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From Paul Benedict <pbened...@apache.org>
Subject Re: [chain][v2] clever context
Date Fri, 09 Sep 2011 21:32:55 GMT
Definitely interesting. I think this might be a special case though;
ClassToInstanceMap is dedicated to class instances which is probably
why they extended Map so that it has extra guarantees. I don't know if
such a pattern is advisable for a regular key/value pair. What are
your thoughts?

On Fri, Sep 9, 2011 at 4:14 PM, Simone Tripodi <simonetripodi@apache.org> wrote:
> indeed, the retrieve method would allow users assigning retrieved
> object to all T that extend V, like the ClassToInstanceMap, take a
> look at the method signatures[1]
>
> [1] http://s.apache.org/Mno
>
> http://people.apache.org/~simonetripodi/
> http://www.99soft.org/
>
>
>
> On Fri, Sep 9, 2011 at 11:02 PM, Paul Benedict <pbenedict@apache.org> wrote:
>> The purpose of Generics is to provide type safety with the implicit
>> casts. Implicit casts because of typing shouldn't cause a
>> ClassCastException. That would break an important principle behind
>> using gnerics. Are you sure Guava is doing what you're proposing?
>> Typing should always be safe; I would be surprised if they would allow
>> unsafe implicit casts.
>>
>> On Fri, Sep 9, 2011 at 3:34 PM, Simone Tripodi <simonetripodi@apache.org> wrote:
>>> Hi Paul,
>>> the type inference becomes more interesting and useful if you think to
>>> more complicated context instances, take as sample the Guava's
>>> ClassToInstanceMap[1] where values extend a specific base type.
>>>
>>> the  <T extends V> in the `retrieve` method reduces anyway the number
>>> of errors, given an hypothetically  Context<String, Number> users
>>> cannot cast to a different type:
>>>
>>> MyPojo myPojo = (MyPojo) context.get( "myKey" );
>>>
>>> I think anyway putting types to Context would make Filter, Command,
>>> Chain, ... classes over engineered IMHO
>>>
>>> best,
>>> Simo
>>>
>>> [1] http://s.apache.org/xfj
>>>
>>> http://people.apache.org/~simonetripodi/
>>> http://www.99soft.org/
>>>
>>>
>>>
>>> On Fri, Sep 9, 2011 at 10:20 PM, Paul Benedict <pbenedict@apache.org> wrote:
>>>> On Fri, Sep 9, 2011 at 3:15 PM, Elijah Zupancic <elijah@zupancic.name>
wrote:
>>>>> Specifying Object for V would be the most likely use case.
>>>>>
>>>>> On Fri, Sep 9, 2011 at 1:12 PM, Paul Benedict <pbenedict@apache.org>
wrote:
>>>>>> On Fri, Sep 9, 2011 at 3:06 PM, Simone Tripodi <simonetripodi@apache.org>
wrote:
>>>>>>> Hi Paul,
>>>>>>> the use of that method is to automatically infer the assigned
type,
>>>>>>> instead of writing
>>>>>>>
>>>>>>>    MyPojo myPojo = (MyPojo) context.get( "myKey" );
>>>>>>>
>>>>>>> the retrieve method allows to
>>>>>>>
>>>>>>>    MyPojo myPojo = context.retrieve( "myKey" );
>>>>>>>
>>>>>>
>>>>>> Hmm... The inference should be automatic unless you specified Object
for type V:
>>>>>> Context<String, String> properties = new ContextBase<String,
String>();
>>>>>> String value = properties.retrieve("myKey");
>>>>>>
>>>>
>>>> I don't have a good answer for the problem. I just think if you
>>>> declare types <K, V> at the class level, those should be the types
>>>> used on the methods too. The problem that I have with <T extends V>
is
>>>> that it assumes a type-safe cast. You are right to say
>>>> ClassCastException was thrown for both of your examples but <T extends
>>>> V> breaks the "rule" that generics should be type-safe casts. It's
>>>> better to have the user create a bum cast and fail then the compiler
>>>> infer a bum cast and fail, imo.
>>>>
>>>> Paul
>>>>
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>>>>
>>>
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>>
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