design pattern qa

8/20/2019 Design Pattern QA

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About Design Pattern

What is the design pattern?

If a problem occurs over and over again, a solution to that problem

has been used effectively. That solution is described as a pattern.

The design patterns are language-independent strategies for solving

common object-oriented design problems. When you make a

design, you should know the names of some common solutions.Learning design patterns is good for people to communicate each

other effectively. In fact, you may have been familiar with some

design patterns, you may not use well-known names to describe

them. SUN suggests GOF (Gang Of Four--four pioneer guys who

wrote a book named "Design Patterns"- Elements of Reusable

Object-Oriented Software), so we use that book as our guide to

describe solutions. Please make you be familiar with these termsand learn how other people solve the code problems.

Do I have to use the design pattern?

If you want to be a professional Java developer, you should know

at least some popular solutions to coding problems. Such solutions

have been proved efficient and effective by the experienced

developers. These solutions are described as so-called design

patterns. Learning design patterns speeds up your experience

accumulation in OOA/OOD. Once you grasped them, you would be

benefit from them for all your life and jump up yourselves to be a

master of designing and developing. Furthermore, you will be able

to use these terms to communicate with your fellows or assessors

more effectively.


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Many programmers with many years experience don't know design

patterns, but as an Object-Oriented programmer, you have to know

them well, especially for new Java programmers. Actually, when

you solved a coding problem, you have used a design pattern. Youmay not use a popular name to describe it or may not choose an

effective way to better intellectually control over what you built.

Learning how the experienced developers to solve the coding

problems and trying to use them in your project are a best way to

earn your experience and certification.

Remember that learning the design patterns will really change how

you design your code; not only will you be smarter but will you

sound a lot smarter, too.

How many design patterns?

Many. A site says at least 250 existing patterns are used in OO

world, including Spaghetti which refers to poor coding habits. The

23 design patterns by GOF are well known, and more are to bediscovered on the way.

Note that the design patterns are not idioms or algorithms or

components.

What is the relationship among these patterns?

Generally, to build a system, you may need many patterns to fit

together. Different designer may use different patterns to solve the

same problem. Usually:

Some patterns naturally fit together

One pattern may lead to another


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Some patterns are similar and alternative

Patterns are discoverable and documentable

Patterns are not methods or framework

Patterns give you hint to solve a problem effectively

Creational Patterns

a.

Abstract Factory

Definition

Provides one level of interface higher than the factory pattern. It is

used to return one of several factories.

Where to use & benefits Creates families of related or dependent objects like Kit.

Provides a class library of products, exposing interface not

implementation.

Needs to isolate concrete classes from their super classes.

A system needs independent of how its products are created,

composed, and represented. Try to enforce a constraint.

An alternative to Facade to hide platform-specific classes

Easily extensible to a system or a family

Related patterns include

o Factory method, which is often implemented with an

abstract factory.


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o Singleton, which is often implemented with an abstract

factory.

o Prototype, which is often implemented with an abstract

factory.o Facade, which is often used with an abstract factory by

providing an interface for creating implementing class.

Example

Suppose you need to write a program to show data in two different

places. Let's say from a local or a remote database. You need to

make a connection to a database before working on the data. In this

case, you have two choices, local or remote. You may use abstract

factory design pattern to design the interface in the following way:


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class DataInfo {}

interface Local {DataInfo[] loadDB(String filename);

}

interface Remote extends Local{void connect2WWW(String url);

}

class LocalMode implements Local {public DataInfo[] loadDB(String name) {

System.out.print("Load from a local database");

return null;}

}

class RemoteMode implements Remote {public void connect2WWW(String url) {

System.out.println("Connect to a remote site");}public DataInfo[] loadDB(String name) {

System.out.println("Load from a remotedatabase ");

return null;}

}

// The Abstract Factoryinterface ConnectionFactory {Local getLocalConnection();Remote getRemoteConnection();

}

class DataManager implements ConnectionFactory {


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boolean local = false;DataInfo[] data;//...public Local getLocalConnection() {

return new LocalMode();}public Remote getRemoteConnection() {

return new RemoteMode();}public void loadData() {

if(local){Local conn = getLocalConnection();data = conn.loadDB("db.db");

}else {Remote conn = getRemoteConnection();

conn.connect2WWW("www.some.where.com");data = conn.loadDB("db.db");

}

}// work on data

public void setConnection(boolean b) {local = b;

}}


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//Use the following Test class to test the aboveclassesclass Test {

public static void main(String[] args) {DataManager dm = new DataManager();DataInfo[] di = null;String dbFileName = "db.db";if (args.length == 1) {

//assume local is set to truedm.setConnection(true);

LocalMode lm =(LocalMode)dm.getLocalConnection();

di = lm.loadDB(dbFileName);} else {

//Note: dm.local = false is defaultsetting

RemoteMode rm =(RemoteMode)dm.getRemoteConnection();

rm.connect2WWW("www.javacamp.org/db/");di = rm.loadDB(dbFileName);

}//use one set of methods to deal with

loaded data.//You don't need to worry about connection

from this point.//Like di.find(), di.search() etc.

}}

C:\ Command Prompt

C:\> java TestConnect to a remote site

Load from a remote database


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C:\ Command Prompt

C:\> java Test localLoad from a local database

Such design is often used in SCJD project assignment. If you have

a multiple places to load data, you just add more methods in the

connection interface without altering other structure, or add a

location variable in.


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b.

Builder

Definition

Construct a complex object from simple objects step by step.

Where to use & benefits

Make a complex object by specifying only its type and

content. The built object is shielded from the details of itsconstruction.

Want to decouple the process of building a complex object

from the parts that make up the object.

Isolate code for construction and representation.

Give you finer control over the construction process.


o Abstract Factory, which focuses on the layer over the

factory pattern (may be simple or complex), whereas a

builder pattern focuses on building a complex object from

other simple objects.

o Composite, which is often used to build a complex object.

Example

To build a house, we will take several steps:

1. build foundation,

2. build frame,

3. build exterior,

4. build interior.


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Let's use an abstract class HouseBuilder to define these 4 steps.

Any subclass of HouseBuilder will follow these 4 steps to build

house (that is to say to implement these 4 methods in the subclass).

Then we use a WorkShop class to force the order of these 4 steps(that is to say that we have to build interior after having finished

first three steps). The TestBuilder class is used to test the

coordination of these classes and to check the building process.

import java.util.*;

class WorkShop {

//force the order of building processpublic void construct(HouseBuilder hb) {

hb.buildFoundation();hb.buildFrame();hb.buildExterior();hb.buildInterior();

}}


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//set steps for building a houseabstract class HouseBuilder {

protected House house = new House();

protected String showProgress() {return house.toString();

}

abstract public void buildFoundation();abstract public void buildFrame();abstract public void buildExterior();abstract public void buildInterior();

}

class OneStoryHouse extends HouseBuilder {

public OneStoryHouse(String features) {house.setType(this.getClass() + " " +

features);}public void buildFoundation() {

//doEngineering()//doExcavating()//doPlumbingHeatingElectricity()//doSewerWaterHookUp()//doFoundationInspection()

house.setProgress("foundation is done");}

public void buildFrame() {//doHeatingPlumbingRoof()//doElectricityRoute()//doDoorsWindows()//doFrameInspection()

house.setProgress("frame is done");


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}

public void buildExterior() {//doOverheadDoors()//doBrickWorks()//doSidingsoffitsGutters()//doDrivewayGarageFloor()//doDeckRail()//doLandScaping()house.setProgress("Exterior is done");

}

public void buildInterior() {//doAlarmPrewiring()//doBuiltinVacuum()//doInsulation()//doDryWall()//doPainting()//doLinoleum()//doCabinet()//doTileWork()//doLightFixtureBlinds()//doCleaning()//doInteriorInspection()house.setProgress("Interior is under

going");}

}

class TwoStoryHouse extends HouseBuilder {

public TwoStoryHouse(String features) {house.setType(this.getClass() + " " +

features);}

public void buildFoundation() {


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//doEngineering()//doExcavating()//doPlumbingHeatingElectricity()//doSewerWaterHookUp()//doFoundationInspection()house.setProgress("foundation is done");

