The document discusses various design patterns including the Gang of Four patterns. It provides examples of inheritance, composition, polymorphism, and several creational patterns (singleton, factory method, abstract factory, prototype, builder) and structural patterns (proxy, flyweight, bridge, facade, decorator, adapter). Each pattern is defined and an example is given in Java code to illustrate how it can be implemented.

1. 1
DESIGN PATTERNS
BY
RENAUD HOYOUX

2. 2 . 1
ABOUT ME
Architecton Cytomine.

3. 2 . 2
Benevolent Dictator of the non-pro t organization Kodo
Wallonie.
ABOUT ME

4. 2 . 3
Coach at CoderDojo Liège.
ABOUT ME

5. 3
DESIGN PATTERNS
In software engineering, a design pattern is a general
repeatable solution to a commonly occurring problem in
software design. A design pattern isn't a nished design that
can be transformed directly into code. It is a description or
template for how to solve a problem that can be used in many
different situations.

6. 4 . 1
OBJECT ORIENTED PROGRAMMING
Inheritance
Composition
Polymorphism

7. 4 . 2
INHERITANCE
class Sandwich {
private int weight;
public void eat() { System.out.println("Miam"); }
public void buy() { System.out.println("it costs X euros"); }
}
class GASandwich extends Sandwich {
public void share() {
System.out.println("I always share when it's not mine");
}
@Override
public void buy() {
System.out.println("Free as in beer not as in speech");
}
}
GASandwich tuna = new GASandwich();
tuna.buy();
tuna.share();
//I can call eat because I inherit from the Sandwich class
tuna.eat();

8. 4 . 3
has-a relationship in classes.
COMPOSITION
public class GAEvent {
private List<GASandwich>;
// ...
}
Every (good) GAEvent has GASandwiches.

9. 4 . 4
POLYMORPHISM
Sandwich ham = new GASandwich();
ham.eat();
ham.buy();
// but I can't call ham.share();
public void doTheRightThing(Sandwich food){
food.buy();
food.eat();
}
Sandwich ham = new GASandwich();
// Yep ! It's work
doTheRightThing(ham);

10. 5
GANG OF FOUR
Creationnal patterns :
Singleton, Factory Method, Abstract Factory, Prototype,
Builder.
Structural patterns :
Proxy, Flyweight, Bridge, Facade, Decorator, Adapter,
Composite.
Behavioral patterns :
Observer, Template Method, Command, Iterator, State,
Mediator, Strategy, Chain of Responsibility, Visitor,
Interpreter, Memento.

11. 6
GANG OF FOUR
Creationnal patterns :
Singleton, Factory Method, Abstract Factory, Prototype,
Builder.

12. 7 . 1
Ensure a class only has one instance, and provide a global
point of access to it.
SINGLETON

13. public class Singleton {
private static Singleton singleton;
//prevent the use of the new keyword
private Singleton() {}
public static synchronized Singleton getSingleton(){
if(singleton == null){
singleton = new Singleton();
}
return singleton;
}
}

14. 7 . 2
7 . 3
Java Optimisation : Bill Pugh Singleton Implementation
public class BillPughSingleton {
private BillPughSingleton(){}
private static class SingletonHelper{
private static final BillPughSingleton INSTANCE
= new BillPughSingleton();
}
public static BillPughSingleton getInstance(){
return SingletonHelper.INSTANCE;
}
}

15. 8 . 1
De ne an interface for creating an object but let subclasses
decide which class to instanciate. The factory method lets a
class defer instantiation to subclasses.
FACTORY METHOD

16. 8 . 2
Basically, we create object without exposing the creation
logic and refer to newly created object using a common
interface.

