1. 02 - Behavioral Design Patterns – 1 Moshe Fresko Bar-Ilan University תשס"ח 2008

2. Behavioral Patterns Behavioral Patterns are concerned with algorithms and the assignment of responsibilities between objects. Not only patterns of objects/classes but also patterns of communication between them. These patterns are: Template Method: An abstract definition of an algorithm. Interpreter: Represents a grammar as a class hierarchy and implements an interpreter as an operation on instances of these classes. Mediator: Provides the indirection needed for loose coupling. Chain of Responsibility: Lets you send requests to an object implicitly through a chain of candidate objects. Observer: Defines and Maintains dependency between objects. (MVC) Strategy: Encapsulates an algorithm in an Object. Command: Encapsulates a request in an Object. State: Encapsulates the states of an Object so that the Object can change its behavior when its state object is changes. Visitor: Encapsulates behavior that would otherwise be distributed across classes. Iterator: Abstracts the way you access and traverse objects in an aggregate.

3. Iterator Moshe Fresko Bar-Ilan University תשס"ו - 2005-2006 Design Patterns Course

4. Iterator Intent Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Motivation An aggregate object such as a list should give you a way to access its elements without exposing its internal structure. Moreover you might want to traverse the list in different ways. We cannot fill the List interface with different traversals we can need. We may want a couple of traversals pending on the same time.

5. Iterator

6. Iterator – Example Structure

7. Iterator Use iterator pattern … To access an aggregate object’s contents without exposing its internal representation. To support multiple traversals of aggregate objects. To provide a uniform interface for traversing different aggregate structures (to support polymorphic iteration).

8. Iterator – General Structure

9. Iterator Participants Iterator Defines an interface for accessing and traversing elements ConcreteIterator Implements the iterator interface Keeps track of the current position in the traversal Aggregate Defines an interface method that creates an iterator object ConcreteAggregate Implements the iterator creation method, and returns an instance of the proper ConcreteIterator

10. Iterator Consequences It supports variants in the traversal of an aggregate Iterators simplify the Aggregate interface More then one traversal can be pending on an aggregate Implementation Who controls the iteration? External Iterator: Client controls the iteration Internal Iterator: The Iterator controls the iteration Who defines the traversal algorithm? The aggregate: This is called a cursor. The iterator. How robust is the iterator? Modifying an aggregate while traversing it will be dangerous for iterator. Robust iterator will not be effected by changes.

11. Java Iterators interface Collection { … Iterator iterator(); … } interface Set extends Collection { … Iterator iterator(); … } interface List extends Collection { … Iterator iterator(); ListIterator listIterator(); ListIterator listIterator(int index); … } Interface Iterator { boolean hasNext() ; Object next() ; void remove() ; } Interface ListIterator extends Iterator { boolean hasNext() ; Object next() ; boolean hasPrevious() ; Object previous() ; int nextIndex() ; int previousIndex() ; void remove() ; void set(Object o) ; void add(Object o) ; }

12. Java Iterator import java.util.*; public class IteratorExample { public static void main(String[] args) { List ints = new ArrayList(); for(int i = 0; i < 10; i++) ints.add(new Integer(i)); Iterator e = ints.iterator(); while(e.hasNext()) System.out.println( ((Integer)e.next()).intValue() ) ; }

13. Chain of Responsibility Moshe Fresko Bar-Ilan University תשס"ו - 2005-2006 Design Patterns Course

14. Chain of Responsibility Use Chain of Responsibility when … More then one object may handle a request, and the handler isn’t known a-priori. You want to issue a request to one of several objects without specifying the receiver explicitly. The set of objects that handle a request should be specified dynamically.

15. Chain of Responsibility General Structure

16. Chain of Responsibility Participants Handler (HelpHandler) Defines and interface for handling requests. Implements the successor link. ConcreteHandler (PrintButton, PrintDialog) Handles requests it is responsible for. Can access its successor. If it does not handle the request, then it forwards it to its successor. Client Initiates the request to a ConcreteHandler object on the chain.

17. Chain of Responsibility Consequences Reduced Coupling Flexibility in assigning responsibilities to Objects Receipt isn’t guaranteed. Implementation Implementing the successor chain can be done with a new implementation or use existing links. Representing Requests may be via an object.

