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Wiki Chapter: CSC/ECE 517 Fall 2011/ch7 7f bn
Wiki Chapter: CSC/ECE 517 Fall 2011/ch7 7f bn


''7f. Pattern classification and importance.  A common classification of the GoF patterns is into "creational," "structural," and "behavioral categories, and, orthogonally, into "class" and "object" patterns.  But the GoF patterns are only a few of the patterns that have been identified in programs.  Look up other ways of classifying patterns, and ranking patterns in terms of importance to programme''
''7f. Pattern classification and importance.  A common classification of the GoF patterns is into "creational," "structural," and "behavioral categories, and, orthogonally, into "class" and "object" patterns.  But the GoF patterns are only a few of the patterns that have been identified in programs.  Look up other ways of classifying patterns.''


__TOC__
__TOC__
==Introduction==
==Introduction==
Money is very important to people worldwide and is something that often must be represented in computer programs.  This raises the question of how money should be represented, which can vary from program to program.  There is much to be considered such as string formatting, the way money is used, and the variety of different currencies in the world.  In Chapter 6 of Skrien's ''Object-Oriented Design Using Java'', he suggests an implementation that represents money as a class, but there are other approaches that can be taken, each with their respective advantages and disadvantages.


==Properties of Money==
Design patterns are general reusable solutions which can be used to tackle recurring problems in software engineering. They have been developed empirically as a result of programmers facing similar challenges frequently. Design patterns act as guides and not the actual solution to the problem at hand. Design patterns typically show relationships between classes or objects. Design patterns are tried and tested solutions and thus can speed up the development process by providing tested, proven development paradigms. A robust design involves having a broad view of the problem at hand rather than coding to patch a problem. This foresight helps predict, avoid and in the worst case tackle problems. This helps procrastinating fire-fighting efforts until implementation phase. Reusing design patterns helps to prevent subtle issues that can cause major problems and improves code readability for coders and architects familiar with the patterns. They give the best practices and are in general applied to Object-oriented design patterns and so are mostly not applicable to functional programming techniques.
Money has certain properties and uses that must be considered in implementation.


===Responsibilities/Using Money===
In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs. Design patterns reside in the domain of modules and interconnections. At a higher level there are architectural patterns that are larger in scope, usually describing an overall pattern followed by an entire system.  
The following are ways in which a representation of money should be able to be used: [http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]
*To represent a positive or negative amount of money.
*Possibly representing multiple components of a currency - i.e. dollars and cents.
*Possibly representing amounts of multiple currencies - i.e. a mix of US Dollars, euros, and yen.'
*To have a string formatting potentially including currency symbols like the dollar sign ($) and/or commas and/or decimal points.
*Addition, subtraction, multiplication, and division.
The degree to which of the above must be met will depend on the depth of the implementing project.


===Different Currencies===
==Class and Object Patterns==
Some applications may only handle only one currency such as the US Dollar, but more generally applied applications, such as banking, must be able to handle different kinds of currencies.


There are currently over 180 currencies in use around the world.[http://en.wikipedia.org/wiki/List_of_circulating_currencies]
Depending on the scope design patterns are broadly classified into 2 of these categories:
===Class patterns:===


Implementation must be aware of how to specifically represent each currency used.  Some currencies use decimal points, such as how the US has 100 cents to a dollar, while others have no decimal, such as yen. Each currency also has a different sign symbol used such as $, €, or ¥. Different currencies also have different rules about how rounding values is handled.
Class patterns specify the relationship between classes and their subclasses.
These patterns tend to use inheritance to establish relationships. Unlike object patterns and object relationships, class patterns generally fix the relationship at compile time.
They are less flexible and dynamic and less suited to polymorphic approaches.


Special consideration must be taken when using different currencies in conjunction with each other.  Foreign exchange rates can be used to convert between different currencies, but the exchange rate often change, so attention must be paid to that.  In some cases, it is also important to keep track of how much money the user has of more than one currency at a time, so that must be handled somehow. [http://www.xencraft.com/resources/multi-currency.html][http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]
===Object Patterns:===


===Immutable vs. Mutable===
Object patterns, the more common of the two, specify the relationships between objects.
Money values can be either mutable, meaning changeable, or immutable, meaning unchanging.  Mutable money values are useful in that a new value does not need to be created for every change.  However, mutable values cause problems for use in hash maps or with concurrency.  The issue with concurrency is a read and write issue in a system with multiple users. With changing values, money not be what the user expects and lead to unpredictable behavior.  Skrien feels this is a significant enough disadvantage to prefer an immutable representation for money.[http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]
In general, the purpose of an object pattern is to allow the instances of different classes to be used in the same place in a pattern. Object patterns avoid fixing the class that accomplishes a given task at compile time. Instead the actual class of the object can be chosen at runtime. Object patterns mostly use object composition to establish relationships between objects.


