Introduction:
Design patterns are recurring solutions to
software design problems you find again and again in real-world application
development. Patterns are about design and interaction of objects, as well as
providing a communication platform concerning elegant, reusable solutions to
commonly encountered programming challenges.
·
Pattern is a word
defined for similar events occuring again and again.
·
They are the best
practices how to solve common known problems
·
It’s a kind of work
around for a problem.
·
Pattern is a defined,
used and tested solution for a know problem.
·
When we face similar
problem in Development, we immediately think of a solution. That solution is
not invented by us. But that solution is always used when you get the same kind
of Problem.
Gang of Four (GoF):-
·
One Day, Four Computer
Scientist (Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides)
·
Met together, and had
a big conversion about the Design Pattern.
·
These gangs of four
people were called Gang of Four.
·
They divided total
design Pattern into 3 part (Creational, Structural, Behavioral) In General,
There may be 100+ design pattern.
·
They tried to find out
the basic design patterns, from which other design patterns might have come
finally they concluded 23 basic designs Pattern.
All other design pattern are extended from
these patterns
Types of Design Patterns.
Design Patterns are divided into 3 parts
·
Creational Patterns(5)
·
Structural Patterns(7)
·
Behavioral
Patterns(11)
Creational Patterns
Can be used to
instantiate objects, directly or indirectly.
1. Abstract Factory: - Creates an instance of several families of
classes. Provide an interface for creating families of related or dependent
objects without specifying their concrete classes.
2. Builder-: Separates object construction from its representation.
Separate the construction of a complex object from its representation so that
the same construction processes can create different representations.
3. Factory Method: - Creates an instance of several derived
classes. Define an interface for creating an object, but let subclasses decide
which class to instantiate. Factory Method lets a class defer instantiation to
subclasses.
4. Prototype-: A fully initialized instance to be copied or cloned. Specify
the kinds of objects to create using a prototypical instance, and create new
objects by copying this prototype.
5. Singleton: - A class of which only a single instance can exist. Ensure a
class only has one instance, and provide a global point of access to it.
Structural Patterns
it can be used to organize your program into groups.
it can be used to organize your program into groups.
This grouping will
provide you clarity and will enable you for easier maintainability.
1. Adapter: - Match interfaces of different classes. Convert the interface
of a class into another interface clients expect. Adapter lets classes work
together that couldn’t otherwise because of incompatible interfaces.
2. Bridge: - Separates an object’s interface from its implementation.
Decouple an abstraction from its implementation so that the two can vary
independently.
3. Composite: - A tree structure of simple and composite
objects. Compose objects into tree structures to represent part-whole
hierarchies. Composite lets clients treat individual objects and compositions
of objects uniformly.
4. Decorator: - Add responsibilities to objects dynamically. Attach
additional responsibilities to an object dynamically. Decorators provide a
flexible alternative to sub classing for extending functionality.
5. Facade: - A single class that represents an entire subsystem. Provide a
unified interface to a set of interfaces in a system. Facade defines a
higher-level interface that makes the subsystem easier to use.
6. Flyweight: - A fine-grained instance used for efficient sharing.
Use sharing to support large numbers of fine-grained objects efficiently. A
flyweight is a shared object that can be used in multiple contexts
simultaneously. The flyweight acts as an independent object in each context it’s
indistinguishable from an instance of the object that’s not shared.
7. Proxy: - An object is representing another object. Provide a surrogate or
placeholder for another object to control access to it.
Behavioral Patterns
It can be used to define the communication and control flow between objects.
It can be used to define the communication and control flow between objects.
1. Chain of Resp: - A way of passing a request between a chain
of objects. Avoid coupling the sender of a request to its receiver by giving
more than one object a chance to handle the request. Chain the receiving
objects and pass the request along the chain until an object handles it.
2. Command: - Encapsulate a command request as an object. Encapsulate a
request as an object, thereby letting you parameterize clients with different
requests, queue or log requests, and support undoable operations.
3. Interpreter: - A way to include language elements in a program. Given a
language, define a representation for its grammar along with an interpreter
that uses the representation to interpret sentences in the language.
4. Iterator: - Sequentially access the elements of a collection. Provide a
way to access the elements of an aggregate object sequentially without exposing
its underlying representation.
5. Mediator: - Defines simplified communication between classes. Define an
object that encapsulates how a set of objects interact. Mediator promotes loose
coupling by keeping objects from referring to each other explicitly, and it
lets you vary their interaction independently.
6. Memento: - Capture and restore an object's internal state. Without
violating encapsulation, capture and externalize an object’s internal state so
that the object can be restored to this state later.
7. Observer: - A way of notifying change to a number of classes. Define a
one-to-many dependency between objects so that when one object changes state,
all its dependents are notified and updated automatically.
8. State: -Alter an object's behavior when its state changes. Allow an
object to alter its behavior when it’s internal state changes. The object will
appear to change its class.
9. Strategy: - Encapsulates an algorithm inside a class. Define a family of
algorithms, encapsulate each one, and make them interchangeable. Strategy lets
the algorithm vary independently from clients that use it.
10. Template: -Defer the exact steps of an algorithm to a subclass. Define the
skeleton of an algorithm in an operation, deferring some steps to subclasses.
Template Method lets subclasses redefine certain steps of an algorithm without
changing.
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Design Patterns in Java by nataraz notes