}

public void buildFrame() {//doHeatingPlumbingRoof()//doElectricityRoute()//doDoorsWindows()//doFrameInspection()house.setProgress("frame is under

construction");}

public void buildExterior() {//doOverheadDoors()//doBrickWorks()//doSidingsoffitsGutters()//doDrivewayGarageFloor()//doDeckRail()//doLandScaping()house.setProgress("Exterior is waiting to

start");}

public void buildInterior() {//doAlarmPrewiring()//doBuiltinVacuum()//doInsulation()//doDryWall()//doPainting()//doLinoleum()

//doCabinet()


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//doTileWork()//doLightFixtureBlinds()//doCleaning()//doInteriorInspection()house.setProgress("Interior is not started

yet");}

}

class House {private String type = null;private List features = new ArrayList();

public House() {

}

public House(String type) {this.type = type;

}


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public void setType(String type) {

this.type = type;}

public String getType() {return type;

}

public void setProgress(String s) {features.add(s);

}

public String toString() {StringBuffer ff = new StringBuffer();String t = type.substring(6);ff.append(t + "\n ");for (int i = 0; i < features.size(); i ++)

{ff.append(features.get(i) + "\n ");

}return ff.toString();

}}

class TestBuilder {

public static void main(String[] args) {

HouseBuilder one = new OneStoryHouse("2bedrooms, 2.5 baths, 2-car garage, 1500 sqft");

HouseBuilder two = new TwoStoryHouse("4bedrooms, 4 baths, 3-car garage, 5000 sqft");

WorkShop shop = new WorkShop();

shop.construct(one);


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shop.construct(two);

System.out.println("Check house buildingprogress: \n");

System.out.println(one.showProgress());System.out.println(two.showProgress());

}}//need jdk1.5 above to compile

C:\ Command Prompt

C:\> javac TestBuilder.java

C:\> java TestBuilderCheck house building progress:

OneStoryHouse 2 bedrooms, 2.5baths, 2-car garage, 1500 sqftfoundation is doneframe is doneExterior is doneInterior is under going

TwoStoryHouse 4 bedrooms, 4 baths,3-car garage, 5000 sqftfoundation is doneframe is under constructionExterior is waiting to startInterior is not started yet

C:\>


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To fine tune the above example, every do method can be designed

as a class. Similar functional class can be designed once and used

by other classes. e.g. Window, Door, Kitchen, etc.

Another example, such as writing a Pizza program. Every gradient

can be designed as a class. One pizza at least consists of several

gradients. Different pizza has different gradients. A builder pattern

may be adopted.


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c.

Factory Method

Definition

Provides an abstraction or an interface and lets subclass or

implementing classes decide which class or method should be

instantiated or called, based on the conditions or parameters given.


Connect parallel class hierarchies.

A class wants its subclasses to specify the object.

A class cannot anticipate its subclasses, which must be created.

A family of objects needs to be separated by using shared

interface.

The code needs to deal with interface, not implemented

classes. Hide concrete classes from the client.

Factory methods can be parameterized.

The returned object may be either abstract or concrete object.

Providing hooks for subclasses is more flexible than creating

objects directly.

Follow naming conventions to help other developers to

recognize the code structure. Related patterns include

o Abstract Factory , which is a layer higher than a factory

method.

o Template method, which defines a skeleton of an

algorithm to defer some steps to subclasses or avoid

subclasses


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o Prototype, which creates a new object by copying an

instance, so it reduces subclasses.

o Singleton, which makes a returned factory method unique.

Examples

To illustrate such concept, let's use a simple example. To paint a

picture, you may need several steps. A shape is an interface.

Several implementing classes may be designed in the following

way.

interface Shape {public void draw();

}

class Line implements Shape {Point x, y;Line(Point a, Point b) {

x = a;

y = b;}public void draw() {

//draw a line;}

}


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class Square implements Shape {Point start;int width, height;Square(Point s, int w, int h) {

start = s;width = w;height = h;

}public void draw() {

//draw a square;}

}class Circle implements Shape {

....}

class Painting {Point x, y;int width, height, radius;Painting(Point a, Point b, int w, int h, int r)

{x = a;y = b;width = w;height = h;radius = r;

}

Shape drawLine() {return new Line(x,y);}Shape drawSquare() {

return new Square(x, width, height);}Shape drawCircle() {

return new Circle(x, radius);

}


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....}

...Shape pic;Painting pt;//initializing pt....if (line)

pic = pt.drawLine();if (square)

pic = pt.drawSquare();if (circle)

pic = pt.drawCircle();

From the above example, you may see that the Shape pic's type

depends on the condition given. The variable pic may be a line or

square or a circle.

You may use several constructors with different parameters to

instantiate the object you want. It is another way to design with

Factory pattern. For example,


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class Painting {...Painting(Point a, Point b) {

new Line(a, b); //draw a line}Painting(Point a, int w, int h) {

new Square(a, w, h); //draw a square}Painting(Point a, int r){

new Circle(a, r); //draw a circle}...

}

You may use several methods to finish the drawing jobs. It is so-

called factory method pattern. for example,

class Painting {...Painting(Point a, Point b) {

draw(a, b); //draw a line}Painting(Point a, int w, int h) {

draw(a, w, h); //draw a square}Painting(Point a, int r){

draw(a, r); //draw a circle}

...}

The above draw() methods are overloaded.

Here is a popular example of Factory design pattern. For example,

you have several database storages located in several places. The


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program working on the database is the same. The user may choose

local mode or remote mode. The condition is the choice by the user.

You may design your program with Factory pattern. When the local

mode is set, you may instantiate an object to work on the localdatabase. If the remote mode is set, you may instantiate an object

which may have more job to do like remote connection,

downloading, etc.

interface DatabaseService {public DataInfo getDataInfo() throws

Exception;

public FieldInfo getFieldInfo() throwsException;

public void write(FieldInfo fi) throwsException;

public void modify(FieldInfo fi) throwsException;

public void delete(FieldInfo fi) throwsException;

//...}class Data implements DatabaseService {

public Data(String fileName) {...};public Data(URL url, String fileName) {....};public DataInfo getDataInfo() throws Exception

{...};public FieldInfo getFieldInfo() throws

Exception {...};public void write(FieldInfo fi) throws

Exception {...};public void modify(FieldInfo fi) throws

Exception {...};public void delete(FieldInfo fi) throws

Exception {...};}class DataManager{


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Data data = null;...if (local) {

data = new Data(localFile);...

}if (remote){

data = new Data(connectRemote,databaseFile);

...}data.write(someInfo);data.modify(someInfo);....

}

To illustrate how to use factory design pattern with class level

implementation, here is a real world example. A company has a

website to display testing result from a plain text file. Recently, thecompany purchased a new machine which produces a binary data

file, another new machine on the way, it is possible that one will

produce different data file. How to write a system to deal with such

change. The website just needs data to display. Your job is to

provide the specified data format for the website.

Here comes a solution. Use an interface type to converge the

different data file format. The following is a skeleton of

implementation.

//Let's say the interface is Displayinterface Display {

//load a filepublic void load(String fileName);


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//parse the file and make a consistent datatype

public void formatConsistency();

}

//deal with plain text fileclass CSVFile implements Display{

public void load(String textfile) {System.out.println("load from a txt

file");}public void formatConsistency() {

System.out.println("txt file formatchanged");

}}

//deal with XML format fileclass XMLFile implements Display {

public void load(String xmlfile) {System.out.println("load from an xml

file");}public void formatConsistency() {

System.out.println("xml file formatchanged");}

}


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//deal with binary format fileclass DBFile implements Display {

public void load(String dbfile) {System.out.println("load from a db file");

}public void formatConsistency() {

System.out.println("db file formatchanged");

}}

//Test the functionalityclass TestFactory {

public static void main(String[] args) {Display display = null;

//use a command line data as a triggerif (args[0].equals("1"))

display = new CSVFile();else if (args[0].equals("2"))

display = new XMLFile();else if (args[0].equals("3"))

display = new DBFile();else

System.exit(1);

//converging code followsdisplay.load("");display.formatConsistency();

}}//after compilation and run it


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C:\>java TestFactory 1load from a txt filetxt file format changed

C:\>java TestFactory 2load from an xml filexml file format changed

C:\>java TestFactory 3load from a db filedb file format changed

In the future, the company may add more data file with different

format, a programmer just adds a new class in accordingly. Such

design saves a lot of code and is easy to maintain.