==Possible Representations==
==Gang Of Four Patterns==
There are several possible ways to represent money in code.


===Floating Point Number===
These 23 GoF patterns are generally considered the foundation for all other patterns. They are categorized in three groups: Creational, Structural, and Behavioral.
Floating point numbers seem like an appealing option since they already include a fractional component, which lends itself straightforward to currencies like the US dollar which also have a fractional component.  However, floating point numbers come with a drawback due to the way floats are formatted. Since floats are formatted from a binary representation, an approximate value is stored in the float.  For instance, 9.48 is stored as 9.479999542236328125.  Performing calculations with these approximated values can lead to inaccurate values, especially with larger numbers. [http://drdobbs.com/java/184405653]


===Two Integers===
===Creational Patterns===
For currencies with a fractional component, using two integers to represent one amount of money is an option.  One integer to represent the main component and one to represent the fractional part. For US currency, this would mean an integer for dollars and an integer for cents. This representation makes displaying monetary values simple, as each component is already split out and accurate. Also, by using integers, calculations with money work out as they should, avoiding the loss of accuracy floating points have.  However, code will need to be implemented to handle when the fraction component carries over.  In the case of dollars and cents, if the cents exceed 100, the cents value should be decreased by 100 and the dollars increased by 1.  This handling of carries adds overhead to calculations.  The main drawbacks of this implementation is the added overhead and storage required. [http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]
In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. Creational design patterns are further categorized into Object-creational patterns and Class-creational patterns. Where, Object-creational patterns deal with Object creation and Class-creational deal with Class-instantiation.


===One Integer, Implied Decimal Point===
====Abstract Factory====
One integer can be used to represent money, even including the fractional component.  The location of the decimal place is simply implied based on the currency.  For instance, a single integer representation of $12.34 would be 1234.  A specialized print method would have to be constructed to display the monetary amount properly to the user, but all calculations work out properly.  This approach has the same advantage that using two integers of maintaining accuracy, but does not have the added overhead or storage. In ''Object-Oriented Design Using Java'', Skrien considers this to be the best use of the primitive types for representing money. [http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]
Creates an instance of several families of classes. Provide an interface for creating families of related or dependent objects without specifying their concrete classes. It is a class that exists to create instances of another class.


===BigDecimal===
Typically, if one wants to construct instances of a class, where the class is selected at run time
Java offers a [http://docs.oracle.com/javase/6/docs/api/java/math/BigDecimal.html BigDecimal] class, which many people use for representing money. BigDecimal is capable of storing any size number since it stores each digit as an entry in an array, which grows as necessary.  It also allows for specification of both the rounding method and the scale, the number of digits past the decimal place, for proper monetary calculations. [http://www.javaranch.com/journal/2003/07/MoneyInJava.html]  On the downside, this specification for rounding and scale is required, which adds code to the implementation. [http://drdobbs.com/java/184405653]  BigDecimal values are also immutable, which avoids read/write and write/write conflicts in applications. [http://www.opentaps.org/docs/index.php/How_to_Use_Java_BigDecimal:_A_Tutorial]  While there are a lot of nice things about BigDecimal, there are some more issues with BigDecimal which can cause problems in representing money.  BigDecimal cannot represent numbers with infinite digits such as 1/9 (.11111...) and throws an ArithmeticException on any calculation that would produce such number. To deal with this, hacks such as the following must be implemented:
 
BigDecimal bd1 = BigDecimal(1);
*Create one AbstractFactory class for each existing class (or group of related classes) you wish to create.
  BigDecimal bd2 = BigDecimal(9);
*Have a polymorphic "create instance" method on each AbstractFactory class, conforming to a common method signature, used to create instances of the corresponding class.
  try {
*Store and pass around instances of the AbstractFactory class to control selection of the class to create.
    return bd1.divide(bd2);
 
} catch(ArithmaticException ae) {
Suppose an abstract class wants to hide its sub class name and its instantiation. If we request one static method of the class that return its sub class object.
     return new BigDecimal(bd1.doubleValue() / bd2.doubleValue());
 
  abstract class AA
  {
  static AA getInstance()
  {
     return new A();
  }
  }
  }
Also, since there isn't a BigDecimal equivalent for SQL databases, so storing BigDecimal values either requires conversion to a different format, which can affect accuracy and/or precision, or the value must be stored as a BLOB, which is cumbersome and inefficient.  In general use, calculations with BigDecimal are significantly slower than with integers, and also take up more storage space. [http://lemnik.wordpress.com/2011/03/25/bigdecimal-and-your-money/]


