Module 4: UML In Action – Design Patterns 1

Module 4: UML In Action - Design Patterns 1

Overview Books Design Patterns – Basics Structural Design Patterns Behavioral Design Patterns Appendix: More on the Observer Pattern More on the Strategy Pattern 2

Books Design Patterns : Elements of Reusable Object-Oriented Software (1995) (The-Gang-of-Four Book) The-Gang-of-Four (GoF) - Gamma, Helm, Johnson , Vlissides Analysis Patterns - Reusable Object Models (1997) Martin Fowler The Design Patterns Smalltalk Companion (1998) Alpert, Brown & Woolf 3

Design Patterns “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice”. --- Christopher Alexander, 1977 This was in describing patterns in buildings and towns. In SE, design patterns are in terms of objects and interfaces, not walls and doors. The manner in which a collection of interacting objects collaborate to accomplish a specific task or provide some specific functionality. 4

Architecture vs. Design Patterns Architecture High-level framework for structuring an application “client-server based on remote procedure calls” “abstraction layering” “distributed object-oriented system based on CORBA” Defines the system in terms of computational components & their interactions Design Patterns Lower level than architectures (Sometimes, called micro-architecture) Reusable collaborations that solve subproblems within an application how can I decouple subsystem X from subsystem Y? Why Design Patterns? Design patterns support object-oriented reuse at a high level of abstraction Design patterns provide a “framework” that guides and constrains object-oriented implementation 5

4 Essential Elements of Design Patterns Name: identifies a pattern Problem: describes when to apply the pattern in terms of the problem and context Solution: describes elements that make up the design, their relationships, responsibilities, and collaborations Consequences: results and trade-offs of applying the pattern 6

How to Describe Design Patterns more fully This is critical because the information has to be conveyed to peer developers in order for them to be able to evaluate, select and utilize patterns. A format for design patterns Pattern Name and Classification Intent Also Known As Motivation Applicability Structure Participants Collaborations Consequences Implementation Sample Code Known Uses Related Patterns 7

Organizing Design Patterns By Purpose (reflects what a pattern does): Creational Patterns Structural Patterns Behavioral Patterns By Scope: specifies whether the pattern applies primarily to classes or to objects. 8

Design Patterns Space Scope Class Object Creational Purpose Structural Factory Method Adapter Abstract Factory Builder Prototype Singleton Adapter Bridge Composite Decorator Façade Flyweight Proxy Behavioral Interpreter Template Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor 9

Some Design Patterns Pattern Name Role Adapter Convert the interface of one class into another interface clients expect. Adapter allows classes to work together that otherwise can’t because of incompatible interfaces. Proxy Provide a surrogate or placeholder for another object. Mediator Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly and let one vary its interaction independently Observer Define a one-to-many dependency between objects so that when one object changes state, all its dependents will be notified and updated automatically. Template Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. 10

Structural Patterns Composite Adapter Façade Proxy 11

Structural Patterns - Composite Intent Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Composite: Applicability Represents part-whole hierarchies of objects. Clients ignore the difference between compositions of objects and individual objects. Clients treat all objects in the composite structure uniformly. 12

Structural Patterns – Composite Class Diagram Component Client operation() getChild( i:int )* Leaf operation() Composite operation() add( c:Component ) remove( c:Component ) getChild( i:int ) operation() { for all g in children g.operation() } 13

Structural Patterns - Composite Object Diagram top : Composite a : Leaf b : Leaf top : Composite d : Leaf c : Leaf e : Leaf 14

