Creational Design Patterns

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Creational Design Patterns

CSC 335: Object-Oriented Programming and Design

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Outline

Three Creational Design Patterns Singleton Factory Abstract Factory Prototype

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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

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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

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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 PatternSingleton

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UML General form as UML

(From http://cvs.m17n.org/~akr/mj/design-pattern/en/design-pattern.html)

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Java Code General Form

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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).

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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.

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Use Example: A PhoneState

class PhoneState { private static PhoneState pS = new PhoneState();

public static PhoneState getInstance() { return pS; } private PhoneState() {} public int getNumPeople() { … }}

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An Alternative Approachclass PhoneState {

private static PhoneState pS;

public static PhoneState getInstance() {

if (pS == null) { pS = new PhoneState(); }

return pS;

}

private PhoneState() {}

public int getNumPeople() {

…

}

}

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OO Design PatternFactory Method

Name: Factory MethodProblem: A Client needs an object and it doesn't

know which of several objects to instantiateSolution: Let an object instantiate the correct

object from several choices. The return type is an abstract class or an interface type.

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Characteristics

A method returns an objectThe return type is an abstract class or interfaceThe interface is implemented by two or more

classes or the class is extended by two or more classes

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General Formhttp://www.dofactory.com/Patterns/PatternFactory.aspx

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•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.

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Example from Java

Border is an interfaceAbstractBorder is an abstract classBorderFactory has a series of static methods

returning different types that implement BorderThis hides the implementation details of the

subclassesThe factory methods directly call the constructors

of the subclasses of AbstractBorder

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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);

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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);

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Lots of Subclasses

javax.swing.border.AbstractBorder java.lang.Object javax.swing.border.AbstractBorder

All Implemented Interfaces: Serializable, Border

Direct Known Subclasses: BasicBorders.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, TitledBorder

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Iterators?

The iterator methods isolate the client from knowing the class to instantiate

List<String> list = new ArrayList<String>();

Iterator<String> 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<String> list = new LinkedList<String>();

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Do we need new?

Objects can be returned without directly using new

double amount = 12345.1234656789457; NumberFormat formatter = NumberFormat.getCurrencyInstance(); System.out.println(formatter.format(amount));

Output if the computer is set to US $12,345.12

Change the computer setting to Germany and we get this: 12.345,12 €

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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

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Behind the scenes

Client: main methodFactory Method: getCurrencyInstance Product: a properly configured instance of DecimalFormat

This is another example of Factory in use