Algorithm Programming Behavioral Design Patterns Bar-Ilan University תשס " ו by Moshe Fresko

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.

Chain of Responsibility Intent Avoid coupling the sender of a request to its receiver by giving more then one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. Motivation A context sensitive Help Facility. To organize help information from most specific to most general The object that provides the help isn’t known explicitly to the help initiator.

Chain of Responsibility (Example Structure)

Chain of Responsibility - Example

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.

Chain of Responsibility General Structure

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.

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.

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

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

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’ | … } *

Interpreter – Example Structure

Interpreter – Possible Structure

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

Interpreter – General Structure

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

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

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’…

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) ; } }

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)) ; } }

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()) ; } }

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 ; } }