Question on layer/peeling

[Peter Levart]

This could be a temporary library trick:

public class Default<any T> {

     private T value;

     private Default() {}

     public static <any T> T value() {
         return new Default<T>().value;
     }
}


with use like:


public class Test {
     public static void main(String[] args) {
         int i = Default.value();
         long l = Default.value();
         Object o = Default.value();
     }
}



...but unfortunately the above Test produces the following runtime 
exception:


Specializing method util/Default$value${0=I}.value()Ljava/lang/Object; 
with class=[] and method=[I]
Specializing util.Default${0=I}; searching for util/Default.class (not 
found)
Specializing util.Default${0=I}; searching for util/Default.class (found)
Exception in thread "main" java.lang.BootstrapMethodError: call site 
initialization exception
         at java.lang.invoke.CallSite.makeSite(CallSite.java:341)
         at 
java.lang.invoke.MethodHandleNatives.linkCallSiteImpl(MethodHandleNatives.java:307)
         at 
java.lang.invoke.MethodHandleNatives.linkCallSite(MethodHandleNatives.java:297)
         at util.Test.main(Test.java:8)
Caused by: java.lang.VerifyError: Bad type on operand stack
Exception Details:
   Location:
     util/Default$value${0=I}.value()I @7: getfield
   Reason:
     Type 'util/Default${0=I}' (current frame, stack[0]) is not 
assignable to 'util/Default'
   Current Frame:
     bci: @7
     flags: { }
     locals: { }
     stack: { 'util/Default${0=I}' }
   Bytecode:
     0000000: bb00 0959 b700 0db4 0012 ac

         at sun.misc.Unsafe.defineAnonymousClass(Native Method)
         at 
java.lang.invoke.GenericMethodSpecializer.metafactory(GenericMethodSpecializer.java:98)
         at java.lang.invoke.CallSite.makeSite(CallSite.java:302)
         ... 3 more


Seems like the specialization of static method is not entirely correct here.

Regards, Peter

On 01/06/2015 03:23 AM, Brian Goetz wrote:
> Yes, this is pretty straightforward.  In the bucket of "things that 
> are easy and small, so we'll ignore them until we solve the ones that 
> are big and difficult.")
>
> The hardest part is picking a syntax (please, no suggestions!)
>
>
> On 1/5/2015 9:19 PM, Vitaly Davidovich wrote:
>> C# has a default (T) keyword to allow generic code to obtain the "zero"
>> value for a type param.  Something like that for java would be nice.
>>
>> Sent from my phone
>> On Jan 5, 2015 9:15 PM, "Michael Barker" <mikeb01 at gmail.com> wrote:
>>
>>> Hi,
>>>
>>> The SotS talks about the use of 'layer' to create an alternative
>>> implementation of methods when the type of an <any T> is known to be a
>>> reference type.  However, the examples only show the use of the layer
>>> keyword on an interface definition, where as I've encountered at 
>>> least one
>>> case where the internal implementation needs to differentiate between a
>>> reference-type and value-type based collection.  The example I'm 
>>> thinking
>>> about is the null-ing out of array elements in a collection (which is
>>> obviously a no-op with a value type, but necessity with reference
>>> types).  Is an interface required in order to define a 'layer' or 
>>> could it
>>> be done within a concrete class?
>>>
>>> E.g. is the following or something similar possible?  If not, how 
>>> would it
>>> be achieved with current spec?
>>>
>>> class ArrayList<any T> {
>>>      T[] values;
>>>      int position;
>>>
>>>      void removeLast() {
>>>          if (position <= 0) {
>>>              return;
>>>          }
>>>
>>>          --position;
>>>          clear(position);
>>>      }
>>>
>>>      private void clear(int index) {
>>>      }
>>>
>>>      layer<ref T> {
>>>          private void clear(int index) {
>>>              values[index] = null;
>>>          }
>>>      }
>>> }
>>>
>>> Mike.
>>>