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d.

Prototype

Definition

Cloning an object by reducing the cost of creation.


When there are many subclasses that differ only in the kind of

objects, A system needs independent of how its objects are created,

composed, and represented.

Dynamic binding or loading a method.

Use one instance to finish job just by changing its state or

parameters.

Add and remove objects at runtime.

Specify new objects by changing its structure. Configure an application with classes dynamically.


o Abstract Factory, which is often used together with

prototype. An abstract factory may store some prototypes

for cloning and returning objects.o Composite, which is often used with prototypes to make a

part-whole relationship.o Decorator , which is used to add additional functionality to

the prototype.

Example


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Dynamic loading is a typical object-oriented feature and prototype

example. For example, overriding method is a kind of prototype

pattern.

interface Shape {public void draw();

}

class Line implements Shape {public void draw() {

System.out.println("line");

}}

class Square implements Shape {public void draw() {

System.out.println("square");}

}

class Circle implements Shape {public void draw() {

System.out.println("circle");}

}


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class Painting {public static void main(String[] args) {

Shape s1 = new Line();Shape s2 = new Square();Shape s3 = new Circle();paint(s1);paint(s2);paint(s3);

}static void paint(Shape s) {

s.draw();}

}----------------------------If we want to make code more readable or do morestuff, we can code the paint method in thefollowing way:

static void paint(Shape s){if ( s instanceof Line)

s.draw();//more job here

if (s instanceof Square)s.draw();//more job here

if (s instanceof Circle)s.draw();

//more job here}nd Prompt

C:\> java Paintinglinesquarecircle


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The paint method takes a variable of Shape type at runtime. The

draw method is called based on the runtime type.

Overloading method is a kind of prototype too.

class Painting {public void draw(Point p, Point p2) {

//draw a line}public void draw(Point p, int x, int y) {

//draw a square}

public void draw(Point p, int x) {//draw a circle

}}

The draw method is called to draw the related shape based on the

parameters it takes.

The prototype is typically used to clone an object, i.e. to make acopy of an object. When an object is complicated or time

consuming to be created , you may take prototype pattern to make

such object cloneable. Assume the Complex class is a complicated,

you need to implement Cloneable interface and override the clone

method(protected Object clone()).

class Complex implements Cloneable {int[] nums = {1,2,3,4,5};public Object clone() {

try {return super.clone();

}catch(CloneNotSupportedException cnse) {System.out.println(cnse.getMessage());return null;

}


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}int[] getNums() {

return nums;}

}class Test {

static Complex c1 = new Complex();static Complex makeCopy() {

return (Complex)c1.clone();}public static void main(String[] args) {

Complex c1 = makeCopy();int[] mycopy = c1.getNums();for(int i = 0; i < mycopy.length; i++)

System.out.print(mycopy[i]);}

}

C:\ Command Prompt

C:\> java Test12345

Cloning is a shallow copy of the original object. If the cloned

object is changed, the original object will be changed accordingly.

See the following alteration.

class Complex implements Cloneable {int[] nums = {1,2,3,4,5};public Object clone() {

try {return super.clone();

}catch(CloneNotSupportedException cnse) {System.out.println(cnse.getMessage());

return null;


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}}int[] getNums() {

return nums;}

}class Test {

Complex c1 = new Complex();Complex makeCopy() {


Test tp = new Test();Complex c2 = tp.makeCopy();int[] mycopy = c2.getNums();mycopy[0] = 5; System.out.println();System.out.print("local array: ");for(int i = 0; i < mycopy.length; i++)

System.out.print(mycopy[i]);System.out.println();

System.out.print("cloned object: ");for(int ii = 0; ii < c2.nums.length; ii++)

System.out.print(c2.nums[ii]);System.out.println();

System.out.print("original object: ");for(int iii = 0; iii < tp.c1.nums.length;iii++)

System.out.print(tp.c1.nums[iii]);}


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C:\> java Test

local array: 52345cloned object: 52345original object: 52345

To avoid such side effect, you may use a deep copy instead of a

shallow copy. The following shows the alteration to the above

example, note that the Complex class doesn't implement Cloneable

interface.

class Complex { int[] nums = {1,2,3,4,5};public Complex clone() {

return new Complex();} int[] getNums() {

return nums;

}}class Test2 {

Complex c1 = new Complex();Complex makeCopy() {


Test2 tp = new Test2();Complex c2 = tp.makeCopy();int[] mycopy = c2.getNums();mycopy[0] = 5;

System.out.println();System.out.print("local array: ");for(int i = 0; i < mycopy.length; i++)

System.out.print(mycopy[i]);


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System.out.println();

System.out.print("cloned object: ");for(int ii = 0; ii < c2.nums.length; ii++)

System.out.print(c2.nums[ii]);System.out.println();

System.out.print("original object: ");for(int iii = 0; iii < tp.c1.nums.length;

iii++)System.out.print(tp.c1.nums[iii]);

}}

C:\> java Test2

local array: 52345cloned object: 52345original object: 12345

e.

Singleton

Definition

One instance of a class or one value accessible globally in anapplication.


Ensure unique instance by defining class final to prevent

cloning.

May be extensible by the subclass by defining subclass final.


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Make a method or a variable public or/and static.

Access to the instance by the way you provided.

Well control the instantiation of a class.

Define one value shared by all instances by making it static. Related patterns include

o Abstract factory, which is often used to return unique

objects.o

Builder , which is used to construct a complex object,

whereas a singleton is used to create a globally accessible

object.

o Prototype, which is used to copy an object, or create an

object from its prototype, whereas a singleton is used to

ensure that only one prototype is guaranteed.

Example

One file system, one window manager, one printer spooler, one

Test engine, one Input/Output socket and etc.

To design a Singleton class, you may need to make the class final

like java.Math, which is not allowed to subclass, or make a variable

or method public and/or static, or make all constructors private to

prevent the compiler from creating a default one.

For example, to make a unique remote connection,

final class RemoteConnection {private Connect con;private static RemoteConnection rc = new

RemoteConnection(connection);private RemoteConnection(Connect c) {

con = c;....

}


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public static RemoteConnectiongetRemoteConnection() {

return rc;}public void setConnection(Connect c) {

this(c);}

}

usage:RemoteConnection rconn =RemoteConnection.getRemoteConnection;rconn.loadData();...

The following statement may fail because of theprivate constructorRemoteConnection con = newRemoteConnection(connection); //failed

//failed because you cannot subclass it (finalclass)class Connection extends RemoteConnection {}

For example, to use a static variable to control the instance;

class Connection {

public static boolean haveOne = false;public Connection() throws Exception{

if (!haveOne) {doSomething();haveOne = true;

}else {throw new Exception("You cannot have a

second instance");

}


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}public static Connection getConnection()

throws Exception{return new Connection();

}void doSomething() {}//...public static void main(String [] args) {

try {Connection con = new Connection();

//ok}catch(Exception e) {

System.out.println("first: "+e.getMessage());

}try {

Connection con2 =Connection.getConnection(); //failed.

}catch(Exception e) {System.out.println("second: "

+e.getMessage());}

}}

C:\ Command Prompt

C:\> java Connectionsecond: You cannot have a secondinstance

For example to use a public static variable to ensure a unique.

class Employee {public static final int companyID = 12345;public String address;


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//...

}class HourlyEmployee extends Employee {

public double hourlyRate;//....

}class SalaryEmployee extends Employee {

public double salary;//...

}class Test {

public static void main(String[] args) {Employee Evens = new Employee();HourlyEmployee Hellen = new

HourlyEmployee();SalaryEmployee Sara = new SalaryEmployee();System.out.println(Evens.companyID ==

Hellen.companyID); //trueSystem.out.println(Evens.companyID ==

Sara.companyID); //true}

}

C:\ Command Prompt

C:\> java Testtruetrue

The companyID is a unique and cannot be altered by

all subclasses.