18. Chain of Responsibility // Chain with a new implementation class HelpHandler { private HelpHandler successor = null ; HelpHandler(HelpHandler successor) { this.successor = successor ; } public void handleHelp() { if (successor!=null) { successor.handleHelp() ; } } // Any relationship (hierarchical or list) like is-a can be used for chaining

19. Interpreter Moshe Fresko Bar-Ilan University תשס"ו - 2005-2006 Design Patterns Course

20. Interpreter Intent Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. Motivation If a particular kind of problem occurs often enough, then it might be worthwhile to express instances of the problem as sentences in a simple language. For example: Regular Expressions Document Retrieval Query

21. Interpreter Regular Expression Example A simple Regular Expression Grammar expression ::= literal | alternation | sequence | repetition | ‘(’ expression ‘)’ alternation ::= expression ‘|’ expression sequence ::= expression ‘&’ expression repetition ::= expression ‘*’ literal ::= ‘a’ | ‘b’ | ‘c’ | … { ‘a’ | ‘b’ | ‘c’ | … } *

22. Interpreter – Example Structure

23. Interpreter – Possible Structure

24. Interpreter Applicability Use Interpreter pattern when there is a language to interpret, and you can represent statements in the language as abstract syntax trees. The interpreter works well, when The grammar is simple Efficiency is not a critical concern

25. Interpreter – General Structure

26. Interpreter Participants AbstractExpression (RegularExpression) Declares an abstract interpret() operation TerminalExpression (LiteralExpression) Implements the interpret() operation for terminal symbols in the grammar NonterminalExpression (AlternationExpression, RepetitionExpression, SequenceExpression) Keeps AbstractExpression for each internal symbol it keeps Implements interpret() operation. Context Contains information that is global to the interpreter Client Builds the abstract syntax tree Calls the interpret() operation

27. Interpreter Consequences It is easy to change and extend the grammar Implementing the grammar is easy Complex grammars are hard to maintain Implementation Creating the abstract syntax tree Defining the interpret() operation Sharing terminal symbols with the Flyweight pattern

28. Interpreter – Example Grammar for Document Search expression ::= literal | alternation | intersection alternation ::= expression OR expression intersection ::= expression AND expression literal ::= ‘a’|‘b’|‘c’|… literal ::= literal ‘a’|‘b’|‘c’…

29. Interpreter – Example // Interface of a document collection interface DocCollection { int[] getDocNumbersForWord(String word) ; } // Interface for searching a document colletion interface DocSearch { int[] getDocNumbers(DocCollection d) ; } // A Literal search class Literal implements DocSearch { String word ; Literal(String word) { this.word = word ; } public int[] getDocNumbers(DocCollection d) { return d.getDocNumbersForWord(this.word) ; } }

30. Interpreter – Example // An alternation search class Alternation implements DocSearch { DocSearch search1=null ; DocSearch search2=null ; Alternation(DocSearch search1, DocSearch search2) { this.search1=search1; this.search2=search2; } public int[] getDocNumbers(DocCollection d) { return Utils.union(search1.getDocNumbers(d),search2.getDocNumbers(d)) ; } } // An intersection search class Intersection implements DocSearch { DocSearch search1=null ; DocSearch search2=null ; Intersection(DocSearch search1, DocSearch search2) { this.search1=search1; this.search2=search2; } public int[] getDocNumbers(DocCollection d) { return Utils.intersection(search1.getDocNumbers(d),search2.getDocNumbers(d)) ; } }

31. Interpreter – Example // The factory for creating the interpreted DocSearch pointer class DocSearchInterpreter { public static DocSearch interpret(String query) { String[] alt = query.split(" OR ") ; DocSearch d = interpretAnd(alt[0]) ; for (int i=1;i<alt.length;++i) d = new Alternation(d,interpretAnd(alt[i])) ; return d ; } private static DocSearch interpretAnd(String query) { String[] alt = query.split(" AND ") ; DocSearch d = interpretOne(alt[0]) ; for (int i=1;i<alt.length;++i) d = new Intersection(d,interpretOne(alt[i])) ; return d ; } private static DocSearch interpretOne(String query) { return new Literal(query.trim()) ; } }

