Classes that are tightly coupled are hard to reuse in isolation, since they depend on each other. Tight coupling leads to monolithic systems, where you can't change or remove a class without understanding and changing many other classes. The system becomes a dense mass that's hard to learn, port, and maintain. Loose coupling increases the probability that a class can be reused by itself and that a system can be learned, ported, modified, and extended more easily. Design patterns use techniques such as abstract coupling and layering to promote loosely coupled systems. Design patterns: Abstract Factory, Bridge, Chain of Responsibility, Command, Facade, Mediator, Observer

The Law of Demeter for Functions The Law of Demeter for functions attempts to minimize coupling between modules in any given program. It tries to prevent you from reaching into an object to gain access to a third object's methods. By writing "shy" code that honors the Law of Demeter as much as possible, we can achieve our objective: While it sounds good in theory, does following the Law of Demeter really help to create more maintainable code? Studies have shown that classes in C++ with larger response sets are more prone to error than classes with smaller response sets (a response set is defined to be the number of functions directly invoked by methods of the class). Because following the Law of Demeter reduces the size of the response set in the calling class, it follows that classes designed in this way will also tend to have fewer errors (see [URL 56] for more papers and information on the Demeter project). Using The Law of Demeter will make your code more adaptable and robust, but at a cost: as a "general contractor," your module must delegate and manage any and all subcontractors directly, without involving clients of your module. In practice, this means that you will be writing a large number of wrapper methods that simply forward the request on to a delegate. These wrapper methods will impose both a runtime cost and a space overhead, which may be significant—even prohibitive—in some applications.