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Note that Singletons are only guaranteed to beunique within a given class

loader. If you use the same class across multipledistinct enterprise

containers, you'll get one instance for eachcontainer.

Whether you need to use synchronized keyword tomanage the method access, it depends on yourproject situation and thread controlling.


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Structural Patterns

a. Adapter

Definition

Convert the existing interfaces to a new interface to achieve

compatibility and reusability of the unrelated classes in one

application. Also known as Wrapper pattern.


Try to match an interface(WindowAdapter, etc.)

Make unrelated classes work together.

Multiple compatibility.

Increase transparency of classes.

Make a pluggable kit. Delegate objects.

Highly class reusable.

Achieve the goal by inheritance or by composition


o Proxy, which provides the same interface as its subject,

whereas an adapter provides a different interface to the

object it adapts.o Decorator , which focuses on adding new functions to an

object, whereas an adapter coordinates two different

objects.

o Bridge, which tries to separate an interface from its

implementation and make an object vary independently,

whereas an adapter tries to change and cooperate the

interface of an object.


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Example

The famous adapter classes in Java API are

WindowAdapter,ComponentAdapter, ContainerAdapter,FocusAdapter, KeyAdapter, MouseAdapter and

MouseMotionAdapter.

As you know, WindowListner interface has seven methods.

Whenever your class implements such interface, you have to

implements all of the seven methods. WindowAdapter class

implements WindowListener interface and make seven empty

implementation. When you class subclass WindowAdapter class,

you may choose the method you want without restrictions. The

following give such an example.

public interface Windowlistener {public void windowClosed(WindowEvent e);public void windowOpened(WindowEvent e);

public void windowIconified(WindowEvent e);public void windowDeiconified(WindowEvent e);public void windowActivated(WindowEvent e);public void windowDeactivated(WindowEvent e);public void windowClosing(WindowEvent e);

}


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public class WindowAdapter implementsWindowListner{

public void windowClosed(WindowEvent e){}public void windowOpened(WindowEvent e){}public void windowIconified(WindowEvent e){}public void windowDeiconified(WindowEvent e){}public void windowActivated(WindowEvent e){}public void windowDeactivated(WindowEvent e){}public void windowClosing(WindowEvent e){}

}

Here is a test program

import javax.swing.*;import java.awt.event.*;class Test extends JFrame {

public Test () {setSize(200,200);setVisible(true);

addWindowListener(new Closer());}public static void main(String[] args) {

new Test();}class Closer extends WindowAdapter {

public void windowClosing(WindowEvent e) {System.exit(0);

}}

}

To reuse classes and make new class compatible with existing ones.

For example, A clean system is already designed, you want to add

more job in, the Extra interface uses adapter pattern to plug in the

existing system.


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interface Clean {public void makeClean();

}class Office implements Clean{

public void makeClean() {System.out.println("Clean Office");

}}class Workshop implements Clean{

public void makeClean() {System.out.println("Clean Workshop");

}}

interface Extra extends Clean{public void takeCare();

}class Facility implements Extra{

public void makeClean() {System.out.println("Clean Facility");

}public void takeCare() {

System.out.println("Care has been taken");}

}In order to reuse Workshop and Office classes,wecreate an adapter interface Extra and

add new job takeCare in the system.

class Test {static void Jobs (Extra job) {

if (job instanceof Clean)((Clean)job).makeClean();

if (job instanceof Extra)((Extra)job).takeCare();

}


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public static void main(String[] args) {Extra e = new Facility();Jobs(e);Clean c1 = new Office();Clean c2 = new Workshop();c1.makeClean();c2.makeClean();e.makeClean();

}

} C:\ Command Prompt

C:\> java TestClean FacilityCare has been takenClean OfficeClean WorkshopClean Facility

By composition, we can achieve adapter pattern. It is also called

wrapper. For example, a Data class has already been designed andwell tested. You want to adapt such class to your system. You may

declare it as a variable and wrapper or embed it into your class.

//well-tested classclass Data {

public void add(Info){}public void delete(Info) {}

public void modify(Info){}//...

}

//Use Data class in your own classclass AdaptData {

Data data;public void add(Info i) {

data.add(i);


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//more job}public void delete(Info i) {

data.delete(i);//more job

}public void modify(Info i) {

data.modify(i);//more job

}//more stuff here//...

}


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b. Bridge

Definition

Decouple an abstraction or interface from its implementation so

that the two can vary independently.


Want to separate abstraction and implementation permanently

Share an implementation among multiple objects

Want to improve extensibility

Hide implementation details from clients


o Abstract Factory, which can be used to create and

configure a particular bridge.o Adapter , which makes unrelated classes work together,

whereas a bridge makes a clear-cut between abstraction

and implementation.

Examples

If you have a question database, you may want to develop a

program to display it based on the user selection. The following is asimple example to show how to use a Bridge pattern to decouple

the relationship among the objects.

import java.util.*;

//abstractioninterface Question {


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public void nextQuestion();public void priorQuestion();public void newQuestion(String q);public void deleteQuestion(String q);public void displayQuestion();public void displayAllQuestions();

}

//implementationclass QuestionManager {

protected Question questDB; //instantiate itlaterpublic String catalog;

public QuestionManager(String catalog) {this.catalog = catalog;

}

public void next() {questDB.nextQuestion();

}


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public void prior() {questDB.priorQuestion();

}

public void newOne(String quest) {questDB.newQuestion(quest);

}

public void delete(String quest) {questDB.deleteQuestion(quest);

}

public void display() {questDB.displayQuestion();

}

public void displayAll() {System.out.println("Question Catalog: " +

catalog);questDB.displayAllQuestions();

}}

//further implementationclass QuestionFormat extends QuestionManager {

public QuestionFormat(String catalog){super(catalog);}

public void displayAll() {

System.out.println("\n~~~~~~~~~~~~~~~~~~~~~~~~");

super.displayAll();


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System.out.println("~~~~~~~~~~~~~~~~~~~~~~~~");

}}

//decoupled implementationclass JavaQuestions implements Question {

private List questions = newArrayList();

private int current = 0;

public JavaQuestions() {//load from a database and fill in the

containerquestions.add("What is Java? ");questions.add("What is an interface? ");questions.add("What is cross-platform? ");questions.add("What is UFT-8? ");questions.add("What is abstract? ");questions.add("What is Thread? ");questions.add("What is multi-threading?

");

}

public void nextQuestion() {

if( current 0 )

current--;

}


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public void newQuestion(String quest) {

questions.add(quest);}

public void deleteQuestion(String quest) {questions.remove(quest);

}

public void displayQuestion() {System.out.println( questions.get(current)

);}

public void displayAllQuestions() {for (String quest : questions) {

System.out.println(quest);}

}}

class TestBridge {public static void main(String[] args) {

QuestionFormat questions = newQuestionFormat("Java Language");

questions.questDB = newJavaQuestions();//can be hooked up with otherquestion class

//questions.questDB = newCsharpQuestions();

//questions.questDB = newCplusplusQuestions();

questions.display();