17. interface DatabaseConnector {
void openConnection();
Result executeQuery(String query);
void closeConnection();
}
public class MySQLConnector implements DatabaseConnector {
@Override
public void openConnection() { /* ... */ }
@Override
public Result executeQuery(String query) { /* ... */ }
@Override
public void closeConnection() { /* ... */ }
//...
}
public class PostgreSQLConnector implements DatabaseConnector {
//...
}
public class MongoDBConnector implements DatabaseConnector {
//...
}

18. 8 . 3
public class DatabaseConnectorFactory {
public DatabaseConnector getDatabaseConnector(String database){
if(database.equals(DB.MONGO)){
return new MongoDBConnector();
} else if(database.equals(DB.POSTGRES)){
return new PostgreSQLConnector();
} else if(database.equals(DB.MYSQL)){
return new MySQLConnector();
}
}
}

19. 8 . 4
String database = CONFIG.database;
DatabaseConnector connector = factory.getDatabaseConnector(database);
connector.openConnection();
Result result = connector.executeQuery(
"SELECT * sandwiches WHERE NOT rotten");
//...
connector.closeConnection();

20. 8 . 5
9 . 1
Provides an interface for creating families of related or
dependent objects without specifying their concrete classes.
This pattern provides an encapsulation mechanism to a group
of individual factories with a common theme.
ABSTRACT FACTORY

21. interface DatabaseConnector {
void openConnection();
Result executeQuery();
void closeConnection();
}
interface SQLConnector extends DatabaseConnector {}
interface DocumentOrientedConnector extends DatabaseConnector {}

22. 9 . 2
public abstract class DatabaseConnectorFactory {
String dbHost;
int dbPort;
public abstract DatabaseConnector getDatabaseConnector(
String database);
}

23. 9 . 3
public class DocumentOrientedConnectorFactory
extends DatabaseConnectorFactory {
@Override
public DocumentOrientedConnector getDatabaseConnector(
String database){
if(database.equals(DB.MONGO)){
return new MongoDBConnector();
} else if(database.equals(DB.COUCHDB)){
return new CouchDBConnector();
}
}
}
public class SQLConnectorFactory extends DatabaseConnectorFactory {
@Override
public SQLConnector getDatabaseConnector(String database){
if(database.equals(DB.POSTGRES)){
return new PostgreSQLConnector();
} else if(database.equals(DB.MYSQL)){
return new MySQLConnector();
}
}
}

24. 9 . 4
DatabaseConnectorFactory factory;
if(sql) {
factory = new SQLConnectorFactory();
} else {
factory = new DocumentOrientedConnectorFactory();
}
DatabaseConnector connector = factory.getDatabaseConnector(database);
connector.openConnection();
//...

25. 9 . 5
10 . 1
Specify the kinds of objects to create using a prototypical
instance, and create new objects by copying this prototype.
PROTOTYPE

26. public class NPC implements Cloneable {
//skin, etc.
//complex constructor
//NB : deep copy is better
public Object clone() {
Object clone = null;
try {
clone = super.clone();
} catch (CloneNotSupportedException e) {
e.printStackTrace();
}
return clone;
}
}

27. 10 . 2
public class NPCCache {
private static Hashtable<String, NPC> npcMap = new Hashtable<>();
public static NPC getNPC(String id) {
NPC cachedNPC = npcMap.get(id);
return (NPC) cachedNPC.clone();
}
// Or you can instanciate Objects only when you need them
// for the first time.
public static void loadCache() {
NPC npc = new NPC(/* ... */);
npcMap.put("walker",npc);
//...
}
}

28. 10 . 3
NPCCache.loadCache();
NPC clonedNPC = NPCCache.getNPC("walker");
// change only skin, x, y & dialogs

29. 10 . 4
11 . 1
Separate the construction of a complex object from its
representation so that the same construction processes can
create different representations.
BUILDER

30. public class Image {
private byte[] bytes;
private ArrayList<MetaData> metadatas;
// setters & getters
}

31. 11 . 2
public abstract class ImageBuilder {
private byte[] bytes;
private ArrayList<MetaData> metadatas;
// setters & getters
public abstract Image createImage();
}

32. 11 . 3
public class JpegBuilder extends ImageBuilder {
public void setCompressionRatio(){
//...
}
@Override
public Image createImage(){
//set metadatas & bytes with the good compression & specifications
}
}
public class Jpeg2000Builder extends ImageBuilder {
//...
}
public class PngBuilder extends ImageBuilder {
//...
}

33. 11 . 4
12
GANG OF FOUR
Structural patterns :
Proxy, Flyweight, Bridge, Facade, Decorator, Adapter,
Composite.

