Proposal: Improved Wildcard Syntax for Java (original) (raw)

Joseph D. Darcy Joe.Darcy at Sun.COM
Thu Mar 19 01:45:18 PDT 2009

Hello.

Howard Lovatt wrote:

Hi Joe,

Emails overlapped, you sent me one whilst I was replying to you, hence second email. 2009/3/18 Joseph D. Darcy <Joe.Darcy at sun.com> wrote: [snip]

There is only one change to the type system; which is to allow covariant assignment, e.g. List lo = new ArrayList(). This isn't a drastic change, arrays already do this. This is in general not type safe. Arrays allow one to do this because array types are reified and there is a store-check at runtime when writing into an array. Since generics in Java are implemented via erasure, this implementation strategy is not possible for Lists and Sets, etc. In the example I sent to Neal I showed how you can write a List that generates an ArrayStoreException, that still uses erasure, by using a factory (java.lang.reflect.Array.newInstance( ... )) to generate a correctly typed array.

That sort of trick can only work on types that are reifiable; List<List> or something with multiple bounds wouldn't work.
Also see the checked collections wrapper methods in java.util.Collections.

The underlying array generates the ArrayStoreException. Therefore it is possible to have generic covariant assignment, erasure, and runtime type safety simultaneously.

Trying to catch this error, incorrect use of an aliased generic, in the type system isn't worth the trouble, arrays have a more useful

That is exactly the sort of issue type systems are designed to catch!

-Joe

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