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questions.next();

questions.newOne("What is object? ");questions.newOne("What is reference

type?");

questions.displayAll();}

}//need jdk1.5 to compile

C:\ Command Prom

C:\> javac TestBridge.javaC:\> java TestBridgeWhat is Java?

~~~~~~~~~~~~~~~~~~~~~~~~Question Catalog: Java LanguageWhat is Java?What is an interface?What is cross-platform?What is UFT-8?What is abstract?What is Thread?What is multi-threading?What is object?What is reference type?~~~~~~~~~~~~~~~~~~~~~~~~

C:\>

Note that the JavaQuestion class can be launched independently

and work as its own system. Here we just show you how to use

Bridge pattern to decouple the interface from its implementation.


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c.

Composite

Definition

Build a complex object out of elemental objects and itself like a

tree structure.

Where to use & benefits Want to represent a part-whole relationship like tree folder

system

Group components to form larger components, which in turn

can be grouped to form still larger components.


o Decorator , which is often used with composite pattern and

with the same parent class.

o Flyweight, which is used with composite pattern to share

components.o Iterator , which is used to traverse the composites.

o Visitor , which localizes operations across composite and

leaf classes.

Example

A component has many elements and itself which has many

elements and itself, etc. A file system is a typical example.

Directory is a composite pattern. When you deal with Directory

object, if isFile() returns true, work on file, if isDirectory() returns

true, work on Directory object.


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class Directory {Directory dir;File[] f;...

boolean isDirectory() {return f == null;

}boolean isFile() {

return f != null;}File getFile(int i) {

if (isFile())return f[i];

return null'}Directory getDirectory() {

if (isDirectory())return dir;

return null;}....

}

For example, General Manager may have several employees and

some of employees are Managers which have several employees.

To illustrate such issue, we design a simple Manager class.

class Employee {String name;double salary;Employee(String n, double s){

name = n;salary = s;

}String getName() {

return name;


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}double getSalary() {

return salary;}public String toString() {

return "Employee " + name;}

}class Manager {

Manager mgr;Employee[] ely;String dept;Manager(Manager mgr,Employee[] e, String d ) {

this(e, d);this.mgr = mgr;

}

Manager(Employee[] e, String d) {ely = e;dept =d;

}String getDept() {

return dept;}


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Manager getManager() {return mgr;

}Employee[] getEmployee() {

return ely;}public String toString() {

return dept + " manager";}

}

class Test {public static void main(String[] args) {

Employee[] e1 = {new Employee("Aaron", 50),new Employee("Betty",

60)};Manager m1 = new Manager(e1, "Accounting");

Employee[] e2 = {new Employee("Cathy", 70),new Employee("Dan", 80),new Employee("Eliz", 90)};

Manager m2 = new Manager(m1, e2,"Production");

System.out.println(m2);Employee[] emp = m2.getEmployee();if (emp != null)

for (int k = 0; k < emp.length; k++)System.out.println(" "+emp[k]+"Salary: $"+ emp[k].getSalary());

Manager m = m2.getManager();System.out.println(" " + m);if (m!= null) {

Employee[] emps = m.getEmployee();if (emps != null)

for (int k = 0; k < emps.length; k++)


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System.out.println(" " +emps[k]+" Salary: $"+ emps[k].getSalary());

}}

}

C:\ Command Prompt

C:\> java TestProduction manager

Employee Cathy Salary: $70.0Employee Dan Salary: $80.0Employee Eliz Salary: $90.0Accounting managerEmployee Aaron Salary: $50.0Employee Betty Salary: $60.0


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d.

Decorator

Definition

Attach additional responsibilities or functions to an object

dynamically or statically. Also known as Wrapper.


Provide an alternative to subclassing. Add new function to an object without affecting other objects.

Make a responsibility easily added and removed dynamically.

More flexibility than static inheritance.

Transparent to the object.


o Adapter pattern, which provides a different interface to

the object it adapts, whereas a decorator changes anobject's responsibilities,

o Proxy pattern, which controls access to the object,

whereas a decorator focuses on adding new functions to

an object,

o Composite pattern, which aggregates an object, whereas a

decorator adds additional responsibilities to an object, and

o Strategy pattern, which changes the guts of an object,

whereas a decorator changes the skin of an object.

o Facade pattern, which provides a way of hiding a complex

class, whereas a decorator adds function by wrapping a

class.

Example


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A JScrollPane object can be used to decorate a JTextArea object or

a JEditorPane object. A window can be decorated with different

borders like BevelBorder, CompoundBorder, EtchedBorder

TitledBorder etc. These border classes working as decorators are provided in Java API.

Decorator pattern can be used in a non-visual fashion. For example,

BufferedInputStream, DataInputStream, and CheckedInputStream

are decorating objects of FilterInputStream class. These decorators

are standard Java API classes.

To illustrate a simple decorator pattern in non-visual manner, wedesign a class that prints a number. We create a decorator class that

adds a text to the Number object to indicate that such number is a

random number. Of course we can subclass the Number class to

achieve the same goal. But the decorator pattern provides us an

alternative way.

import java.util.Random;class Number {public void print() {

System.out.println(new Random().nextInt());}

}

class Decorator {

public Decorator() {System.out.print("Random number: ");//add

a description to the number printednew Number().print();

}}

class SubNumber extends Number{

public SubNumber() {


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super();System.out.print("Random number: ");print();

}}

class Test {public static void main(String[] args) {

new Decorator();new SubNumber();

}}java TestRandom number: 145265744Random number: 145265755

e.

Façade

Definition

Make a complex system simpler by providing a unified or general

interface, which is a higher layer to these subsystems.


Want to reduce complexities of a system.

Decouple subsystems , reduce its dependency, and improve

portability.

Make an entry point to your subsystems.


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Minimize the communication and dependency between

subsystems.

Security and performance consideration.

Shield clients from subsystem components. Simplify generosity to specification.


o Abstract Factory, which is often used to create an

interface for a subsystem in an independent way, and can

be used as an alternative way to a facade.o Singleton, which is often used with a facade.

o Mediator , which is similar to facade, but a facade doesn't

define new functionality to the subsystem.

Example

JDBC design is a good example of Façade pattern. A database

design is complicated. JDBC is used to connect the database and

manipulate data without exposing details to the clients.

Security of a system may be designed with Façade pattern. Clients'

authorization to access information may be classified. General

users may be allowed to access general information; special guests

may be allowed to access more information; administrators and

executives may be allowed to access the most important

information. These subsystems may be generalized by one

interface. The identified users may be directed to the relatedsubsystems.

interface General {public void accessGeneral();

}interface Special extends General {

public void accessSpecial();

}


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interface Private extends General {public void accessPrivate();

}

class GeneralInfo implements General {public void accessGeneral() {//...}//...

}class SpecialInfo implements Special{

public void accessSpecial() {//...

}public void accessGeneral() {}//...

}class PrivateInfo implements Private, Special {

public void accessPrivate() {// ...}public void accessSpecial() {//...}public void accessGeneral() {// ...}

//...}

class Connection {//...

if (user is unauthorized) throw newException();

if (user is general) return new GeneralInfo();


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if (user is special) return new SpecialInfo();if (user is executive) return new

PrivateInfo();//...

}

The above code example illustrates that the whole system is not

exposed to the clients. It depends on the user classification.

Mr. SudHakar Chavali proposes a better design, similar to the

above, but avoids repeated code. Look at code below.interface General {

public void accessGeneral();}

interface Special extends General {

public void accessSpecial();

}

interface Private extends General {

public void accessPrivate();

}

class GeneralInfo implements General {

public void accessGeneral() {

//...}

//...

}


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class SpecialInfo extends GeneralInfo implementsSpecial{

public void accessSpecial() {

//...}

}

class PrivateInfo extends SpecialInfo implementsPrivate {

public void accessPrivate() {

// ...

}

//...}

To avoid repeated code, SpecialInfo become subclass of

GeneralInfo and PrivateInfo becomes subclass of SpecialInfo.

When a person is exposed to special information, that person is

allowed to access general information also. When a person is

exposed to private information, that person is allowed to accessgeneral information and special information also.

When you design a mortgage process system, you may consider the