// "Component" abstract class Component {protected string name; // Constructor public Component(string name) {this.name name;} http://www.dofactory.com/Patterns/PatternComposite.aspx public abstract void Add(Component c); public abstract void Remove(Component c); public abstract void Display(int depth); using System; using System.Collections; } namespace DoFactory.GangOfFour.Composite.Structural { // "Composite" class Composite : Component {private ArrayList children new ArrayList(); // MainApp test application // Constructor public Composite(string name) : base(name) { } class MainApp { static void Main() { // Create a tree structure Composite root new Composite("root"); root.Add(new Leaf("Leaf A")); root.Add(new Leaf("Leaf B")); public override void Add(Component component) {children.Add(component);} public override void Remove(Component component) {children.Remove(component);} public override void Display(int depth) {Console.WriteLine(new String('-', depth) name); Composite comp new Composite("Composite X"); comp.Add(new Leaf("Leaf XA")); comp.Add(new Leaf("Leaf XB")); // Recursively display child nodes foreach (Component component in children) {component.Display(depth 2);} root.Add(comp); root.Add(new Leaf("Leaf C")); // Add and remove a leaf Leaf leaf new Leaf("Leaf D"); root.Add(leaf); root.Remove(leaf); // Recursively display tree root.Display(1); // Wait for user Console.Read(); } } } } -root ---Leaf A ---Leaf B ---Composite X -----Leaf XA -----Leaf XB ---Leaf C // "Leaf" class Leaf : Component {// Constructor public Leaf(string name) : base(name) { } public override void Add(Component c) {Console.WriteLine("Cannot add to a leaf");} public override void Remove(Component c) {Console.WriteLine("Cannot remove from a leaf");} public override void Display(int depth) 15 {Console.WriteLine(new String('-', depth) name);} } }

Structural Patterns – Composite Participants Component Declares the interface for objects in the composition. Implements default behavior for the interface common to all classes, as appropriate. Declares an interface for accessing and managing its child components. Optionally defines an interface for accessing a components parent. Leaf Represents leaf objects in the composition. Defines behavior for primitive objects in the composition. Composite Defines behavior for components having children. Stores child components. Implements child-related operations. Client Manipulates objects in the composition through the Component interface. 16

Structural Patterns – Composite Collaborations Clients use the Component class interface to interact with objects in the composite structure. If the recipient is a Leaf, then the request is handled directly. If the recipient is a Composite, then it usually forwards requests to its child components, possibly performing additional operations before and/or after forwarding. 17

Structural Patterns - Adapter Intent Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces. Applicability Reuse of an existing class is desired, but the interface does not match the need. Design of a reusable class that cooperates with unrelated or unforeseen classes, but classes don’t have compatible interfaces. 18

Structural Patterns - Adapter Class Diagram Client Target request() Adaptee specialOperation() Adapter adaptee.specificOperation() request() 19

Structural Patterns - Adapter Participants Target — defines the domain-specific interface that the client uses. Client — collaborates with objects conforming to the Target interface. Adaptee — defines an existing interface that needs adapting. Adapter — adapts the interface of Adaptee to the Target interface. Collaborations Clients call operations on an Adapter instance. In turn, the Adapter calls Adaptee operations that carry out the request. 20

Structural Patterns - Façade Intent Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier to use. Applicability Provides a simple interface to a complex subsystem. Decouples the details of a subsystem from clients and other subsystems. Provides a layered approach to subsystems. 21

Structural Patterns - Façade Class Diagram subsystem Facade 22

Structural Patterns - Façade Participants Façade Knows which classes are responsible for each request. Delegates client requests to appropriate objects. Subsystem classes Implement subsystem functionality. Handle work assigned by the Façade object. Have no knowledge of the façade. Collaborations Clients communicate with the subsystem sending requests to the Façade. Reduces the number of classes the client deals with. Simplifies the subsystem. Clients do not have to access subsystem objects directly. 23

Structural Patterns - Proxy Intent Provide a surrogate or placeholder for another object to control access to it. Applicability Remote proxy — provides a local representative for an object in a different address space. Virtual proxy — creates expensive objects on demand. Protection proxy — controls access to the original object. Smart reference — replacement for a bare pointer Reference counting Loading persistent object on access Transactional locking 24

