Dynamically Selecting & Creating Instance of Implementation at Runtime

Question

I'm looking to create a instance of an implementation of a interface where the class name of the interface is both given at runtime via properties and also unknown at compile-time (the binary may be provided by another developer in a separate package). I am wondering if this is both possible, and if it is, how I can accomplish this.
For example:
package com.someone.else;
public class ExternalImplementation extends Object; // This is a silly example.
package com.me;  // In my code, in the body of some method, I would have:
String className = "ExternalImplementation"; // This would be gotten through a Properties object and may be just the class name or whatever it takes to specify the correct class.
Object o = /* And here, I want to create an instance of an ExternalImplementation, even though it doesn't exist at compile-time. */;Thanks for your time!
~ Anthop

Answer 1

Ah yes, sorry - Spoon's is correct. I was mentally absent for half of my answer :) Note that it would also be enough to do:
SomeInterface instance = (SomeInterface)Class.forInstance("ExternalImplementation").newInstance();Less typing and will give you the same error if the external imlementation doesn't in fact implement SomeInterface.