Coupling....

I've been thinking about coupling for a while now.  It's more than just one class or process knowing about another class or process, that happens all the time.  It's about using the minimum amount of coupling in a program for it's operation.  

Coupling is not a 'one size fits all' issue where there is a right and a wrong way to use it.  It's more of understanding when to use it, when it happens by accident and in general, being aware of the repercussions when too much coupling exists.

In my mind, coupling exists when there is a specific relationship between two classes or processes.  This relationship must be explicit where one class must be coupled by another into for some operation to be viable.  This is not the same as just passing data from one object to another where the data couple be in the form of a third data object (a model, strings, streams, etc) where the two objects are not really bound together but there data is.  This is about objects like a View having access to a model in the MVC pattern but the Model does not have access to the view.   

Coupling can be reduced by providing dynamic patters to abstract away direct coupling, but this has side effects.  Using multiple levels of indirection to avoid coupling can lead to logic that is hard to understand or maintain by other programmers later in the projects lifecycle.  Abstracting connections without a firm business needs may make the code overly complex and hide the classes relationships.   You can see this patten when using reflection that may then load a configuration file that loads a class that...... you get the picture.  Not fun to modify or debug.

On the other side the coin is how indirect coupling can be created by over use of patterns like dependance injection where annotations spread through the source files with little thought on the tangled relationships that ensue throughout the code.  Dependance injections is wonderful but not if it obfuscates the is-a and has-a relationships into a mass of dependancies.  Dependance injection does not replace the design and understanding of the class relationships and operation.

Coupling issue are not limited to source code.  The same could be said of build projects, unit tests and other supporting aspects of a project.