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 programmers

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.

 class Reader {
  private Builder m_builder;

  public Reader( Builder b ) { m_builder = b; }

  public void construct( String file_name ) {
     try {
        BufferedReader br = new BufferedReader(
                       new FileReader( file_name ) );
        String line, cell = "";
        String[] tokens;
        boolean first_line = true;
        while ((line = br.readLine()) != null) {
           tokens = line.split( "\\s" );
           int i = 0;
           if (first_line) {
              m_builder.set_width_and_height(
                             Integer.parseInt( tokens[0] ),
                             Integer.parseInt( tokens[1] ) );
              i = 2;
              first_line = false;
           }
           for ( ; i < tokens.length; ++i)
              if (tokens[i].equals( "" )) {
                 m_builder.build_cell( cell );
                 cell = "";
                 m_builder.start_row();
              } else if (tokens[i].equals( "" )) {
                 m_builder.build_cell( cell );
                 cell = "";
              } else {
                 cell += " " + tokens[i];
              }
        }
        m_builder.build_cell( cell );
        br.close();
     } catch( Exception ex ) { ex.printStackTrace(); }
   }  
 }

 interface Builder {
  void set_width_and_height( int width, int height );
  void start_row();
  void build_cell( String value );
  Component get_result();
 }

 class JTable_Builder implements Builder {
  private JTable     m_table;
  private TableModel m_model;
  private int i = 0, j = 0;

  public void set_width_and_height( int width, int height ) {
     m_table = new JTable( height, width );
     m_model = m_table.getModel();
  }
  public void start_row() {
     ++i;
     j = 0;
  }
  public void build_cell( String value ) {
     m_model.setValueAt( value, i, j++ );
  }
  public Component get_result() { return m_table; }
 }

 class GridLayout_Builder implements Builder {
  private JPanel m_panel = new JPanel();

  public void set_width_and_height( int width, int height ) {
     m_panel.setLayout( new GridLayout( height, width ) );
     m_panel.setBackground( Color.white );
  }
  public void start_row() { }
  public void build_cell( String value ) {
     m_panel.add( new Label( value ) );
  }
  public Component get_result() { return m_panel; }
 }

 class GridBagLayout_Builder implements Builder {
  private JPanel m_panel = new JPanel();
  private GridBagConstraints c = new GridBagConstraints();
  private int i = 0, j = 0;

  public void set_width_and_height( int width, int height ) {
     m_panel.setLayout( new GridBagLayout() );
     m_panel.setBackground( Color.white );
  }
  public void start_row() {
     ++i;
     j = 0;
  }
  public void build_cell( String value ) {
     c.gridx = j++;
     c.gridy = i;
     m_panel.add( new Label( value ), c );
  }
  public Component get_result() { return m_panel; }
 }

 public class BuilderDemo {
  public static void main( String[] args ) {
     Builder target = null;
     try {
        target = (Builder) Class.forName(
                              args[0] ).newInstance();
     } catch( Exception ex ) { ex.printStackTrace(); }
     Reader parser = new Reader( target );
     parser.construct( "BuilderDemo.dat" );

     JFrame frame = new JFrame( "BuilderDemo - " + args[0] );
     frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
     frame.getContentPane().add( target.get_result() );
     frame.pack();
     frame.setVisible( true );
   }  
 }

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.

Abstract Class

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.

The following is the abstract class proposed by Skrien in Object-Oriented Design Using Java:

public abstract class Money implements Comparable<Money> {
  public long getAmount()
  public String toString()
  public int compareTo(Money m)
  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()
}

[1]

Single Class

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 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:

public class Money implements Comparable<Money> {
  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() { ... }
}

[2]

Resources

• Need to go through these for information
  • PDF from class lecture
  • Java's Currency class docs
  • A standalone class to tackle the problem (in Java)
  • Representing money (Java)
  • Working with money in Java
  • Time and Money Java project - doesn't seem to have much documentation, though
  • Joda Money class library for Java
  • BigDecimal and Your Money (a criticism of using BigDecimal format)
  • Tutorial on using BigDecimal
  • JUnit Test Infected: Programmers Love Writing Tests (has a Money class example
  • Currency Internationalization (i18n), Multiple Currencies and Foreign Exchange (FX) -nothing programming-wise, but a good overview of the considerations for currency
  • Yet Another C++ Money Class
  • Representing currencies (txt document with example and details)