oop - Mediator Vs Observer Object-Oriented Design Patterns

Question

I have been reading the Gang Of Four, in order to solve some of my problems and came across the Mediator pattern.

I had earlier used Observer in my projects for making some GUI application. I am a bit confused as I do not find great difference between the two. I browsed to find the difference but could not find any apt answer for my query.

Could some one help me to differentiate between the two with some good example which clearly demarcates the two?

Answer 1

The Observer pattern: Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

The Mediator pattern: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Source: dofactory

Example:

The observer pattern: Class A, can have zero or more observers of type O registered with it. When something in A is changed it notifies all of the observers.

The mediator pattern: You have some number of instances of class X (or maybe even several different types:X, Y & Z), and they wish to communicate with each other (but you don't want each to have explicit references to each other), so you create a mediator class M. Each instance of X has a reference to a shared instance of M, through which it can communicate with the other instances of X (or X, Y and Z).