when to use pattern

Here are some common causes of redesign along with the design pattern(s) that address them:
1. Creating an object by specifying a class explicitly. Specifying a class name when you create an object
commits you to a particular implementation instead of a particular interface. This commitment can
complicate future changes. To avoid it, create objects indirectly.
Design patterns: Abstract Factory (87), Factory Method (107), Prototype (117).
2. Dependence on specific operations. When you specify a particular operation, you commit to one way
of satisfying a request. By avoiding hard-coded requests, you make it easier to change the way a
request gets satisfied both at compile-time and at run-time.
Design patterns: Chain of Responsibility (223), Command (233).
3. Dependence on hardware and software platform. External operating system interfaces and
application programming interfaces (APIs) are different on different hardware and software
platforms. Software that depends on a particular platform will be harder to port to other platforms. It
may even be difficult to keep it up to date on its native platform. It's important therefore to design
your system to limit its platform dependencies.
Design patterns: Abstract Factory (87), Bridge (151).
4. Dependence on object representations or implementations. Clients that know how an object is
represented, stored, located, or implemented might need to be changed when the object changes.
Hiding this information from clients keeps changes from cascading.
Design patterns: Abstract Factory (87), Bridge (151), Memento (283), Proxy (207).
5. Algorithmic dependencies. Algorithms are often extended, optimized, and replaced during
development and reuse. Objects that depend on an algorithm will have to change when the algorithm
changes. Therefore algorithms that are likely to change should be isolated.
Design patterns: Builder (97), Iterator (257), Strategy (315), Template Method (325), Visitor (331).
6. Tight coupling. 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 (87), Bridge (151), Chain of Responsibility (223), Command (233),
Facade (185), Mediator (273), Observer (293).
7. Extending functionality by subclassing. Customizing an object by subclassing often isn't easy. Every
new class has a fixed implementation overhead (initialization, finalization, etc.). Defining a subclass
also requires an in-depth understanding of the parent class. For example, overriding one operation
might require overriding another. An overridden operation might be required to call an inherited
operation. And subclassing can lead to an explosion of classes, because you might have to introduce
many new subclasses for even a simple extension.

Object composition in general and delegation in particular provide flexible alternatives to inheritance
for combining behavior. New functionality can be added to an application by composing existing
objects in new ways rather than by defining new subclasses of existing classes. On the other hand,
heavy use of object composition can make designs harder to understand. Many design patterns
produce designs in which you can introduce customized functionality just by defining one subclass
and composing its instances with existing ones.
Design patterns: Bridge (151), Chain of Responsibility (223), Composite (163), Decorator (175),
Observer (293), Strategy (315).
8. Inability to alter classes conveniently. Sometimes you have to modify a class that can't be modified
conveniently. Perhaps you need the source code and don't have it (as may be the case with a
commercial class library). Or maybe any change would require modifying lots of existing subclasses.
Design patterns offer ways to modify classes in such circumstances.
Design patterns: Adapter (139), Decorator (175), Visitor (331).
These examples reflect the flexibility that design patterns can help you build into your software. How crucial
such flexibility is depends on the kind of software you're building. Let's look at the role design patterns play
in the development of three broad classes of software: application programs, toolkits, and frameworks.