What's the proper way to use reflection to instantiate objects of unknown classes at runtime?

I am working on a Configuration Loader class so that I can change the parameters of my program via an external text file (config.txt) rather than having to recompile my code with every change I make.

It has been suggested that I use Java's Reflection to do this, but I'm a little confused as to how I might actually implement this.

I have been able to successfully extract the class name and the arguments for its constructor from my text file, but how do I go from this to an instantiated object?

here's what I have of my method so far:

public void loadObject(String classString, HashMap hm)
  {
  String className = props.getProperty(classString);
  Class c = Class.forName(className);
  }

classString is a string containing the name of the class, and hm is a hashmap where the class' constructor parameters map to their intended values.

I.e., for class Foo (int xPos, float yPos), "xPos" would map to a string of the intended int, and "yPos" maps to a string of the intended float. I want to be able to return, new Foo(hm.get"xPos".toInt, hm.get"yPost".toFloat), but I'm unsure how to dynamically use a constructor like that (the issue is, there are multiple possible classes -- perhaps it's a bar instead of a foo, for instance).

I know that its possible to do an if/else based off the classString, and simply call the proper constructor after identifying it that way, but I am looking to create a more extensible code that doesn't have to be rewritten every time I add a new class to the program.

All of the possible objects inherit from a single parent object.