep 27 '16 at 18:04

score 14 · Answer 3 · answered Jul 26 '11 at 11:33

14

Or do the following:

...

  Integer[] integerArray = new Integer[integerList.size()];
  integerList.toArray(integerArray);

  return integerArray;

}

answered Jul 26 '11 at 11:33

Michael

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Is this integerArray which is Integer[] equal to int[] ? I guess it is not. Need to do unboxing. – Akh Aug 28 '13 at 23:06
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Only works if _integerList_ is a List or an ArrayList.. not for "classic" Arrays tho :( – f1v3 Sep 13 '17 at 14:35
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you could also use `Arrays.asList(objectArray).toArray(new Integer[objectArray.length])`, but then, this does exactly the same as [namalfernandolk's answer](https://stackoverflow.com/a/8220245/6723250) with unneccessary overhead – Xerus Oct 17 '17 at 15:53

score 5 · Answer 4 · edited Feb 05 '12 at 23:38

5

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;

you try to cast an Array of Object to cast into Array of Integer. You cant do it. This type of downcast is not permitted.

You can make an array of Integer, and after that copy every value of the first array into second array.

edited Feb 05 '12 at 23:38

thkala

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answered Jul 12 '09 at 11:59

Tommaso Taruffi

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S R Chaitanya · Answer 5 · 2018-03-06T07:24:11.273

When casting is done in Java, Java compiler as well as Java run-time check whether the casting is possible or not and throws errors in case not.

When casting of Object types is involved, the instanceof test should pass in order for the assignment to go through. In your example it results
Object[] a = new Object[1]; boolean isIntegerArr = a instanceof Integer[]
If you do a sysout of the above line, it would return false;
So trying an instance of check before casting would help. So, to fix the error, you can either add 'instanceof' check
OR
use following line of code:(Arrays.asList(a)).toArray(c);
Please do note that the above code would fail, if the Object array contains any entry that is other than Integer.

casting Object array to Integer array error

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