Re: Indirect references to classes?
From: Hal Vaughan (hal_at_thresholddigital.com)
Date: 10/12/03
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Date: Sat, 11 Oct 2003 23:54:41 GMT
Thanks for both responses. I had no idea where to look or under what topic
to search. Both posts gave me info on classes I had not learned about yet.
It also showed my reference book (Java 2 in Plain English, by Overland and
Morrison, published by M & T Books) is behind, since the class Class is not
in it at all (this books claims to be a full reference book to Java classes
and API).
I came up with two methods (acutally an overloaded method) to act as a
constructor for a class where I only have the name, or name and parameters.
They're pasted in below, along with sample calling code:
static Object getObject(String className) {
Object object = null;
try {
Class classDefinition = Class.forName(className);
object = classDefinition.newInstance();
} catch (Exception e) {System.out.println("Error w/ constructor: " + e);}
return object;
}
static Object getObject(String className, Object[] args) {
Object object = null;
Class[] argTypes = new Class[args.length];
for (int i = 0; i < args.length; i++) {
argTypes[i] = args[i].getClass();
}
try {
Class classDefinition = Class.forName(className);
Constructor classConstructor = classDefinition.getConstructor(argTypes);
object = classConstructor.newInstance(args);
} catch (Exception e) {System.out.println("Error w/ constructor[]: " + e);}
return object;
}
Be sure to add
import java.lang.reflect.Constructor;
import java.lang.Object;
at the start of the class you put these in.
For calling, if the constructor takes no args, call like this:
ClassName myObject = (ClassName) getObject("NameOfClassIWant");
For calling a constructor that takes args, call like this:
Object[] myParms = new Object[] {list, of, parameter, objects};
ClassName myObject = (ClassName) getObject("NameOfClassIWant", myParms);
Once you have myObject, you can use and abuse it like any other object.
Thanks, guys for the info.
I hope this helps a few others out there, since it provides a working way to
do this.
Hal
Hal Vaughan wrote:
> I'm not sure what the correct name for this would be. I'd think it's
> either an indirect reference, or a pointer, or something like that.
>
> I'm working on a program that would call a series of classes in different
> orders and there are times where I may add extra classes that weren't part
> of the original design. I'd like to be able to store a list of the
> classes, in the order that they are used, in a text file. Then I could
> have the first class read in the text file and call the classes in order
> as
> it goes through the text file. I don't want to have to hardcode a case
> statement, calling each class if the line matches the name, since I want
> to be able to easily add more classes in the future without re-compiling
> or altering the original class.
>
> If I have a classname stored in a String, is there any way to call that
> class from the name stored in the string?
>
> I know it can be done in Perl (I forget what it's called -- I think it's
> "dereferencing"), like this:
>
> &$function
>
> and it will call the function that matches the name stored in $function.
>
> Can I do something like this in Java?
>
> Thanks!
>
> Hal
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