process of checking client's bank, credit and other loan information.

Facade design may be a choice.


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f.

Flyweight

Definition

Make instances of classes on the fly to improve performance

efficiently, like individual characters or icons on the screen.


Need to instantiate a large amount of small and fine-grained

classes.

Need icons to represent object.

An object extrinsic state can be shared by classes.

Reduce the number of objects created, decrease memory

footprint and increase performance.

Increase runtime cost associated with transferring, finding, or

computing extrinsic data. Related patterns include

o Composite, which supports recursive structures, whereas

an flyweight is often applied on it.

o Factory Method, which produces specific object upon

requirement, whereas an flyweight uses it to reduce

objects.

o State, which allows an object to alter its behavior when its

internal state is changed, whereas a flyweight is best

implemented on it.

o Strategy, which allows an algorithm vary independently to

suit its needs, whereas a flyweight is based on such

strategy.

Examples


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In order to share an object, we may declare an interface and an

intrinsic state through which flyweights can receive and act on it. If

you want to show a file system with folders to show the directories

or subdirectories, you don't need to load all the files or directoriesat one loading time. You may show the upper level folders first. If

the user clicks a folder, then load its subdirectories and files. The

shared trigger is mouse-clicked. The composite pattern may be

combined to define the flyweight system.

class Folder {void draw(..) {}

}class FolderFactory {

...if (selected) {

return aFolder;else

return aFile;...

}

...

To show how to use flyweight to reduce object creation, we will

make a program to draw 1000 circles with 6 different colors.

Before we customize it to a flyweight design, it is coded as follows:


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import java.awt.*;import java.awt.Color;import java.awt.event.*;import javax.swing.*;import javax.swing.event.*;

class Test extends JFrame{private static final Color colors[] = {

Color.red, Color.blue,

Color.yellow, Color.orange,

Color.black, Color.white };private static final int WIDTH_ = 400,

HEIGHT = 400,NUMBER_OF_CIRCLES =

1000;

public Test() {Container contentPane = getContentPane();

JButton button = new JButton("Draw Circle");final JPanel panel = new JPanel();

contentPane.add(panel,BorderLayout.CENTER);

contentPane.add(button, BorderLayout.SOUTH);

setSize(WIDTH,HEIGHT);

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);setVisible(true);

button.addActionListener(newActionListener() {

public void actionPerformed(ActionEvent

event) {


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Graphics g = panel.getGraphics();

for(int i=0; i < NUMBER_OF_CIRCLES;++i) {

g.setColor(getRandomColor());int r = getRandomR();g.drawOval(getRandomX(),

getRandomY(), r, r); }

}});

}

private int getRandomX() {return (int)(Math.random()*WIDTH );

}private int getRandomY() {

return (int)(Math.random()*HEIGHT);}private int getRandomR() {

return (int)(Math.random()*(HEIGHT/10));}private Color getRandomColor() {

returncolors[(int)(Math.random()*colors.length)];

}public static void main(String[] args) {

Test test = new Test();}}

Copy, paste above code, and run it to see the functionality. and

Prompt

C:\> java Test


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Note that the above program doesn't take advantage of reusability

of OOP. We will customize it and make a Circle object, so the

Circle object can be reused.

class Circle {private Color color;

public Circle(Color color) {this.color = color;

}public void draw(Graphics g, int x, int y, int

r) { g.setColor(color);g.drawOval(x, y, r, r);

}}

Then we rewrite the program. It is possible for people to rewrite

with Circle object in the following way:

import java.awt.*;import java.awt.Color;import java.awt.event.*;import javax.swing.*;import javax.swing.event.*;

class Test extends JFrame{private static final Color colors[] = {

Color.red, Color.blue,


Color.black, Color.white };private static final int WIDTH = 400,

HEIGHT = 400,NUMBER_OF_CIRCLES =

1000;


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public Test() {

Container contentPane = getContentPane();



contentPane.add(button, BorderLayout.SOUTH);setSize(WIDTH ,HEIGHT);



public void actionPerformed(ActionEventevent) {

Graphics g = panel.getGraphics();

for(int i=0; i < NUMBER_OF_CIRCLES; ++i){

Circle circle = newCircle(getRandomColor());

circle.draw(g, getRandomX(), getRandomY(),getRandomR());//1000 object created.

}}

});}

private int getRandomX() {return (int)(Math.random()*WIDTH );

}

private int getRandomY() {return (int)(Math.random()*HEIGHT);


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}private int getRandomR() {

return (int)(Math.random()*(HEIGHT/10));}private Color getRandomColor() {



Test test = new Test();}

}

From the above code, you may note that 1000 circle object has

been created. It is memory consuming.

To improve it, we will create a CircleFactory class to customize it

by using flyweight design pattern. Since we just draw circle with

different colors, we can store color info in a hashmap. If a circle

has been drawn, the new circle will be checked with color. If thecircle with the same color has been found in the hashmap, the circle

will share the instance which is picked up from the hashmap

instead of creating a new one. We will reuse the object with

different state, that is to say we will share the instance and draw the

circle with different start position and radius on the fly.

class CircleFactory {

//store colorprivate static final HashMap circleByColor =

new HashMap();

public static Circle getCircle(Color color) {Circle circle =

(Circle)circleByColor.get(color);

if(circle == null) {


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circle = new Circle(color);circleByColor.put(color, circle);System.out.println("Creating " + color +

" circle");//see how many objects we create oncommand line

}return circle;

}}

So our test program will be coded as follows:

import java.awt.*;import java.util.HashMap;import java.awt.Color;import java.awt.event.*;import javax.swing.*;import javax.swing.event.*;

class Test extends JFrame{

private static final Color colors[] = {Color.red, Color.blue,


Color.black, Color.white };private static final int WIDTH = 400,

HEIGHT = 400,

NUMBER_OF_CIRCLES =1000;

public Test() {Container contentPane = getContentPane();



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contentPane.add(button, BorderLayout.SOUTH);setSize(WIDTH,HEIGHT);



public void actionPerformed(ActionEventevent) {

Graphics g = panel.getGraphics();for(int i=0; i < NUMBER_OF_CIRCLES;

++i) {Circle circle =

CircleFactory.getCircle(getRandomColor());circle.draw(g, getRandomX(),

getRandomY(),getRandomR()); //Since we have 6 different colors,

we have 6 objects created.}

}});


Test test = new Test();

}private int getRandomX() {return (int)(Math.random()*WIDTH );

}private int getRandomY() {

return (int)(Math.random()*HEIGHT);}private int getRandomR() {

return (int)(Math.random()*(HEIGHT/10));


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}private Color getRandomColor() {


}}

class CircleFactory {private static final HashMap circleByColor =

new HashMap();

public static Circle getCircle(Color color) {Circle circle =

(Circle)circleByColor.get(color);

if(circle == null) {circle = new Circle(color);circleByColor.put(color, circle);System.out.println("Creating " + color +

" circle");}return circle;

}}

class Circle {private Color color;

public Circle(Color color) {this.color = color;

}public void draw(Graphics g, int x, int y, int

r) {g.setColor(color);g.drawOval(x, y, r, r);

}


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}

Copy, paste above code and run it. You will see the printout from

the command line, that you only have 6 objects created, not 1000

objects because you only have 6 colors. Such a big reduction of

object creation will improve your program performance

dramatically.

C:\ Command Prompt

C:\> java TestCreatingjava.awt.Color[r=255,g=0,b=0]circleCreatingjava.awt.Color[r=0,g=0,b=0] circleCreatingjava.awt.Color[r=255,g=200,b=0]

circleCreatingjava.awt.Color[r=255,g=255,b=0]circleCreatingjava.awt.Color[r=0,g=0,b=255]circleCreating

java.awt.Color[r=255,g=255,b=255]circle

Flyweight design is effective with instantiating a large amount of

small and fine-grained classes by combining with factory design

pattern.


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If you have jdk1.5 installed, you may need to use a tool to check if

you save the memory by running your commands as follows:

C:\ Command Prompt

C:\> java -Dcom.sun.management.jmxremote Test

And open another console as follows:

C:\ Command Prompt

C:\> jconsole

The jconsole tool will hook up your program Test to give your

statistic numbers about your program, such as threads, heap,

memory, VM, etc.

String class is designed with Flyweight design pattern. It has

similar structure as above example. When you create a string

constant, such constant is stored in a pool. When the second string

is created, it will be checked to see if it has been created. If it is

true, the second string instance will be picked up from the string

pool instead of creating a new one. This is why the following code

makes sense, but bothers many people.String s1 = "hello";String s2 = "hello"; //store in a string pool.String s3 = new String("hello");

System.out.println(s1==s2); //true, share the samememmory addressSystem.out.println(s1==s3); //false


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g.

Proxy

Definition

Use a simple object to represent a complex one or provide a

