Re: Type of a generic class?
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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