1 Creational Design Patterns CSC 335: Object-Oriented Programming and Design

2 Outline  Three Creational Design Patterns  Singleton  Factory  Abstract Factory  Prototype

3 To use new or to not use new? That is the question.  Since most object-oriented languages provide object instantiation with new and initialization with constructors  There may be a tendency to simply use these facilities directly without forethought to future consequences  The overuse of this functionality often introduces inflexibility in the system

4 Creational Patterns  Creational patterns describe object-creation mechanisms that enable greater levels of reuse in evolving systems: Builder, Singleton, Prototype  The most widely used is Factory  This pattern calls for the use of a specialized object solely to create other objects

5 Recurring Problem Some classes have only one instance. For example, there may be many printers in a system, but there should be only one printer spooler How do we ensure that a class has only one instance and that instance is easily accessible? Solution Have constructor return the same instance when called multiple times Takes responsibility of managing that instance away from the programmer It is simply not possible to construct more instances OO Design Pattern Singleton

6 UML General form as UML (From

7 Java Code General Form // NOTE: This is not thread safe! public class Singleton { private static Singleton uniqueInstance; // other useful instance variables here private Singleton() {} public static Singleton getInstance() { if (uniqueInstance == null) { uniqueInstance = new Singleton(); } return uniqueInstance; } // other useful methods here }

8 Participant Singleton Defines a constructor that returns the (single) instance of this class. Store this instance in the class ( uniqueInstance ). Defines operations ( SingletonOperation() ) and data ( getSingletonData() ) for this instance. Optionally, may also define a static method ( Instance() ) to return the instance, instead of a constructor (in this case, the constructor would be a private method).

9 Implementing Singleton in Java Make constructor(s) private so that they can not be called from outside. Declare a single static private instance of the class. Write a public getInstance() or similar method that allows access to the single instance.

10 Patterns IP-10 Use Example: A PhoneState class PhoneState { private static PhoneState pS = new PhoneState(); public static PhoneState getInstance() { return pS; } private PhoneState() {} public int getNumPeople() { … }

11 Patterns IP-11 An Alternative Approach class PhoneState { private static PhoneState pS; public static PhoneState getInstance() { if (pS == null) { pS = new PhoneState(); } return pS; } private PhoneState() {} public int getNumPeople() { … }

12 OO Design Pattern Factory Method  Name: Factory Method  Problem: A Client needs an object and it doesn't know which of several objects to instantiate  Solution: Let an object instantiate the correct object from several choices. The return type is an abstract class or an interface type.

13 Characteristics  A method returns an object  The return type is an abstract class or interface  The interface is implemented by two or more classes or the class is extended by two or more classes

14 General Form

15 Product (Page) defines the interface of objects the factory method creates ConcreteProduct implements the Product interface Creator declares the factory method, which returns an object of type Product. Creator may also define a default implementation of the factory method that returns a default ConcreteProduct object. may call the factory method to create a Product object. ConcreteCreator (Report, Resume) overrides the factory method to return an instance of a ConcreteProduct.

16 Example from Java  Border is an interface  AbstractBorder is an abstract class  BorderFactory has a series of static methods returning different types that implement Border  This hides the implementation details of the subclasses  The factory methods directly call the constructors of the subclasses of AbstractBorder

17 One type setSize(250, 100); JPanel toBeBordered = new JPanel(); Border border = BorderFactory.createMatteBorder(2, 1, 5, 9, Color.RED); toBeBordered.add(new JLabel("" + border.getClass())); toBeBordered.setBorder(border); getContentPane().add(toBeBordered, BorderLayout.CENTER);

18 Another type setSize(250, 100); JPanel toBeBordered = new JPanel(); Border border = BorderFactory.createEtchedBorder(); toBeBordered.add(new JLabel("" + border.getClass())); toBeBordered.setBorder(border); getContentPane().add(toBeBordered, BorderLayout.CENTER);

19 Lots of Subclasses javax.swing.border.AbstractBorder java.lang.Object javax.swing.border.AbstractBorder All Implemented Interfaces: SerializableSerializable, BorderBorder Direct Known Subclasses: BasicBorders.ButtonBorderBasicBorders.ButtonBorder, BasicBorders.FieldBorder, BasicBorders.MarginBorder, BasicBorders.MenuBarBorder, BevelBorder, CompoundBorder, EmptyBorder, EtchedBorder, LineBorder, MetalBorders.ButtonBorder, MetalBorders.Flush3DBorder, MetalBorders.InternalFrameBorder, MetalBorders.MenuBarBorder, MetalBorders.MenuItemBorder, MetalBorders.OptionDialogBorder, MetalBorders.PaletteBorder, MetalBorders.PopupMenuBorder, MetalBorders.ScrollPaneBorder, MetalBorders.TableHeaderBorder, MetalBorders.ToolBarBorder, TitledBorderBasicBorders.FieldBorder BasicBorders.MarginBorderBasicBorders.MenuBarBorder BevelBorderCompoundBorderEmptyBorderEtchedBorder LineBorderMetalBorders.ButtonBorder MetalBorders.Flush3DBorderMetalBorders.InternalFrameBorder MetalBorders.MenuBarBorderMetalBorders.MenuItemBorder MetalBorders.OptionDialogBorderMetalBorders.PaletteBorder MetalBorders.PopupMenuBorderMetalBorders.ScrollPaneBorder MetalBorders.TableHeaderBorderMetalBorders.ToolBarBorder TitledBorder

20 Iterators?  The iterator methods isolate the client from knowing the class to instantiate List list = new ArrayList (); Iterator itr = list.iterator(); System.out.println(itr.getClass().toString());  What type is itr ? class java.util.AbstractList$Itr  What type is itr with this change? List list = new Linked List ();

21 Do we need new?  Objects can be returned without directly using new double amount = ; NumberFormat formatter = NumberFormat.getCurrencyInstance(); System.out.println(formatter.format(amount)); Output if the computer is set to US $12, Change the computer setting to Germany and we get this: ,12 €

22 What Happened?  getCurrencyInstance returns an instance of DecimalFormat where methods like setCurrency help build the appropriate object  It encapsulates the creation of objects  Can be useful if the creation process is complex, for example if it depends on settings in configuration files or the jre or the OS

23 Behind the scenes  Client: main method  Factory Method: getCurrencyInstance  Product: a properly configured instance of DecimalFormat  This is another example of Factory in use