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// This interface handles adding, deleting and updating
// all observers 

public interface Subject {
	
	public void register(Observer o);
	public void unregister(Observer o);
	public void notifyObserver();
	
}

http://www.newthinktank.com/2012/08/observer-design-pattern-tutorial/

// The Observers update method is called when the Subject changes

public interface Observer {
	
	public void update(double ibmPrice, double aaplPrice, double googPrice);
	
}