Diamond and Generic Methods

[Paul Benedict]

That's interesting. Because inference is supposed to be a help to
explicit parametrization....

This would be obviously rejected by the compiler:
identity(new java.util.ArrayList<T>());

Yet, as the compiler knows, the inference says T is Object so use Object.

While totally understandable, I also think that will be a learning
curve for some people. Java 5 taught lots of people that you can't
instantiate using <T> but the new Java 7 code can give the appearance
you suddenly can.

Paul

On Thu, May 5, 2011 at 10:18 AM, Maurizio Cimadamore
<maurizio.cimadamore at oracle.com> wrote:
> On 05/05/11 16:06, Jesper Öqvist wrote:
>> So, after searching the archives and scouring the JSR334 v0.875 draft, I
>> have a question about Diamond. While the specification draft does not
>> explicitly disallow it, it seems weird that this works (Java7 developer
>> preview b139):
>>
>> class C {
>> <T>  void identity(T a) { return a; }
>>     void m() {
>>       identity(new java.util.ArrayList<>());
>>     }
>> }
>>
>> Since there is a distinction between raw types and parameterized types
>> with inferred type arguments (in the discussion part of the Diamond
>> section in JSR334), it would be interesting to know what type is
>> instantiated in the above example.
> Hi
> The above code is equivalent to the following:
>
> class C {
> static<Z>  ArrayList<Z>  makeArrayList() { return new ArrayList<Z>(); }
>
> <T>  void identity(T a) { return a; }
>    void m() {
>      identity(makeArrayList());
>    }
> }
>
> In both cases, an ArrayList<Object> is created - note that, because of
> the restrictions in Java method type-inference, the information on
> formal argument type is not used to infer a better type. In this
> particular case it might not seem a big problem, but in the following
> case you  get a compile-time error:
>
> class C {
> static<Z>  ArrayList<Z>  makeArrayList() { return new ArrayList<Z>(); }
>
> <T>  T identity(T a) { return a; }
>    void m() {
>      //error - ArrayList<Object>  found - expected ArrayList<String>
>      ArrayList<String>  s = identity(makeArrayList());
>    }
> }
>
>
> Maurizio
>
>> /Jesper
>>
>
>
>