placeholder for another object to control access to it.


If creating an object is too expensive in time or memory. Postpone the creation until you need the actual object.

Load a large image (time consuming).

Load a remote object over network during peak periods.

Access right is required to a complex system.


o Adapter pattern, which provides a different interface to

the object it adapts, whereas a proxy provides the sameinterface as its subject, and

o Decorator pattern, which focuses on adding new functions

to an object, whereas a proxy controls access to the object.

Example

When loading a large image, you may create some light object to

represent it until the image is loaded completely. Usually a proxy

object has the same methods as the object it represents. Once the

object is loaded, it passes on the actual object. For example,

abstract class Graphic {public abstract void load();public abstract void draw();

...


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}class Image extends Graphic{

public void load() {...

}public void draw() {...}...

}class ImgProxy extends Graphic {

public void load() {if(image == null) {

image = new Image(filename);}...

public void draw() {...

}...

}

Behavioral Patterns

a.

Chain of Responsibility

Definition

Let more than one object handle a request without their knowing

each other. Pass the request to chained objects until it has been

handled.


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One request should be handled by more than one object.

Don't know which object should handle a request, probably

more than one object will handle it automatically.

Reduce coupling.

Flexible in handling a request.


o Composite, which a chain of responsibility pattern is often

applied in conjunction with.

Example

The Java Servlet filter framework is an example of chain of

resposibility design. Note that the chain.doFilter() is the method

that should be called to make the chain roll. If the subclass missed

it, the whole chain would be stopped or blocked.

Java exception handling is another example of chain ofresponsibility design. When an error occurs, the exception call will

look for a handling class. If there is no handler, the super Exception

class will be called to throw the exception. Otherwise, the handler

class will handle it.

Here comes a simple example, just to show how chain of

responsibility works. Whenever you spend company's money, you

need get approval from your boss, or your boss's boss. Let's say, the

leadership chain is:

Manager-->Director-->Vice President-->President


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The following is a command line program to check who is

responsible to approve your expenditure.

import java.io.*;abstract class PurchasePower {

protected final double base = 500;protected PurchasePower successor;


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public void setSuccessor(PurchasePower successor){this.successor = successor;

}

abstract public voidprocessRequest(PurchaseRequest request);}

class ManagerPPower extends PurchasePower {private final double ALLOWABLE = 10 * base;

public void processRequest(PurchaseRequestrequest ) {

if( request.getAmount() < ALLOWABLE )System.out.println("Manager will

approve $"+ request.getAmount());else

if( successor != null)successor.processRequest(request);

}}

class DirectorPPower extends PurchasePower {private final double ALLOWABLE = 20 * base;

public void processRequest(PurchaseRequestrequest ) {

if( request.getAmount() < ALLOWABLE )System.out.println("Director willapprove $"+ request.getAmount());

elseif( successor != null)

successor.processRequest(request);}

}


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class VicePresidentPPower extends PurchasePower {private final double ALLOWABLE = 40 * base;

public void processRequest(PurchaseRequestrequest) {

if( request.getAmount() < ALLOWABLE )System.out.println("Vice President

will approve $" + request.getAmount());elseif( successor != null )

successor.processRequest(request);}

}

class PresidentPPower extends PurchasePower {private final double ALLOWABLE = 60 * base;

public void processRequest(PurchaseRequestrequest){

if( request.getAmount() < ALLOWABLE )System.out.println("President will

approve $" + request.getAmount());else

System.out.println( "Your request for$" + request.getAmount() + " needs a boardmeeting!");

}

}

class PurchaseRequest {

private int number;private double amount;private String purpose;


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public PurchaseRequest(int number, doubleamount, String purpose){

this.number = number;this.amount = amount;this.purpose = purpose;

}

public double getAmount() {return amount;

}public void setAmount(double amt){

amount = amt;}

public String getPurpose() {return purpose;

}public void setPurpose(String reason) {

purpose = reason;}

public int getNumber(){return number;

}public void setNumber(int num) {

number = num;}

}

class CheckAuthority {public static void main(String[] args){

ManagerPPower manager = newManagerPPower();

DirectorPPower director = newDirectorPPower();


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VicePresidentPPower vp = newVicePresidentPPower();

PresidentPPower president = newPresidentPPower();

manager.setSuccessor(director);director.setSuccessor(vp);vp.setSuccessor(president);

//enter ctrl+c to kill.try{

while (true) {System.out.println("Enter the

amount to check who should approve yourexpenditure.");

System.out.print(">");double d = Double.parseDouble(new

BufferedReader(newInputStreamReader(System.in)).readLine());

manager.processRequest(newPurchaseRequest(0, d, "General"));

}}catch(Exception e){

System.exit(1);}

}}

C:\ Command Prompt


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C:\> javac CheckAuthority.javaC:\> java CheckAuthority

Enter the amount to check whoshould approve your expenditure.>500Manager will approve $500.0Enter the amount to check whoshould approve your expenditure.>5000

Director will approve $5000.0Enter the amount to check whoshould approve your expenditure.>11000Vice President will approve$11000.0Enter the amount to check whoshould approve your expenditure.

>30000Your request for $30000.0 needs aboard meeting!Enter the amount to check whoshould approve your expenditure.>20000President will approve $20000.0Enter the amount to check who

should approve your expenditure.>C:\>

You may redo it using interface instead of abstract class.

The composite pattern is often used with chain of responsibility.

That means a class may contain the related class that may handle

the request.


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b. Command

Definition

Streamlize objects by providing an interface to encapsulate a

request and make the interface implemented by subclasses in order

to parameterize the clients.


One action can be represented in many ways, like drop-down

menu, buttons and popup menu.

Need a callback function, i.e., register it somewhere to be

called later.

Specify and execute the request at different time Need to undo an action by storing its states for later retrieving.

Decouple the object with its trigger

Easily to be extensible by not touching the old structure.


o Composite, which aggregates an object. You may

combine it into a composite command pattern. In general,

a composite command is an instance of the composite.o Memento, which keeps state of an object. Command

supports undo and redo.

Example

The simple example of Command pattern is to design a Command

interface and with an execute method like this:


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public interface Command {public void execute();

}

Then, design multiple implementation classes and see how powerful the execute() method has been called dynamically.

In order to take advantage of Java built-in interfaces, we will design

a window with a drop down menu, button commands and popup

menu with command pattern.

As we know, JButton, JMenuItem and JPopupMenu have

constructors accept Action type variable. Action interface extends

ActionListener, which has the following hierarchy.

public interface EventLister {...

}

public interface ActionListener extendsEventListener {

...}

public interface Action extends ActionListener {...

}

There is an abstract class called AbstractAction which implementsAction interface. It has the following design.

public abstract class AbstractAction extendsObject

implements Action, Cloneable,Serializable


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We will create several command classes to subclass the

AbstractAction class and pass them to the constructors of JButton,

JMenuItem and JPopupMenu classes. There is a request method

called actionPerformed(), every command classes must implementit in order to make it work. To show the concept, we just design

two actions: submit and exit. You may expand such design to your

need in your future project.

Such action can be attached to any component, AWT or Swing.

The caption, and Icon have been designed as well as tooltips.

class ExitAction extends AbstractAction {private Component target;public ExitAction(String name, Icon icon,

Component t){putValue(Action.NAME, name);putValue(Action.SMALL_ICON, icon);putValue(Action.SHORT_DESCRIPTION, name + "

the program");

target = t;} public void actionPerformed(ActionEvent evt) {

int answer =JOptionPane.showConfirmDialog(target, "Are yousure you want to exit? ", "Confirmation",

JOptionPane.YES_NO_OPTION);

if ( answer == JOptionPane.YES_OPTION) {System.exit(0);

}}

}

Similar to the above exit action, the submit action is as follows:

class SubmitAction extends AbstractAction {


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private Component target;public SubmitAction(String name, Icon icon,


the program");target = t;

} public void actionPerformed(ActionEvent evt) {

