Here's a neat trick for factory/ classes. Say you have a class, ConnectionManager, that you want to be the only guy to instantiate a Connection class. Sure, sure, you can make the constructor package-private, but I'm not that trusting a guy. So...

public class Configuration 
{
	/**  A constructor that ensures only ConfigurationManager can create configurations */
	Configuration( ConfigurationManager.ConstructionParameters construct )
	{
		this.name = construct.name;
	}
}

public final class ConfigurationManager
{
	/**  An small helper class that ensures only ConfigurationManager can instantiate Configuration */
	static class ConstructionParameters
	{
		final String name;
		private ConstructionParameters( String n ) { name = n; }
	}

}

Okay, not that impressive. But now I know no-one is creating those Configurations except for my manager, even within this package.