===Class===
class A extends AA
In ''Object-Oriented Design Using Java'', recommends using a class to represent money, rather than just using a long integer alone. He gives two reasons for using a class. The first is encapsulation - by encapsulating money into a class, all the operations specific to working with money can be implemented to work as desired in all cases, which also avoids code duplication. The second reason is that the compiler is better able to detect coding errors, such as adding money to something that isn't money. [http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf] Implementing Money as a class is a generally good idea, but there are multiple ways to go about it.
  {
   
  }


====Abstract Class====
class Client
If multiple kinds of currencies are needed to be represented, one implementation is to create an abstract Money class and have each possible currency be a subclass of the abstract class.  Since there are over 180 different currencies worldwide, it can be a challenge to implement subclasses for each one required.
{
AA aa=AA.getInstance();
aa.method();
  }


The following is the abstract class proposed by Skrien in Object-Oriented Design Using Java:
====Builder====
public abstract class Money implements Comparable<Money> {
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 so that
  public long getAmount()
*An object with strict properties (e.g. immutable, or say, maxTemperature >= minTemperature) can be configured in less strict steps.
  public String toString()
*Avoid hard-to-remember/understand chatty constructors with many arguments, and
  public int compareTo(Money m)
*Where possible allow the caller to reuse steps for creating similar instances.
  public boolean equals(Object o)
 
  public int hashCode()
====Factory Method====
  public Money plus(Money)
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.
  public Money minus(Money)
 
  public Money times(double factor)
====Prototype====
  public Money dividedBy(double divisor)
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.
  public Money negate()
====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.
[http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]


====Single Class====
===Structural Patterns===
Rather than implementing a variety of subclasses for each currency, a single class can be used to represent money, with currency specified within.  This is a good idea, since currencies basically operate the same calculation-wise, but have variations in decimal locations and the symbols used to present them.  Java has a [http://docs.oracle.com/javase/6/docs/api/java/util/Currency.html Currency class] which holds the information for a given currency.


The following is the Single Class implementation given by Skrien in ''Object-Oriented Design Using Java'':
====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.
====Bridge====
Separates an object’s interface from its implementation. Decouple an abstraction from its implementation so that the two can vary independently.
====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.
====Decorator====
Add responsibilities to objects dynamically.  Attach additional responsibilities to an object dynamically. Decorators provide a  flexible alternative to subclassing for extending functionality.
====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.
====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.
====Proxy====
An object representing another object. Provide a surrogate or placeholder for another object to control access to it.


public class Money implements Comparable<Money> {
===Behavioral Patterns===
  private long amount;
  private Currency currency;
  public Money(long amount, Currency currency) {
    this.amount = amount;
    this.currency = currency;
  }
  public long getAmount() { return amount; }
  public Currency getCurrency() {
    return currency; }
  public String toString() { ...above... }
  public int compareTo(Money o) { ... }
  public boolean equals(Object o) { ... }
  public int hashCode() { ... }
  public Money plus(Money) { ... }
  public Money minus(Money) { ... }
  public Money times(double factor) { ... }
  public Money dividedBy(double divisor) { ... }
  public Money negate() { ... }
}
[http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf]


====Mixed Money====
====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.
====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.
====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.
====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.
====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.
====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.
====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.
====State====
Alter an object's behavior when its state changes. Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
====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.
====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 the algorithm’s structure.
====Visitor====
Defines a new operation to a class without change. Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.


====Using an Interface====




==Resources==
==Resources==
*Need to go through these for information
**[http://en.wikipedia.org/wiki/Software_design_pattern Software Engineering Design Patterns]
**[http://courses.ncsu.edu/csc517//common/lectures/notes/lec20.pdf PDF from class lecture]
**[http://sourcemaking.com/design_patterns Class and Object Patterns]
**[http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Currency.html Java's Currency class docs]
**[http://www.dofactory.com/Patterns/Patterns.aspx Patterns]
**[http://drdobbs.com/java/184405653 A standalone class to tackle the problem (in Java)]
**[http://geekswithblogs.net/subodhnpushpak/archive/2009/09/18/the-23-gang-of-four-design-patterns-.-revisited.aspx Design Patterns]
**[http://www.javapractices.com/topic/TopicAction.do?Id=13 Representing money (Java)]
**[http://c2.com/cgi/wiki?GangOfFour Gang of Four]
**[http://www.javaranch.com/journal/2003/07/MoneyInJava.html Working with money in Java]
**[http://home.earthlink.net/~huston2/dp/builder.html Binders Pattern]
**[http://timeandmoney.sourceforge.net/ Time and Money Java project - doesn't seem to have much documentation, though]
**[http://joda-money.sourceforge.net/ Joda Money class library for Java]
**[http://lemnik.wordpress.com/2011/03/25/bigdecimal-and-your-money/ BigDecimal and Your Money (a criticism of using BigDecimal format)]
**[http://www.opentaps.org/docs/index.php/How_to_Use_Java_BigDecimal:_A_Tutorial Tutorial on using BigDecimal]
**[http://junit.sourceforge.net/doc/testinfected/testing.htm JUnit Test Infected: Programmers Love Writing Tests (has a Money class example]
**[http://www.xencraft.com/resources/multi-currency.html Currency Internationalization (i18n), Multiple Currencies and Foreign Exchange (FX)] -nothing programming-wise, but a good overview of the considerations for currency
**[http://www.di-mare.com/adolfo/p/money.htm Yet Another C++ Money Class ]
**[http://edenti.deis.unibo.it/utils/Java-tips/Currency%20representation.txt Representing currencies (txt document with example and details)]

