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From: "Lew" <lew@1:261/38.remove-yy0-this>
Subject: Re: Type of a generic class?
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Date: Fri, 03 Aug 2012 18:54:14 GMT
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  To: Donkey Hottie
From: Lew <lewbloch@gmail.com>

Donkey Hottie wrote:
> I have this class called Global. It is trying to be a simplistic
> simulation of global as in MUMPS/M language. It is a persistent
> variable, that is accessible everywhere, and retains it's value over
> time. I store them in a database.
>
> First problem I have is to translate the type to a lower level
> application API call. I can not leave the cast or type conversion to
> compiler only.
>
> For that I figured out that I may need a variable of Class<T>, I'm using
> the variables isAssignableFrom(Class) to find out the correct API call.

Not good.

> Could there be a simpler way?

Store a 'Class<T>' reference (matching the generic type) as a final variable. 
This is a "run-time type token" (RTTT).

> the final Class<T> as a member variable. Is that really needed? How

Yes.

> could I use some typeinfo (reflection API?) instead?

You mean a different reflection API. The 'Class' methods are part of 
reflection.

> If I could use serialization and store the objects that way maybe into
> BLOBs there would not be problems, but currently I can not do that.

How would a more complex, I/O-based solution be better?

> I would like to get rid of that "klass" argument for the Global<T>. Any
> ideas?

Why do you want to get rid of it?

It's the right way to do what you want.

> Class is a simple version containg only the important parts.
>
> public class Global<T extends Object>
> {
>     final String name ;
>     final Connection conn ;
>     final Class<T> klass;
>
>     public Global(String name, Connection conn, Class<T> klass)
>     {
>         this.name = name ;
>         this.conn = conn ;
>         this.klass = klass;
>     }
>
>     @SuppressWarnings("unchecked")

DON'T DO THAT!

You don't need it. If you did, you should comment why the expression is type 
safe despite the suppression.

And you should annotate the declaration of the variable, not the method.

>     public T get() throws Exception
>     {
>         T rc = null;
>
>         if (klass.isAssignableFrom(Boolean.class))

This is an antipattern.

>         {
>             rc = (T)SystemProperties.getSystemBoolean(name, conn);
>         }
>         else if(klass.isAssignableFrom(Date.class))
>         {
>             rc = (T)SystemProperties.getSystemDate(name, conn);
>         }
>         else if (klass.isAssignableFrom(Long.class))
>         {
>             rc = (T)SystemProperties.getSystemLong(name, conn);
>         }
>         else if (klass.isAssignableFrom(Integer.class))
>         {
>             rc = (T)SystemProperties.getSystemInt(name, conn);
>         }
>         else if (klass.isAssignableFrom(String.class))
>         {
>             rc = (T)SystemProperties.getSystemString(name, conn);
>         }
>         return rc ;
>     }
> }

You should override 'get()' in type-specific subtypes of your 'Global'. 
If-chains
of reflection are a reliable indicator of a bad architecture. Use polymorphism 
instead.

--
Lew

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