Indirect references to classes?

[Hal Vaughan]

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

[Raymond DeCampo]

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?

Yes, Java is well suited for this task. The main tool for this is
reflection. Reflection lets you discover information about classes at
runtime.

There are a number of approaches, I recommend the following. Create an
interface that has the method on it you wish to call. Each class you
create would implement the interface. Use the Class.forName() method to
load each class named in your file. If you wish to be cautious you can
use Class.isAssignableFrom() to ensure the loaded Class implements the
interface. Use the Class.newInstance() method to create a instance of
the class. Cast the instance to you interface and invoke the method.

Ray

[VIjay Raghunathan]

You can do this using reflection or polymorphism. So you have this
text-file with a list of classes in them (the classname). Now one of
the pre-requisites for this to work is that your ClassLoader has to
have the class in it's classpath. Call
Class clazz =
classLoader.loadClass("my.fully.qualified.class.na me");
This will return an object of type Class. Create a new instance by
using
Object o =clazz.newInstance();
Now you can use reflection to call a method on this object.

Alternatively once you have the object you can cast it to some type
(that you define) and then call the method on that type.
MyType myObject = (MyType) o;
myObject.doSomethingDynamically();

The second way is better because it provides some degree of
Type-Checking (although not much), although the first method allows
you to run a method thats not hard-coded at runtime.

[Hal Vaughan]

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