34. 13 . 1
Provide a surrogate or placeholder for another object to
control access to it.
PROXY

35. 13 . 2
We create object having original object to interface its
functionality to outer world.
interface Image {
void display();
}

36. public class RealImage implements Image {
private String fileName;
public RealImage(String fileName){
this.fileName = fileName;
loadFromDisk(fileName);
}
@Override
public void display() {
System.out.println("Displaying " + fileName);
}
}
public class ProxyImage implements Image{
private RealImage realImage;
private String fileName;
@Override
public void display() {
if(realImage == null){
realImage = new RealImage(fileName);
}
realImage.display();
}
}

37. 13 . 3
Image image = new ProxyImage("test_10mb.jpg");
//image will be loaded from disk
image.display();
System.out.println("");
//image will not be loaded from disk
image.display();

38. 13 . 4
14 . 1
Use sharing to support large numbers of ne grained objects
ef ciently.
FLYWEIGHT
Flyweight pattern tries to reuse already existing similar kind
objects by storing them and creates new object only when no
matching object is found.

39. 14 . 2
Example : In a Role Playing Game, we have salesman Non
Playable Characters :
They don't move.
Event reactions are the same.
Only speci c caracteristics (prices, items, skin) vary.

40. public class NPC {
//skin, etc.
}
public class SalesmanNPC extends NPC { /* ... */ }

41. 14 . 3
public class NPCFactory {
private static final HashMap<String, NPC> memo = new HashMap();
public static NPC getNPC(String type) {
NPC npc = memo.get(type);
if(npc == null) {
npc = new NPC(/*...*/); // or SalesmanNPC ...
memo.put(type, npc);
}
return npc;
}
}

42. 14 . 4
SalesmanNPC npc = (SalesmanNPC) NPCFactory.getNPC("salesman");
// set X & Y, dialog, items
npc.draw();

43. 14 . 5
15 . 1
Decouple an abstraction from its implementation so that the
two can vary independently
BRIDGE
Prefer composition over inheritance.

44. 15 . 2

45. interface IColor {
void fill();
}
public class Red implements IColor {
@Override
public void fill(){
System.out.println("fill shape with red color");
}
}
public class Green implements IColor {
@Override
public void fill(){
System.out.println("fill shape with green color");
}
}

46. 15 . 3
public abstract class Shape {
// Composition
protected IColor color;
public abstract void draw();
}
public class Triangle extends Shape {
@Override
public void draw(){
System.out.print("Draw triangle and ");
}
}
public class Square extends Shape {
@Override
public void draw(){
System.out.print("Draw square and ");
}
}

47. 15 . 4
Shape triangle = new Triangle(new Green());
triangle.draw();
Shape square = new Square(new Red());
square.draw();
Draw triangle and fill shape with green color
Draw square and fill shape with red color

48. 15 . 5
16 . 1
Provide a uni ed interface to a set of interfaces in a system.
Facade de nes a higher-level interface that makes the
subsystem easier to use. this pattern adds an interface to
existing system to hide its complexities.
FACADE

49. 16 . 2
Facade pattern hides the complexities of the system by
involving a single class which provides simpli ed methods
required by client and delegates calls to methods of existing
system classes.
This is the basic principle of an API...

50. 17 . 1
Attach additionnal responsibilities to an object dynamically.
Decorators provide a exible alternative to subclassing for
extending functionality.
DECORATOR
Instead of modifying the existing functionalities, we will
extend them.
One class takes in another class, both of which extend the
same abstract class, and adds functionality.

51. interface Shape {
void draw();
}
public class Rectangle implements Shape {
@Override
public void draw() {
System.out.println("Shape: Rectangle");
}
}
public class Square implements Rectangle {
@Override
public void draw() {
System.out.println("Shape: Square");
}
}

52. 17 . 2
public abstract class ShapeDecorator implements Shape {
protected Shape decoratedShape;
public ShapeDecorator(Shape decoratedShape){
this.decoratedShape = decoratedShape;
}
public abstract void draw();
}

53. 17 . 3
public class RedBorderShape extends ShapeDecorator {
public RedBorderShape(Shape decoratedShape) {
super(decoratedShape);
}
@Override
public void draw() {
decoratedShape.draw();
setRedBorder(decoratedShape);
}
//...
}
public class SmoothBorderShape extends ShapeDecorator {
public SmoothBorderShape(Shape decoratedShape) {
super(decoratedShape);
}
@Override
public void draw() {
decoratedShape.draw();
setSmooothBorder(decoratedShape);
}
//...
}