Structural Patterns - Proxy Class Diagram abstract Subject Client request() . RealSubject request() . Proxy request() . request() { . realSubject.request() . } 25

Structural Patterns - Proxy Object Diagram aClient: aProxy : Proxy subject : RealSubject 26

Structural Patterns - Proxy Participants Subject: Defines the common interface for RealSubject and Proxy. Proxy: Maintains reference to real subject Can be substituted for a real subject Controls access to real subject May be responsible for creating and deleting the real subject Special responsibilities Marshaling for remote communication Caching data Access validation RealSubject: Defines the real object that the proxy represents. Client: Accesses the RealSubject through the intervention of the Proxy. Collaborations Proxy forwards requests to RealSubject when appropriate, depending on the kind of proxy. 27

Behavioral Patterns Observer Strategy Command State Visitor 28

Behavioral Patterns - Observer Intent Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Applicability An abstraction has two aspects, one dependent on the other. When changing one object requires changing others, and you don’t know how many objects need changed. When an object needs to notify others without knowledge about who they are. 29

Behavioral Patterns - Observer Class Diagram Subject * attach( observer ) detach( observer ) notify() interface Observer observers update() for all o in observers o.update() observerState : subject.getState() ConcreteSubject subjectState getState() ConcreteObserver subject update() 30

Behavioral Patterns - Observer Participants Subject Observer Defines an updating interface for objects that should be identified of changes. ConcreteSubject Knows its observers, but not their “real” identity. Provides an interface for attaching/detaching observers. Stores state of interest to ConcreteObserver objects. Sends update notice to observers upon state change. ConcreteObserver Maintains reference to ConcreteSubject (sometimes). Maintains state that must be consistent with ConcreteSubject. Implements the Observer interface. Collaborations ConcreteSubject notifies observers when changes occur. ConcreteObserver may query subject regarding state change. 31

Behavioral Patterns - Observer Sequence Diagram subject : ConcreteSubject observer1 : ConcreteObserver observer2 : ConcreteObserver attach( observer1 ) attach( observer2 ) notify() update() getState() update() getState() 32

Behavioral Patterns - Strategy Pattern Intent: defines a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Motivation: when there are many algorithms for solving a problem, hard-wiring all algorithms in client’s code may have several problems: Clients get fat and harder to maintain Different algorithms may be appropriate at different time It is difficult to add new algorithms 33

Behavioral Patterns - Strategy Pattern Context strategy ContextInterface() Strategy AlgorithmInterface() ConcreteStrategyA ConcreteStrategyB ConcreteStrategyC AlgorithmInterface() AlgorithmInterface() AlgorithmInterface() 34

Behavioral Patterns - Participants of Strategy Strategy: declares an interface common to all supported algorithm. Context uses this interface to call the algorithm defined by a ConcreteStrategy. ConcreteStrategy: implements the algorithm using the Strategy interface Context: maintains a reference to a Strategy object and defines an interface that let Strategy access its data 35

Behavioral Patterns - Sorting Example Requirement: we want to sort a list of integers using different sorting algorithms, e.g. quick sort, selection sort, insertion sort, etc. E.g., {3, 5, 6, 2, 44, 67, 1, 344, . } {1, 2, 3, 5, 6, 44, 67, 344, . } One way to solve this problem is to write a function for each sorting algorithm, e.g. quicksort(int[] in, int[] res) insertionsort(int[] in, int[] res) mergesort(int[] in, int[] res) A better way is to use the Strategy pattern 36

Behavioral Patterns - Strategy Pattern Main() { stdRec.SetSortStr(sortStrInfo); stdRec.Sort()} Main Main() How is stdRec implemented? How is –sortStrategy implemented? stdRec -sortStrategy SortedList -list: ArrayList SetSortStr(sortStr:SortStrategy) Sort() SortStrategy Sort(list:ArrayList) Sort() {sortStrategy.Sort(list)} QuickSort Sort(list:ArrayList) InsertionSort Sort(list:ArrayList) MergeSort Sort(list:ArrayList) 37