Latest revision as of 04:50, 1 December 2011

Wiki Chapter: CSC/ECE 517 Fall 2011/ch7 7f bn

7f. Pattern classification and importance. A common classification of the GoF patterns is into "creational," "structural," and "behavioral categories, and, orthogonally, into "class" and "object" patterns. But the GoF patterns are only a few of the patterns that have been identified in programs. Look up other ways of classifying patterns.

Introduction

Design patterns are general reusable solutions which can be used to tackle recurring problems in software engineering. They have been developed empirically as a result of programmers facing similar challenges frequently. Design patterns act as guides and not the actual solution to the problem at hand. Design patterns typically show relationships between classes or objects. Design patterns are tried and tested solutions and thus can speed up the development process by providing tested, proven development paradigms. A robust design involves having a broad view of the problem at hand rather than coding to patch a problem. This foresight helps predict, avoid and in the worst case tackle problems. This helps procrastinating fire-fighting efforts until implementation phase. Reusing design patterns helps to prevent subtle issues that can cause major problems and improves code readability for coders and architects familiar with the patterns. They give the best practices and are in general applied to Object-oriented design patterns and so are mostly not applicable to functional programming techniques.

In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs. Design patterns reside in the domain of modules and interconnections. At a higher level there are architectural patterns that are larger in scope, usually describing an overall pattern followed by an entire system.

Class and Object Patterns

Depending on the scope design patterns are broadly classified into 2 of these categories:

Class patterns:

Class patterns specify the relationship between classes and their subclasses. These patterns tend to use inheritance to establish relationships. Unlike object patterns and object relationships, class patterns generally fix the relationship at compile time. They are less flexible and dynamic and less suited to polymorphic approaches.

Object Patterns:

Object patterns, the more common of the two, specify the relationships between objects. In general, the purpose of an object pattern is to allow the instances of different classes to be used in the same place in a pattern. Object patterns avoid fixing the class that accomplishes a given task at compile time. Instead the actual class of the object can be chosen at runtime. Object patterns mostly use object composition to establish relationships between objects.

Gang Of Four Patterns

These 23 GoF patterns are generally considered the foundation for all other patterns. They are categorized in three groups: Creational, Structural, and Behavioral.

Creational Patterns

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. Creational design patterns are further categorized into Object-creational patterns and Class-creational patterns. Where, Object-creational patterns deal with Object creation and Class-creational deal with Class-instantiation.

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. It is a class that exists to create instances of another class.

Typically, if one wants to construct instances of a class, where the class is selected at run time

  • Create one AbstractFactory class for each existing class (or group of related classes) you wish to create.
  • Have a polymorphic "create instance" method on each AbstractFactory class, conforming to a common method signature, used to create instances of the corresponding class.
  • Store and pass around instances of the AbstractFactory class to control selection of the class to create.

Suppose an abstract class wants to hide its sub class name and its instantiation. If we request one static method of the class that return its sub class object. 

abstract class AA
{
  static AA getInstance()
  {
    return new A();
  }
}

class A extends AA
{

}

class Client
{
AA aa=AA.getInstance();
aa.method();
}

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 so that

  • An object with strict properties (e.g. immutable, or say, maxTemperature >= minTemperature) can be configured in less strict steps.
  • Avoid hard-to-remember/understand chatty constructors with many arguments, and
  • Where possible allow the caller to reuse steps for creating similar instances.

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.

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.

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

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.

Bridge

Separates an object’s interface from its implementation. Decouple an abstraction from its implementation so that the two can vary independently.

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.

Decorator

Add responsibilities to objects dynamically. Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

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.

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.

Proxy

An object representing another object. Provide a surrogate or placeholder for another object to control access to it.

Behavioral Patterns

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.

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.

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.

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.

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.

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.

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.

State

Alter an object's behavior when its state changes. Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

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.

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 the algorithm’s structure.

Visitor

Defines a new operation to a class without change. Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.


Resources

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