54. 17 . 4
//A simple square
Shape square = new Square();
square.draw();
//A red square
Shape redSquare = new RedBorderShape(square);
redSquare.draw();
//A rectangle with smooth border
Shape smoothRectangle = new SmoothBorderShape(new Rectangle());
smoothRectangle.draw();
//Even a red rectangle with smooth border
Shape redSmoothRectangle = new SmoothBorderShape(
new RedBorderShape(
new Rectangle()));
redSmoothRectangle.draw();

55. 17 . 5
17 . 6
Real example :
//First open an inputstream of it:
FileInputStream fis = new FileInputStream("/objects.gz");
//We want speed, so let's buffer it in memory:
BufferedInputStream bis = new BufferedInputStream(fis);
//The file is gzipped, so we need to ungzip it:
GzipInputStream gis = new GzipInputStream(bis);
//we can now use the read method.

56. 18 . 1
Convert the interface of a class into another interface that
client expect. The adapter pattern lets classes work together
that couldn't otherwise because of incompatible interfaces.
ADAPTER

57. interface MediaPlayer {
void play(String audioType, String fileName);
}
interface AdvancedMediaPlayer {
void play(String fileName);
}

58. 18 . 2
public class VlcPlayer implements AdvancedMediaPlayer{
@Override
public void play(String fileName) {
System.out.println("Playing vlc file. Name: "+ fileName);
}
}
public class Mp4Player implements AdvancedMediaPlayer{
@Override
public void play(String fileName) {
System.out.println("Playing mp4 file. Name: "+ fileName);
}
}

59. 18 . 3
public class MediaAdapter implements MediaPlayer {
AdvancedMediaPlayer advancedMusicPlayer;
public MediaAdapter(String audioType){
if(audioType.equalsIgnoreCase("vlc") ){
advancedMusicPlayer = new VlcPlayer();
} else if (audioType.equalsIgnoreCase("mp4")){
advancedMusicPlayer = new Mp4Player();
}
}
@Override
public void play(String audioType, String fileName) {
advancedMusicPlayer.play(fileName);
}
}

60. 18 . 4
public class AudioPlayer implements MediaPlayer {
MediaAdapter mediaAdapter;
@Override
public void play(String audioType, String fileName) {
//inbuilt support to play mp3 music files
if(audioType.equalsIgnoreCase("mp3")){
System.out.println("Playing mp3 file. Name: " + fileName);
}
//mediaAdapter is providing support to play other file formats
else if(audioType.equalsIgnoreCase("vlc")
|| audioType.equalsIgnoreCase("mp4")){
mediaAdapter = new MediaAdapter(audioType);
mediaAdapter.play(audioType, fileName);
} //else ...
}
}

61. 18 . 5
AudioPlayer audioPlayer = new AudioPlayer();
audioPlayer.play("mp3", "beyond the horizon.mp3");
audioPlayer.play("mp4", "alone.mp4");
audioPlayer.play("vlc", "far far away.vlc");
audioPlayer.play("avi", "mind me.avi");

62. 18 . 6
19 . 1
Compose objects into tree structures to represent part-
whole hierarchies. The composite pattern lets client treat
individual objects and compositions of objects uniformly.
COMPOSITE

63. 19 . 2
Example : File in Java

64. public class Employee {
private String name;
//...
private List<Employee> subordinates = new ArrayList<>();
public void add(Employee e) {
subordinates.add(e);
}
public void remove(Employee e) {
subordinates.remove(e);
}
public List<Employee> getSubordinates(){
return subordinates;
}
}

65. 19 . 3
20
GANG OF FOUR
Behavioral patterns :
Observer, Template Method, Command, Iterator, State,
Mediator, Strategy, Chain of Responsibility, Visitor,
Interpreter, Memento.

66. 21 . 1
De ne a one-to-many dependency between objects so that
when one object changes state, all its dependents are noti ed
and updated automatically.
OBSERVER

67. 21 . 2
Example : A Graphical User Interface with 3 visualizations of
a number : binary, octal and hexadecimal. Whenever the
number changes, these visualizations must change too.

