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Type of a generic class?

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  Type of a generic class? "Donkey Hottie" <donkey.hottie@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
    Re: Type of a generic class? "Lew" <lew@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
    Re: Type of a generic class? "markspace" <markspace@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
      Re: Type of a generic class? "Donkey Hottie" <donkey.hottie@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
        Re: Type of a generic class? "Lew" <lew@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
        Re: Type of a generic class? "markspace" <markspace@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
          Re: Type of a generic class? "Donkey Hottie" <donkey.hottie@1:261/38.remove-yy0-this> - 2012-08-03 18:54 +0000
    Re: Type of a generic class? "Donkey Hottie" <donkey.hottie@1:261/38.remove-5qr-this> - 2012-08-04 18:41 +0000

#17044 — Type of a generic class?

From: Donkey Hottie <donkey@fredriksson.dy.fi>


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. 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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#17061

  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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#17062

  To: Donkey Hottie
From: markspace <-@.>

On 8/2/2012 12:04 PM, Donkey Hottie wrote:

> 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.


I'm going to ignore other obvious problems and simply focus on the big picture 
here.  You idea how to accomplish this looks *TERRIBLE*.  Why aren't you using 
some kind of ORM?  At least use a light-weight library for translating data 
base entities into objects.

<http://commons.apache.org/dbutils/>

Also JPA will do some kinds of automatic instantiation for you:

<http://openjpa.apache.org/builds/1.0.4/apache-openjpa-1.0.4/docs/manual/jpa_ov
erview_mapping_inher.html>

I didn't check to see if those are the most recent docs (I don't use this sort 
of feature).  Google for "JPA table inheritance" and check through the results 
carefully.


> Class is a simple version containg only the important parts.


Honestly, while we all appreciate the attempt, you example is far from 
complete.  We can guess at a few things, but you should think much more 
carefully at what the real problem is and design something to illustrate it.

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#17070

  To: markspace
From: Donkey Hottie <donkey@fredriksson.dy.fi>

03.08.2012 00:08, markspace kirjoitti:
> On 8/2/2012 12:04 PM, Donkey Hottie wrote:
>
>> 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.
>
>
> I'm going to ignore other obvious problems and simply focus on the big
> picture here.  You idea how to accomplish this looks *TERRIBLE*.  Why
> aren't you using some kind of ORM?  At least use a light-weight library
> for translating data base entities into objects.
>

I am not trying to translate database tables to objects. I have an ORM for 
that.

I am trying to convert a simple { String name, String value } pair in database 
to a Global<T>(name) so that it will convert the Java simple datatypes to a 
string and back. This is for system parameters, configuration parameters.

It is indeed kind of spooky and strange idea, after all what is so wrong about 
simple

    String value = SystemProperties.getString("ParamName") ;

It is OK, but I'm kind of toying with idea of strong typing of System 
Parameters: A Date can not be stored as Long or Double etc..

The back end of the system parameters is simply

CREATE TABLE SystemParameters
(
    name varchar(64) not null primary_key,
    value varchar(512)
) ;

That's it. That is have the table is given to me. But I want to make the Java 
code bit more pedantic on the types of the parameters it uses and maintains.

This idea just occurred to me when I stubled against MUMPS language when 
learning InterSystems Ensemble. It is based on their Cacho NoSQL/SQL hybrid 
database, and while being an ancient language, MUMPS/M has lots of interesting 
features, including threading and database persistence in the old Programming 
Language Core!

I just thought I might as well copy their Global variable idea and include it 
in my toolbox of Java commons. It is not an ORM. I do not know about dbutils, 
but JPA I am very familiar with. It maps Java Objects to Relational Database 
records. There one attribute can have just one type. I want many types to my 
value attribute, but only for Java code. Just to force the apps to store at 
least correct type of values for properties, so that compiler stops my fellow 
programmer if he is about to generate a runtime error later somewhere, for 
example if he is trying to store 'NOW' for a value that is needed as Date.

