Omnipresent unicorn game keeper...

Unicorn.java

import java.util.HashMap;
import java.util.Map;

public class Unicorn {  
	// The locations of unicorns in our magical, one-dimensional land
	private static final int UNICORN_ONE_LOCATION = 4;
	private static final int UNICORN_TWO_LOCATION = 10;
	// Let's create a game keeper for the magical land to keep track of where all the Unicorns are!
	private static final Map<Integer, Unicorn> UNICORN_GAME_KEEPER = new HashMap<Integer, Unicorn>();

	static {
		// We'll whisper in his ear and tell him where they are hiding
		UNICORN_GAME_KEEPER.put(UNICORN_ONE_LOCATION, new Unicorn());
		UNICORN_GAME_KEEPER.put(UNICORN_TWO_LOCATION, new Unicorn());
	}

	// Keep track of whether this unicorn has been patted
	private boolean patted;

	public static boolean pat() {
		// Where is this patting action being performed?
		StackTraceElement[] stackTrace = Thread.currentThread().getStackTrace();
		int location = stackTrace[stackTrace.length - 1].getLineNumber();
		
		// Let's ask the game keeper if the disembodied hand doing the patting touches a unicorn
		Unicorn unicorn = UNICORN_GAME_KEEPER.get(location);
		if (unicorn != null && !unicorn.patted) {
			// Yup, it did!
			unicorn.patted = true;
			return true;
		}
			
		return false;
	}
}