68. interface IObserver<T> {
void update(T i);
}
interface IObservable<T> {
void attach(IObserver observer);
//void remove(IObserver observer);
//void clean();
//...
public void notifyAllObservers(T state);
}

69. 21 . 3
class Subject implements IObservable<Integer> {
private List<IObserver> observers = new ArrayList<IObserver>();
private int state;
public void setState(int state) {
this.state = state;
notifyAllObservers(this.state);
}
public void attach(IObserver observer){
observers.add(observer);
}
public void notifyAllObservers(Integer state){
for (IObserver observer : observers) {
observer.update(state);
}

70. 21 . 4
class BinaryObserver implements IObserver<Integer>{
@Override
public void update(Integer i) {
System.out.println("Binary String: "+Integer.toBinaryString(i));
}
}
class HexaObserver implements IObserver<Integer>{
@Override
public void update(Integer i) {
System.out.println("Hex String: "+Integer.toHexString(i)
.toUpperCase());
}
}

71. 21 . 5
Subject subject = new Subject();
subject.attach(new BinaryObserver());
subject.attach(new HexaObserver());
System.out.println("First state change: 15");
subject.setState(15);
System.out.println("Second state change: 10");
subject.setState(10);
First state change: 15
Binary String: 1111
Hex String: F
Second state change: 10
Binary String: 1010
Hex String: A

72. 21 . 6
21 . 7
A FUNCTIONAL INTERFACE
@FunctionalInterface
interface IObserver<T> {
void update(T i);
}
Subject subject = new Subject();
subject.attach(i ­> {
System.out.println("Binary String: "+Integer.toBinaryString(i));
});
subject.attach(i ­> {
System.out.println("Hex String: "+Integer.toHexString(i)
.toUpperCase());
});
System.out.println("First state change: 15");
subject.setState(15);
System.out.println("Second state change: 10");
subject.setState(10);

73. 22 . 1
De ne the skeleton of an algorithm in an operation, deferring
some steps to subclasses. The template method lets
subclasses rede ne certains steps of an algorithm without
changing the algorithm's structure.
TEMPLATE METHOD

74. 22 . 2
Well ... It's the purpose of an abstract class.

75. 23 . 1
Encapsulate a request as an object, thereby letting you
parameterize clients with different requests, queue or log
request, and support undoable operations.
COMMAND

76. 23 . 2
Data Driven pattern

77. class Stock {
private String name = "ABC";
private int quantity = 10;
public void buy(){
System.out.println("Stock : "+quantity+" "+name+" bought");
}
public void sell(){
System.out.println("Stock : "+quantity+" "+name+" sold");
}
}

78. 23 . 3
interface Order {
void execute();
}
abstract class StockOrder implements Order {
protected Stock stock;
abstract void execute();
}

79. 23 . 4
class BuyStock extends StockOrder {
public BuyStock(Stock stock){
this.stock = stock;
}
public void execute() {
stock.buy();
}
}
class SellStock extends StockOrder {
public SellStock(Stock stock){
this.stock = stock;
}
public void execute() {
stock.sell();
}
}

80. 23 . 5
class Broker {
private List<Order> orderList = new ArrayList<Order>();
public void takeOrder(Order order){
orderList.add(order);
}
public void placeOrders(){
for (Order order : orderList) {
order.execute();
}
orderList.clear();
}
}
Stock abcStock = new Stock();
BuyStock buyStockOrder = new BuyStock(abcStock);
SellStock sellStockOrder = new SellStock(abcStock);
Broker broker = new Broker();
broker.takeOrder(buyStockOrder);
broker.takeOrder(sellStockOrder);
broker.placeOrders();

81. 23 . 6
24 . 1
Provide a way to access the elements of an aggregate object
sequentially without exposing its underlying representation.
ITERATOR

82. The concerned classes (ex : People) return a subclasse
(PeopleIterator) which implements the Iterator interface.
interface Iterator<T> {
boolean hasNext();
T next();
//first();
// ...
}

83. 24 . 2
24 . 3
Remember : In Java,
List are Iterable<T>.
Set are Iterable<T>.
Arrays are not even if the foreach is well de ned.
ArrayList<String> list = new ArrayList<>();
String[] array = new String[1];
list.add("test1")
array[0] = "test1"
list.add("test2")
array[1] = "test2"
for(String s : list){
System.out.println(s);
}
for(String s : array){
System.out.println(s);
}

