Generics: Why Class clazzIntArray = int[].class; does not work

Question

Following lines:

Class<Integer> clazzInteger = Integer.class;
Class<Integer> clazzInt = int.class;

Are valid and will compile/run even when:

if(clazzInt.equals(clazzInteger)) {
  System.out.println("clazzInt equals clazzInteger");
}else {
  System.out.println("clazzInt and clazzInteger are not equal");
}

Will print clazzInt and clazzInteger are not equal. But Class<int> clazzInt = int.class; do not work of course.

So why this analogy cannot be applied to array types?

Class<int[]> clazzIntArray = int[].class;
Class<Integer[]> clazzIntArray = int[].class;  // type mismatch: 
//cannot convert from Class<int[]> to Class<Integer[]>

But

Class<int[]> clazzIntArray = int[].class; // this is ok

I'm now baffled why Class<Integer[]> clazzIntArray = int[].class is invalid? What Class<int[]> means? And why the analogy between array and non array types does not work?

int[] and Integer[] are different classes. Instance of `int[]` is not an instance of `Integer[]`, and you cannot even cast between them iirc — amit, Aug 13 '15 at 07:53
Related: http://stackoverflow.com/questions/880581/how-to-convert-int-to-integer-in-java — Maroun, Aug 13 '15 at 07:58
@MarounMaroun Why should it? They're different classes, and one isn't a subtype of the other. It doesn't appear to be a candidate for autoboxing either, although no doubt the explanation lies somewhere around there. — user207421, Aug 13 '15 at 08:09

score 1 · Answer 1 · answered Aug 13 '15 at 07:52

1

Under the hood, Autoboxing/Unboxing happens on individual elements inside the array. not with whole array type.

Java cannot magically convert whole primitive array to a Wrapper array. An individual element and an Array which consists of individual elements is the point here.

For ex : Array is a Basket and Fruit's inside the basket are elements (int's/Integer's)

answered Aug 13 '15 at 07:52

Suresh Atta

120,458
37
198
307

Does it mean that Autoboxing is applied also on generics type parameters ? – plastique Aug 13 '15 at 10:06

score 1 · Accepted Answer · answered Aug 13 '15 at 19:30

1

Autoboxing has nothing to do with it. The Java Language Specification specifies (in JLS 15.8.2) exactly what type T.class has:

If T is a reference type, T.class has type Class<T>
If T is a primitive type, T.class has type Class<wrapper class of T>

That's it. int.class has type Class<Integer> because the specification says so. int[].class has type Class<int[]> because the specification says so. Class<int[]> and Class<Integer[]> are not compatible types in Java.

answered Aug 13 '15 at 19:30

newacct

119,665
29
163
224

Thanks for answer, but `int.class` and 'Integer.class' are not equal. Does it means that `int.class` is somewhat subclass of `Integer.class` ? – plastique Aug 14 '15 at 06:09
@plastique: I didn't say `int.class` and `Integer.class` were equal (they are not). I said the expression `int.class` has type `Class` (so does the expression `Integer.class`). – newacct Aug 14 '15 at 09:26
Ok, thank for clarification. Still, there has to be some relationship between `int.class` and `Integer.class`. Both has type `Class` but their types are not equal. So sorry but i have to ask again is 'int.class' extending 'Integer.class' ? – plastique Aug 14 '15 at 09:55
@plastique: Sorry but your question doesn't make any sense. How can one value "extend" another one? – newacct Aug 14 '15 at 20:08

score 0 · Answer 3 · answered Aug 13 '15 at 07:53

Autoboxing only works on single "instances" - an int can be autoboxed to an Integer and an Integer can be outboxed to an int. This is not true for arrays though. E.g., the statement int[] arr = new Integer[10]; will not compile. The same holds for manipulating these classes via reflection.

Generics: Why Class clazzIntArray = int[].class; does not work

3 Answers3