Design patterns for multi-threaded messaging server

I’m designing an instant messaging server as a personal exercise to improve my understanding and application of multi-threading and design patterns in Java. I’m still designing, there’s no code yet.

Understanding meaning of arrows in Observer pattern

Why are there two abstract interfaces in the observer pattern?

I’m reading about design patterns from both “Head First Design Patterns” and the original “Elements of reusable software etc” and I’m finding some confusion right at the start, in the Observer pattern.

Is there a way to not use the Observer pattern when creating GUI systems?

In Java, the Swing library implements the Observer/Subscriber-Publisher design pattern to design the way the program responds to events on the GUI (button clicks, etc).

Is there a way to not use the Observer pattern when creating GUI systems?

In Java, the Swing library implements the Observer/Subscriber-Publisher design pattern to design the way the program responds to events on the GUI (button clicks, etc).

How does Observer create loosely-coupled design?

I completely understand the Observer design pattern, what it is for and how to implement it. I also understand what the term ‘loosely-coupled’ means.

Observer pattern: “Web of observers” – Is this ever in use?

I had an idea (which I’m sure already exists), to create a sort of ‘network of observers/subjects’. I would like to describe how it works and than ask several questions about it.

Observer pattern: “Web of observers” – Is this ever in use?

I had an idea (which I’m sure already exists), to create a sort of ‘network of observers/subjects’. I would like to describe how it works and than ask several questions about it.

I’m always reimplementing observer/subject code in Java. Is there a better option?

I’m always writing observer/subject interfaces in a particular Java project, e.g.:

Are EventHandler and IObservable interchangable?

I have an object which will periodically raise an event based on an action performed in an application. This will be heard by any listener(s) and acted upon accordingly.