84. 25 . 1
Allow an object to alter its behaviour when its internal state
changes. The object will appear to change its class.
STATE
Example : Screen in a video game

85. public abstract class State {
public abstract void switch(TV context);
}
public class Off extends State {
@Override
public void switch(TV context){
context.setState(new On());
}
}
public class On extends State {
@Override
public void switch(TV context){
context.setState(new Off());
}
}

86. 25 . 2
public class TV {
public State state;
public void setState(State state) {
this.state = state;
}
public void pressButton(){
state.switch(this);
}
}
Off initialState = new Off();
TV tv = new TV(initialState);
//press
tv.pressButton();
//again
tv.pressButton();

87. 25 . 3
26 . 1
De ne an object that encapsulates how a set of objects
interacts. The mediator pattern promotes loose coupling by
keeping objects from referring to each other explicitly, and it
lets you vary their interaction independently.
MEDIATOR

88. public class Person {
protected Mediator mediator;
public void send(String msg){
mediator.send(this, msg);
}
public void notify(String msg){
System.out.println(msg);
}
}
public class Boss extends Person {}

89. 26 . 2
public class Mediator {
private List<Person> people;
private Boss boss;
// ...
public void send(Person from, String msg){
if(from instanceof Boss){
for(Person p : people) p.notify("Boss says : "+msg);
} else {
boss.notify(from +" : "+msg);
}
}
}

90. 26 . 3
27 . 1
De ne a family of algorithms, encapsulate each one, and
make them interchangeable. The strategy pattern lets the
algorithm vary independently from client to client.
STRATEGY

91. interface Strategy {
Path shortestPath(Graph graph, Node begin, Node end);
}
public class Dijkstra implements Strategy{
@Override
public Path shortestPath(Graph graph, Node begin, Node end) {
//...
}
}
public class AStar implements Strategy{
@Override
public Path shortestPath(Graph graph, Node begin, Node end) {
//...
}
}

92. 27 . 2
public class GraphManager {
private Graph graph;
private Strategy strategy;
public GraphManager(Strategy strategy){
this.strategy = strategy;
}
//set strategy
public int executeStrategy(Node begin, Node end){
return strategy.shortestPath(this.graph, begin, end);
}
}
GraphManager context = new GraphManager(new Dijkstra());
context.executeStrategy(begin, end);
context.setStrategy(new AStar());
context.executeStrategy(begin, end);

93. 27 . 3
28 . 1
Avoid coupling the sender of a request to its receiver by
giving more than one object a chance to handle the request.
Chain the receiving objects and pass the request along the
chain until an object handles it.
CHAIN OF RESPONSIBILITY

94. public abstract class AbstractLogger {
public static int INFO = 1;
public static int DEBUG = 2;
public static int ERROR = 3;
protected int level;
protected AbstractLogger nextLogger;
public void logMessage(int level, String message){
if(this.level <= level){
write(message);
}
if(nextLogger !=null){
nextLogger.logMessage(level, message);
}
}

95. 28 . 2
public class ConsoleLogger extends AbstractLogger {
public ConsoleLogger(int level){
this.level = level;
}
@Override
protected void write(String message) {
System.out.println("Standard Console::Logger: " + message);
}
}
public class ErrorLogger extends AbstractLogger {
public ErrorLogger(int level){
this.level = level;
}
@Override
protected void write(String message) {
System.out.println("Error Console::Logger: " + message);
}
}

96. 28 . 3
//creation
AbstractLogger loggerChain = new ErrorLogger(AbstractLogger.ERROR);
AbstractLogger fileLogger = new FileLogger(AbstractLogger.DEBUG);
AbstractLogger consoleLogger = new ConsoleLogger(AbstractLogger.INFO);
//chaining
loggerChain.setNextLogger(fileLogger);
fileLogger.setNextLogger(consoleLogger);
loggerChain.logMessage(AbstractLogger.INFO,
"This is an information.");
loggerChain.logMessage(AbstractLogger.DEBUG,
"This is an debug level information.");
loggerChain.logMessage(AbstractLogger.ERROR,
"This is an error information.");