JOptionPane.showMessageDialog(target,"submit action clicked ");

}}

You can modify the program to add more commands in. These

command classes are decoupled from any program. It is very good

for maintenance.

The whole workable program is as follows. You can run it to see

the powerful command design pattern.

import javax.swing.*;import java.awt.event.*;import java.awt.*;

class ExitAction extends AbstractAction {private Component target;public ExitAction(String name, Icon icon,



}


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public void actionPerformed(ActionEvent evt) {int answer =

JOptionPane.showConfirmDialog(target, "Are yousure you want to exit? ", "Confirmation",

JOptionPane.YES_NO_OPTION);if ( answer == JOptionPane.YES_OPTION) {

System.exit(0);}

}}


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class SubmitAction extends AbstractAction {

private Component target;public SubmitAction(String name, Icon icon,



}public void actionPerformed(ActionEvent evt) {

JOptionPane.showMessageDialog(target,"submit action clicked ");

}}

class Test extends JFrame{Test() {

Action ea = new ExitAction("Exit", null,

this);

Action sa = new SubmitAction("Submit",

null, this);

JMenuBar jbr = new JMenuBar();JMenu dropmenu= new JMenu("File");JMenuItem submitmenu = new JMenuItem(sa);

JMenuItem exitmenu = new JMenuItem(ea);dropmenu.add(submitmenu);

dropmenu.add(exitmenu);jbr.add(dropmenu);setJMenuBar(jbr);

final JPopupMenu pop = newJPopupMenu("PopMenu");

pop.add(sa);


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pop.add(ea);addMouseListener(new MouseAdapter() {

public void mousePressed(MouseEvent e){

showPopup(e);}

public void mouseReleased(MouseEvent e){

showPopup(e);}

private void showPopup(MouseEvent e) {if (e.isPopupTrigger()) {

pop.show(e.getComponent(),e.getX(),

e.getY());}

}

});JPanel jp = new JPanel();JButton subbtn = new JButton(sa);JButton exitbtn = new JButton(ea);jp.add(subbtn);jp.add(exitbtn);

Container con = getContentPane();con.add(jp, "South");

setTitle("Command pattern example");setSize(400,200);setVisible(true);

}

public static void main(String[] args) {


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new Test();}

}java Test

A windows pops up.

Pay attention to the action buttons. The instances can be

parameterized to JButton, JMenuItem and JPopupMenu

constructors. The powerful action design (Java built-in Action

interface) makes objects like ExitAction, SubmitAction be used

everywhere. Design once, use everywhere.

c.

Interpreter

DefinitionProvides a definition of a macro language or syntax and parsing

into objects in a program.


Need your own parser generator.

Translate a specific expression. Handle a tree-related information.


o Composite, which is an instance in an interpreter.

o Flyweight, which shows how to share symbols with

abstract context.

o Iterator , which is used to traverse the tree structure.


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o Visitor , which is used to maintain behavior of each note in

tree structure.

ExampleGiven any string expression and a token, filter out the information

you want. The below is a simple parser program. the myParser

method can be used to parse any expression. The composite, visit

and iterator patterns have been used.

import java.util.*;

class Parser{private String expression;private String token;private List result;private String interpreted;

public Parser(String e, String t) {expression = e;

token = t;}


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public void myParser() {StringTokenizer holder = new

StringTokenizer(expression, token);String[] toBeMatched = new

String[holder.countTokens()];int idx = 0;while(holder.hasMoreTokens()) {

String item = holder.nextToken();int start = item.indexOf(",");if(start==0) {

item = item.substring(2);}

toBeMatched[idx] = item;idx ++;}result = Arrays.asList(toBeMatched);

}public List getParseResult() {

return result;}public void interpret() {

StringBuffer buffer = new StringBuffer();ListIterator list =

result.listIterator();while (list.hasNext()){

String token = (String)list.next();if (token.equals("SFO")){

token = "San Francisco";}else if(token.equals("CA")) {token = "Canada";

}//...buffer.append(" " + token);

}interpreted = buffer.toString();

}


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public String getInterpretedResult() {return interpreted;


String source ="dest='SFO',origin='CA',day='MON'";

String delimiter = "=,'";Parser parser = new Parser(source,

delimiter);parser.myParser();parser.interpret();String result =

parser.getInterpretedResult();System.out.println(result);

}}java Parserdest San Francisco origin Canada day MON


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d.

Iterator

DefinitionProvide a way to move through a list of collection or aggregated

objects without knowing its internal representations.


Use a standard interface to represent data objects.

Use s standard iterator built in each standard collection, likeList, Sort, or Map.

Need to distinguish variations in the traversal of an aggregate.

Similar to Enumeration class, but more effective.

Need to filter out some info from an aggregated collection.


o Composite, which supports recursive structures, whereas

an iterator is often applied on it.o Factory Method, which provides appropriate instances

needed by an iterator subclasses.

o Memento, which is often used with an Iterator. The

iterator stores the memento internally.

Example

Employee is an interface, Manager, PieceWorker, HourlyWorker

and CommissionWorker are implementation classes of interface

Employee. EmployeeTest class will create a list and use a built-in

iterator of ArrayList class to traverse the members of the list.

import java.util.*;interface Employee {

public abstract double earnings();


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}class Manager implements Employee {

private double weeklySalary;private String name;public Manager(String name, double s) {

this.name = name;setWeeklySalary(s);

}

void setWeeklySalary(double s) {if (s > 0) {weeklySalary = s;

} elseweeklySalary = 0;

}public double earnings() {

return weeklySalary;}public String getName() {

return name;}public String toString() {

return "Manager: " + getName();}

}

class PieceWorker implements Employee {

private double wagePerPiece;private int quantity;private String name;public PieceWorker(String name, double w, int

q) {this.name = name;setWagePerPiece(w);setQuantity(q);

}


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void setWagePerPiece(double w) {

if (w > 0)wagePerPiece = w;

elsewagePerPiece = 0;

}

void setQuantity(int q) {if ( q > 0)

quantity = q;else

quantity = 0;}public String getName() {

return name;}public double earnings() {

return quantity * wagePerPiece;}

public String toString() {return "Piece worker: " + getName();

}}

class HourlyWorker implements Employee {

private double hourlyWage;private double hours;private String name;public HourlyWorker(String name, double w,

double h) {this.name = name;setHourlyWage(w);setHours(h);

}


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void setHourlyWage(double w) {

if (w > 0)hourlyWage = w;

elsehourlyWage = 0;

}

void setHours(double h) {if ( 0


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setTotalSales(totalSales);}void setSalary(double s) {

if( s > 0)salary = s;

elsesalary = 0;

}void setCommission(double c) {

if ( c > 0)commission = c;

elsecommission = 0;

}void setTotalSales(double ts) {

if (ts > 0 )totalSales = ts;

elsetotalSales = 0;

}public String getName() {

return name;}public double earnings() {

return salary + commission/100*totalSales;}public String toString() {

return "Commission worker:"+ getName();}

}

class EmployeeTest {public static void main(String[] args) {

java.util.List list = new ArrayList();

list.add(new Manager("Bill", 800.00));


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list.add(new CommissionWorker("Newt",400.0, 3.75, 159.99));

list.add(new PieceWorker("Al", 2.5, 200));list.add(new HourlyWorker("Babara", 13.75,

40));list.add(new Manager("Peter", 1200.00));list.add(new CommissionWorker("Margret",

600.0,5.5, 200.25));list.add(new PieceWorker("Mark", 4.5,

333));list.add(new HourlyWorker("William",

31.25, 50));

System.out.println("Use built-initerator:");

Iterator iterator = list.iterator();while(iterator.hasNext()) {

Employee em =(Employee)iterator.next();

System.out.print(em + " earns $");System.out.println(em.earnings());

}}

}%java EmployeeTestUse built-in iterator:Manager: Bill earns $800.0

Commission worker:Newt earns $405.999625Piece worker: Al earns $500.0Hourly worker: Babara earns $550.0Manager: Peter earns $1200.0Commission worker:Margret earns $611.01375Piece worker: Mark earns $1498.5Hourly worker: William earns $1562.5


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The above example also shows a dynamic binding feature which is

popular in Object-Oriented realm.

If you want to pick up a specific object from the aggregated list,

you may use the following code.

while(iterator.hasNext()) {Employee em = (Employee)iterator.next();if (em instanceof Manager) {

System.out.print(em + " earns $");System.out.println(em.earnings());

}}

The above list can also be replaced by an array and achieve the

same result.

e.

Mediator

Definition

Define an object that encapsulates details and other objects interact

with such object. The relationships are loosely decoupled.


Partition a system into pieces or small objects.

Centralize control to manipulate participating objects(a.k.a

colleagues)

Clarify the complex relationship by providing a board

committee.

Limit subclasses.


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Improve objects reusabilities.

Simplify object protocols.

The relationship between the control class and other

partic

design pattern qa

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