2. Gang of Four
o Pattern-based design was introduced
into architecture and engineering in
the 1950's
o It wasn't until a group of four
researchers combined forces that
pattern-based design became well-
known and commonplace
o This group was known as the gang of four (GoF)
3. Gang of Four
o The gang of four (GoF) is:
o Erich Gamma
o Richard Helm
o Ralph Johnson
o John Vlissides
o They are the authors of the famous
text "Design Patterns: Elements of
Reusable Object-Oriented Software"
4. Pattern??
o A pattern is a recurring solution to a
standard problem, in a context.
o A pattern is a three-part rule, which
expresses a relation between a
certain context, a problem, and a
solution.
5. Design Pattern Advantages
o Using patterns offers a few key
advantages:
o Facilitate a proven solution
o Provide a common vocabulary
6. Types of Patterns
o Creational patterns
o Deal with initializing and configuring classes and
objects.
o Structural patterns
o Deal with decoupling interface and
implementation of classes and objects
o Composition of classes or objects
o Behavioural patterns
o Deal with dynamic interactions among societies
of classes and objects.
o Distribute responsibility.
7. Creational Design Patterns
o Abstract Factory
o Factory for building related objects
o Builder
o Separates an object construction from its
representation.
o Factory Method
o Creates an instance of several derived classes.
o Prototype
o Factory for cloning new instances from a
prototype.
o Singleton
o A class of which a single object can exist.
8. Structural Patterns
o Adapter
o Match interfaces of different classes.
o Bridge
o Separates an Object’s interface from its
implementation.
o Composite
o A tree structure of simple and composite objects.
o Decorator
o Add responsibilities of Objects dynamically.
o Facade
o A single class that represents the entire
subsystem.
10. Behavioural Patterns
o Chain of Responsibility
o Passes a request between a chain of Objects.
o Command
o Encapsulate a command request as an object.
o Iterator
o Elements of a collection are accessed
sequentially.
o Interpreter
o Language elements are included.
o Mediator
o Defines simplified communication between
classes.
11. Behavioural Patterns(Cont.)
o Memento
o Capture and restores an object’s internal state.
o Observer
o A way of notifying change to a number of
classes.
o State
o Alter an object’s behaviour when its state
changes.
o Strategy
o Encapsulates an algorithm inside a class.
o Template Method
o Defer the exact steps of an algorithm to a
subclass.
14. Benefits of Design Patterns
o Design patterns enable large-
scale reuse of software
architectures and also help
document systems.
o Patterns help improve developer
communication.
o Pattern names form a common
vocabulary.
o Patterns help ease the transition
to Object Oriented technology.
15. Drawbacks of Design Patterns
o Patterns do not lead to direct
code reuse.
o Patterns are deceptively simple.
o Teams may suffer from pattern
overload.
o Patterns are validated by
experience and discussion rather
than by automated testing.
o Integrating patterns is a human-
intensive activity.
16. Suggestions for Effective Use
o Do not recast everything as a
pattern
o Instead, develop strategic domain patterns and
reuse existing tactical patterns.
o Clearly document when patterns
apply and do not apply.
o Directly involve pattern authors
with application developers and
domain experts.