97. 28 . 4
29 . 1
Represent an operation to be performed on the elements of
an object structure. The visitor pattern lets you de ne a new
operation without changing the classes of the elements on
which it operates.
VISITOR

98. interface Visitable{
void accept(Visitor visitor);
}
public class Book implements Visitable{
private double price;
private double weight;
public double getPrice() {
return price;
}
public double getWeight() {
return weight;
}
//accept the visitor
@Override
public void accept(Visitor vistor) {
visitor.visit(this);
}
}

99. 29 . 2
interface Visitor{
void visit(Visitable visitable);
}
public class PostageVisitor implements Visitor {
private double totalPostageForCart;
public double getTotalPostage() {
return totalPostageForCart;
}
//collect data about the book
@Override
public void visit(Visitable visitable) {
if(Visitable instanceof Book) {
//assume we have a calculation here related to weight and price
//free postage for a book over 10
if(book.getPrice() < 10.0) {
totalPostageForCart += book.getWeight() * 2;
}
}
// else ... other visitables

100. 29 . 3
public class ShoppingCart {
//normal shopping cart stuff
private ArrayList<Visitable> items;
public double calculatePostage() {
//create a visitor
PostageVisitor visitor = new PostageVisitor();
//iterate through all items
for(Visitable item: items) {
item.accept(visitor);
}
double postage = visitor.getTotalPostage();
return postage;
}
}

101. 29 . 4
30 . 1
Given a language, de ne a representation for its grammar
along with an interpreter that uses the representation to
interpret sentences in the language.
INTERPRETER
Example : SQL parsing

102. interface Expression {
boolean interpret(String context);
}
public class TerminalExpression implements Expression {
private String data;
public TerminalExpression(String data){
this.data = data;
}
@Override
public boolean interpret(String context) {
if(context.contains(data)){
return true;
}
return false;
}
}

103. 30 . 2
public class OrExpression implements Expression {
private Expression expr1 = null;
private Expression expr2 = null;
public OrExpression(Expression expr1, Expression expr2) {
this.expr1 = expr1;
this.expr2 = expr2;
}
@Override
public boolean interpret(String context) {
return expr1.interpret(context) || expr2.interpret(context);
}
}
public class AndExpression implements Expression {
private Expression expr1 = null;
private Expression expr2 = null;
public AndExpression(Expression expr1, Expression expr2) {
this.expr1 = expr1;
this.expr2 = expr2;
}
@Override
public boolean interpret(String context) {
return expr1.interpret(context) && expr2.interpret(context);
}
}

104. 30 . 3
public class InterpreterPatternDemo {
//Rule: Robert and John are male
public static Expression getMaleExpression(){
Expression robert = new TerminalExpression("Robert");
Expression john = new TerminalExpression("John");
return new OrExpression(robert, john);
}
//Rule: Julie is a married women
public static Expression getMarriedWomanExpression(){
Expression julie = new TerminalExpression("Julie");
Expression married = new TerminalExpression("Married");
return new AndExpression(julie, married);
}
public static void main(String[] args) {
Expression isMale = getMaleExpression();

105. 30 . 4
31 . 1
Without violating encapsulation, capture and externalize an
object's internal state so that the object can be restored to
this state later.
MEMENTO

106. public class Memento<T> {
private T state;
public Memento(T state){
this.state = state;
}
public T getState(){
return state;
}
}

107. 31 . 2
public class Originator {
//all internal states
// ...
public Memento<Originator> saveToMemento(){
//...
}
public void restoreFromMemento(Memento<Originator> memento){
//...
}
}
public class CareTaker {
private List<Memento<Originator>> mementoList = new ArrayList<>();
public void add(Memento<Originator> state){
mementoList.add(state);
}
public Memento<Originator> get(int index){
return mementoList.get(index);
}
}

108. 31 . 3
Originator originator = new Originator();
CareTaker careTaker = new CareTaker();
// originator changes;
careTaker.add(originator.saveToMemento());
// originator changes;
careTaker.add(originator.saveToMemento());
// originator changes;
originator.restoreFromMemento(careTaker.get(0));

109. 31 . 4
32
ARCHITECTURAL PATTERNS
Data Access Object
MVC
Multitier
Microservices
Message-driven architecture
...

110. 33
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