Not a big deal, but this is my idea here.

> <http://commons.apache.org/dbutils/>
>
> Also JPA will do some kinds of automatic instantiation for you:
>
> <http://openjpa.apache.org/builds/1.0.4/apache-openjpa-1.0.4/docs/manual/jpa_
overview_mapping_inher.html>
>
>
> I didn't check to see if those are the most recent docs (I don't use
> this sort of feature).  Google for "JPA table inheritance" and check
> through the results carefully.
>
>
>> Class is a simple version containg only the important parts.
>
>
> Honestly, while we all appreciate the attempt, you example is far from
> complete.  We can guess at a few things, but you should think much more
> carefully at what the real problem is and design something to illustrate
> it.
>
>
>

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#17073

  To: Donkey Hottie
From: Lew <lewbloch@gmail.com>

Donkey Hottie wrote:
> markspace kirjoitti:
>> ...

> I am trying to convert a simple { String name, String value } pair in

How about a Map<String, Global<?>>

Also, you should move that if-chain reflection out of your app and use 
polymorphism instead.

> database to a Global<T>(name) so that it will convert the Java simple
> datatypes to a string and back. This is for system parameters,
> configuration parameters.
>
>> It is indeed kind of spooky and strange idea, after all what is so wrong
>> about simple
>     String value = SystemProperties.getString("ParamName") ;
>
> It is OK, but I'm kind of toying with idea of strong typing of System
> Parameters: A Date can not be stored as Long or Double etc..

Using strings to look up types is not strong typing.

>
> The back end of the system parameters is simply
>
>
>
> CREATE TABLE SystemParameters
> (
>     name varchar(64) not null primary_key,
>     value varchar(512)
> ) ;
>
>
> That's it. That is have the table is given to me. But I want to make the
> Java code bit more pedantic on the types of the parameters it uses and
> maintains.

JPA does that for you, too.

Anyway, your use of reflection is anti-pedantic.

--
Lew

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#17080

  To: Donkey Hottie
From: markspace <-@.>

On 8/2/2012 4:27 PM, Donkey Hottie wrote:

> It is indeed kind of spooky and strange idea, after all what is so wrong
> about simple
>
>      String value = SystemProperties.getString("ParamName") ;
>
> It is OK, but I'm kind of toying with idea of strong typing of System
> Parameters: A Date can not be stored as Long or Double etc..


To do something like this, I'd arrange some implementation for the parameter:

class Parameter {
   public void processParam( String param ) {
      double d = Double.parse( param ); // or whatever...
      ...
   }
}

And then use a Map as Lew suggests or some other method of binding the 
Parameter class to the string "ParamName".

(You could, for example use Class.forName():
   Parameter proc = Class.forName(
"my.project.parameter."+"ParamName").newInstance();

or you could use annotations:

@Parameter("ParamName")
class Parameter {...

or any other crazy thing you like.  It would probably be better than what you 
have now.)


> The back end of the system parameters is simply
>
> CREATE TABLE SystemParameters
> (
>      name varchar(64) not null primary_key,
>      value varchar(512)
> ) ;


I've written this table definition.  In fact it was 23 odd years ago, as in 
intern while still in college.  I don't know if it's good or bad practice, but 
it seems a common idea.

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#17093

  To: markspace
From: Donkey Hottie <donkey@fredriksson.dy.fi>

03.08.2012 06:08, markspace kirjoitti:
>
>> The back end of the system parameters is simply
>>
>> CREATE TABLE SystemParameters
>> (
>>      name varchar(64) not null primary_key,
>>      value varchar(512)
>> ) ;
>
>
> I've written this table definition.  In fact it was 23 odd years ago, as
> in intern while still in college.  I don't know if it's good or bad
> practice, but it seems a common idea.
>

Not much worse that a property file common in Java. But a database table allows 
automatic parameters per a SaaS tenant.

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#17169

  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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