Tight coupling

Less coupling makes code easier to extend and maintain. Tight coupling makes code more rigid and brittle and more likely to break when changes are needed.

Coupling takes different forms: 

• Subclass coupling is dependent on the implementation and hierarchy of the parent class.
• Control dependencies. Code that tells its dependences what to do. If the control API of the dependency changes, the dependent code will break.
• State shape dependencies. Code that only uses a subset of the data structure.
• Event/message coupling. Code that communicates with other units via message passing, events, etc.