Behavioral Patterns Command Intent Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Applicability Parameterize objects by an action In place of “callbacks” Specify, queue, and execute requests at different times Supports undo when Command maintains state information necessary for reversing command. Added support for logging Command behavior. Support high-level operations built on primitive operations (transactions). 38

Behavioral Patterns - Command Class Diagram Client Invoker abstract Command * execute() Receiver action() ConcreteCommand state execute() receiver.action() 39

Behavioral Patterns - 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 a corresponding operation on Receiver. Client (Application): Creates a Command object and sets its Receiver. Invoker: Asks the Command to carry out a request. Receiver: Knows how to perform the operation associated with a request. Can be any class. Collaborations Creates a ConcreteCommand object and sets its Receiver. An Invoker stores the ConcreteCommand. Invoker calls execute() on command. ConcreteCommand invokes operation on its receiver. 40

Behavioral Patterns - Command Sequence Diagram aClient : Client aCommand : ConcreteCommand anInvoker : Invoker aReceiver: create( aReceiver ) store( aCommand ) execute() action() 41

Behavioral Patterns - State Intent Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Applicability An object’s behavior depends on its state, and it must change its behavior at run-time depending on its state. Operations have large, multipart conditional statements that depend on the object’s state. Usually represented by constants. Some times, the same conditional structure is repeated. 42

Behavioral Patterns - State Class Diagram Context state request() abstract State handle() ConcreteStateA handle() ConcreteStateB handle() state.handle(); 43

Behavioral Patterns - State Participants Context State Defines interface of interest to clients. Maintains an association with a subclass of State, that defines the current state. Defines an interface for encapsulating the behavior with respect to state. ConcreteStatex Each subclass implements a behavior associated with a particular state of the Context. Collaborations Context delegates state-specific behavior to the current concrete State object. The state object may need access to Context information; so the context is usually passed as a parameter. Clients do not deal with State object directly. Either Context or a concrete State subclass can decide which state succeeds another. 44

Behavioral Patterns - Visitor Intent Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Applicability An object structure contains many disparate classes, and operations need to be performed based on concrete classes. Many distinct operations need to be performed on an object structure. An object structure rarely changes, but new operations need to be defined over the structure. 45

Behavioral Patterns - Visitor Class Diagram Client abstract Element * ObjectStructure accept( v : Visitor ) abstract Visitor visitA( element : ConcreteElementA ) visitB( element : ConcreteElementB ) ConcreteElementA accept( v : Visitor ) operationA() ConcreteVisitor1 visitA( element : ConcreteElementA ) visitB( element : ConcreteElementB ) ConcreteVisitor2 v.visitA( this ) visitA( element : ConcreteElementA ) visitB( element : ConcreteElementB ) ConcreteElementB accept( v : Visitor ) operationB() v.visitB( this ) 46

Behavioral Patterns - Visitor Participants Visitor — declares a visit operation for each class within the object structure aggregation. ConcreteVisitor — implements each operation declared by Visitor. Provides algorithm context. Element — defines an accept operation taking a Visitor as an argument. ConcreteElementX — implements an accept operation taking a Visitor as an argument. ObjectStructure Enumerates its elements; potentially disparate classes. May provide a high level interface for visitor to visit its elements. Potentially a composite or just a general collection. Collaborations A client creates an instance of a concrete Visitor subclass. Client requests the ObjectStructure to allow the visitor to visit each. When visited, Element invokes the appropriate operation on Visitor; overloading to know the element type. 47

Behavioral Patterns - Visitor Sequence Diagram aStruct : ObjectStructure elemA : ConcreteElementA accept( v ) elemB : ConcreteElementB v : Visitor visitConcreteElementA( elemA ) operationA() accept( v ) visitConcreteElementB( elemB ) operationB() 48