32. Interpreter – Example // Some utilities for union and // intersection of sorted integer lists class Utils { public static int[] union(int[] a, int[] b) { List l = new ArrayList() ; int i=0, j=0; while (i<a.length && j<b.length) { if (a[i]==b[j]) { l.add(new Integer(a[i])) ; i++ ; j++ ; continue ; } else if (a[i]<b[j]) { l.add(new Integer(a[i])) ; i++ ; continue ; } else { l.add(new Integer(b[j])) ; j++ ; continue ; } } for (;i<a.length;++i) l.add(new Integer(a[i])) ; for (;j<b.length;++j) l.add(new Integer(b[j])) ; return arrayFromList(l) ; } public static int[] intersection(int[] a, int[] b) { List l = new ArrayList() ; int i=0, j=0; while (i<a.length && j<b.length) { if (a[i]==b[j]) { l.add(new Integer(a[i])) ; i++ ; j++ ; continue ; } else if (a[i]<b[j]) { i++ ; continue ; } else { j++ ; continue ; } } return arrayFromList(l) ; } private static int[] arrayFromList(List l) { int[] r=new int[l.size()] ; for (int i=0;i<r.length;++i) r[i]=((Integer)l.get(i)).intValue() ; return r ; } }

33. Command Moshe Fresko Bar-Ilan University תשס"ו - 2005-2006 Design Patterns Course

34. Command Pattern Intent: Encapsulate a request as an Object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Motivation: The Command Pattern lets toolkit objects make requests of unspecified application objects by turning the request itself into an Object. This object can be stored and passed around.

35. Command - Motivation Menus can be implemented easily. Each choice in a Menu instance is an instance of MenuItem class.

36. Command – Motivation

39. Command – Applicability Use Command pattern when you want to Parametrize objects by an action to perform. (callback function or Interface) Specify, queue and execute requests at different times. Support undo. Support logging. Structure a system around high-level operations built on primitive operations.

40. Command – Structure

41. Command – Participants Command Declares an interface for executing an operation. ConcreteCommand Defines a binding between a receiver object and an action. Implements Execute by invoking the corresponding operations on Receiver. Client Creates a ConcreteCommand object and sets its receiver. Invoker Asks the Command to carry out the request. Receiver Knows how to perform the operations associated with carrying out a request.

42. Command – Interactions

43. Command – Consequences Command decouples the object that invokes the operation from the one that knows how to perform it. Commands are first-class objects. They can be manipulated and extended. You can assemble commands into a composite command. It's easy to add new Commands, because you don't have to change existing classes.

44. Command – Implementation How intelligent should a Command be? Supporting undo and redo. Avoiding error accumulation in the undo process. Using C++ templates to avoid creating a Command subclass for every kind of action and receiver.

45. Command – Sample Code class Command { public: virtual ~Command(); virtual void Execute() = 0; protected: Command(); };

46. Command – Sample Code class OpenCommand : public Command { private: Application* _application; char* _response; public: OpenCommand(Application*) { _application = a; } virtual void Execute() { const char* name = AskUser(); Document* document = new Document(name); _application->Add(document); document->Open(); } protected: virtual const char* AskUser(); };

47. Command – Sample Code class PasteCommand : public Command { private: Document* _document; public: PasteCommand(Document*) { _document = doc; } virtual void Execute() { _document->Paste(); } };

48. Command – Sample Code template class SimpleCommand : public Command { public: typedef void (Receiver::* Action)(); SimpleCommand(Receiver* r, Action a) : _receiver(r), _action(a) { } virtual void Execute() { (_receiver->*_action)(); } private: Action _action; Receiver* _receiver; };

49. Command – Sample Code MyClass* receiver = new MyClass; //... Command* aCommand = new SimpleCommand (receiver, &MyClass::Action); //... aCommand->Execute();

50. Command – Sample Code class MacroCommand : public Command { private: List * _cmds; public: MacroCommand(); virtual ~MacroCommand(); virtual void Add(Command*) { _cmds->Append(c); } virtual void Remove(Command*) { _cmds->Remove(c); } virtual void Execute() { ListIterator i(_cmds); for (i.First(); !i.IsDone(); i.Next()) { Command* c = i.CurrentItem(); c->Execute(); } } };