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From: "Donkey Hottie" <donkey.hottie@1:261/38.remove-5qr-this>
Subject: Re: Type of a generic class?
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  To: Donkey Hottie
From: Donkey Hottie <donkey@fredriksson.dy.fi>

02.08.2012 22:04, Donkey Hottie kirjoitti:
>
> 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.

Actually I renamed this to a SystemProperty, and will implement the Global<T 
extends java.io.Serializable> later. It will act just like MUMPS Global and 
does not need any clutterin oddities. Any serializable objects just serialized 
to a BLOB.

I have a separate use case for that too.

public class Global<T extends java.io.Serializable> {

    private static final Log LOG = LogFactoryUtil.getLog(Global.class);
    final String name;
    final Connection conn;

    public Global(String name, Connection conn)
    {
        this.name = name;
        this.conn = conn;
    }

    public T get() throws Exception
    {
        T rc = null;
        LOG.info(String.format("Returning Global value %s=%s ",
                this.name, String.valueOf(rc)));
        throw new Exception("Not yet implemented!");
    }

    public void set(T value) throws Exception
    {
        LOG.info(String.format("Setting Global value %s=%s ",
                this.name, String.valueOf(value)));
        throw new Exception("Not yet implemented!");
    }
}


>
> 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.
> Could there be a simpler way?
>
> the final Class<T> as a member variable. Is that really needed? How
> could I use some typeinfo (reflection API?) instead?
>
> 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.
>
> I would like to get rid of that "klass" argument for the Global<T>. Any
> ideas?
>
> 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")
>     public T get() throws Exception
>     {
>         T rc = null;
>         if (klass.isAssignableFrom(Boolean.class))
>         {
>             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 ;
>     }
> }
>

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