How to Select & Use Design Patterns How to Select ( 20 in the book, and still growing fast?, more on Internet) Scan Intent Sections Study How Patterns Interrelate Study Patterns of Like Purpose Examine a Cause of Redesign Consider What Should Be Variable in Your Design How to Use Read the pattern once through for an overview : appears trivial, but not Go back and study the structure, participants, and collaborations sections Look at Sample Code: concrete example of pattern in code Choose names for pattern participants Define the classes Define application specific names for operations in the pattern Implement the operations to carry out the responsibilities and collaborations in the pattern 49

Appendix: More on the Observer Pattern Decouples a subject and its observers Widely used in Smalltalk to separate application objects from interface objects Known in the Smalltalk world as Model-View-Controller (MVC) Rationale: the interface is very likely to change while the underlying business objects remain stable Defines a subject (the Observable) that is observed Allows multiple observers to monitor state changes in the subject without the subject having explicit knowledge about the existence of the observers Observer Subject Observer Observer 50

Appendix: More on the Observer Pattern The Model-View-Controller (MVC) Developed at Xerox Parc to provide foundation classes for Smalltalk-80 The Model, View and Controller classes have more than a 10 year history Fundamental Principle separate the underlying application MODEL (business objects) from the INTERFACE (presentation objects) Rationale for MVC: Design for change and reuse Business Objects (the Model in MVC) Expert Interface Novice Interface MVC and Observer Pattern In Smalltalk, objects may have dependents When an object announces “I have changed”, its dependents are notified It is the responsibility of the dependents to take action or ignore the notification 51

Appendix: More on the Observer Pattern java.util.Observable Observable/subject objects (the Model in Model-View) can announce that they have changed Methods: – void setChanged() – void clearChanged() – boolean hasChanged() WHAT IF Observers query a Subject periodically? query Subject setChanged() Harry Observer hasChanged() True/false 52

Appendix: More on the Observer Pattern Implementing & Checking an Observable Implementing an Observable import java.util.*; import java.io.*; Checking an Observable public static void main (String args [ ] ) { Harry harry new Harry (false); public class Harry extends Observable { private boolean maritalStatus false; public Harry (boolean isMarried) { maritalStatus isMarried; } harry.updateMaritalStatus (true); if (harry.hasChanged() ) System.out.println ("Time to call harry"); } public void updateMaritalStatus (boolean change) { maritalStatus change; } // set flag for anyone interested to check this.setChanged(); 53

Appendix: More on the Observer Pattern Implementing the Observer Pattern Step 1: Observers register with Observable addObserver (this) Observer1 addObserver (observer2) Observer2 Harry Step 2. Observable notifies Observers notifyObservers(Object arg) Harry update(Observable o, Object arg) Observer1 Observable (Harry) may also send himself a notifyObservers() msg - no params 55

Appendix: More on the Observer Pattern java.util.Observable The superclass of all ‘observable’ objects to be used in the Model View design pattern Methods are provided to: void addObserver(anObserver) int countObservers() void deleteObserver (anObserver) void deleteObservers () Interface Defines the update() method that allows an object to ‘observe’ subclasses of Observable Objects that implement the interface may be passed as parameters in: addObserver(Observer o) 56

Summary Design Patterns Creational Patterns Structural Patterns: Adapter, Composite, Façade, Proxy Behavioral Patterns: Command, Observer, State, Visitor Appendix: More on the Observer Pattern The Model-View-Controller (MVC) java.util.Observable 57

Points to Ponder List as many design patterns as you can think of in the Model-ViewController (MVC). How many design patterns can exist? In the order of tens? hundreds? or thousands? Justify your reasoning. What would be the major differences between design patterns and architectural patterns? What style of architecture is closest to the Observer pattern in the manner objects interact with each other? Map the Observer pattern into the Java Event Model. What are the essential tradeoffs between 1) Observers query a Subject periodically and 2) using the Observer pattern? 58