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From: Lew <lewbloch@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Cannot create a generic array of <type>
Date: Tue, 18 Oct 2011 23:28:48 -0700 (PDT)
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Warren Tang wrote:
> The following code:
>=20
>   ArrayList<Integer>[] indexedValues =3D new ArrayList<Integer>[5];
>=20
> generates an error:
>=20
>   Cannot create a generic array of ArrayList<Integer>
>=20
> Could someone explain this to me?

Sure.  The short answer is that generics and arrays do not mix.  Slightly l=
onger, the Java Language Specification explicitly forbids arrays of non-rei=
fiable types:

"An array creation expression specifies the element type, the number of lev=
els of nested arrays, and the length of the array for at least one of the l=
evels of nesting. ... It is a compile-time error if the element type is not=
 a reifiable type".
<http://java.sun.com/docs/books/jls/third_edition/html/arrays.html#10.3>

Generic types like 'ArrayList<Integer>' are not reifiable, that is, you can=
not actually create such a type at runtime due to type erasure.

The longer answer is that arrays are reifiable, therefore their element typ=
es must also be reifiable.  Unlike generics, arrays carry their base type w=
ith them at run time - they "know" their type.  Generic types do not.  (You=
 can get around this with a run-time type token, or RTTT, but that's a topi=
c for another discussion.)  So when you try to make an array of a generic t=
ype you deprive the array of information it needs.  This the compiler won't=
 allow.

Back to the short answer: don't mix arrays and generics.  There are a few c=
onversion classes in the Collections framework that let you convert between=
 them, but use them sparingly and usually to go from array-land to collecti=
ons-land to stay.

Arrays.asList
static <T> List<T> asList(T... a)
<http://download.oracle.com/javase/7/docs/api/java/util/Arrays.html#asList(=
T...)>

--=20
Lew