CS 4240: Introduction to Design Patterns

CS 4240: Introduction to Design Patterns Readings: Chap. 5 of Design Patterns Explained Also, Chap. 6 and 7 Also, on-line info on Composite pattern (and others) at http://www.oodesign.com

Readings • Chapter 5 of our textbook • Handouts on various patterns

Background, Definition • From Alexander’s work in architecture • Alexander: “a solution to a problem in context.” • Design pattern: a description of a problem that reoccurs and an outline of an approach to solving that problem • Generally domain, language independent • Also, analysis patterns

Design Patterns: Essential Elements • Pattern name • A vocabulary of patterns is beneficial • Intent • Problem • When to apply the pattern, what context. • How to represent, organize components • Conditions to be met before using • Solution • Design elements: relationships, responsibilities, collaborations • A template for a solution that you implement • Consequences • Results and trade-offs that result from using the pattern • Needed to evaluate design alternatives

Essential Elements (con’td) • Participants and collaborators • Implementation • Generic Structure

Patterns Are (and Aren’t) • Name and description of a proven solution to a problem • Documentation of a design decision • They’re not: • Reusable code, class libraries, etc. (At a higher level) • Do not require complex implementations • Always the best solution to a given situation • Simply “a good thing to do”

Describing Design Patterns • The GoF defined a standard format • Generally followed • Not just a UML diagram! • Pattern Format (13 sections): • Pattern name and classification • Intent: what’s it do? Rationale? • Also known as • Motivation • A scenario that illustrates a sample problem and how this patterns helps solve it. • Applicability • For which situations can this be applied? • Structure • Graphical representation (e.g. UML)

Pattern Format (cont’d) • Participants • Classes and objects, their responsibilities • Collaborations • How participants interact • Consequences • Implementation • Pitfalls, hints, techniques, language issues • Sample code • Code fragments that illustrate the pattern • Known uses • From real systems • Related patterns • Similar patterns, collaborating patterns

Example 1: Singleton Pattern • Global variables are bad! • Why? • We often want to use global variables! • Why?

Example 1: Singleton Pattern • Context: Only one instance of a class is created. Everything in the system that needs this class interacts with that one object. • Controlling access: Make this instance accessible to all clients • Solution: • The class has a static variable called theInstance (etc) • The constructor is made private (or protected) • Clients call a public operation getInstance()that returns the one instance • This may construct the instance the very first time or be given an initializer

Singleton: Java implementation public class MySingleton { private static MySingleton theInstance = new MySingleton(); private MySingleton() { // constructor … } public static MySingleton getInstance() { return theInstance; }}

Static Factory Methods • Singleton patterns uses a static factory method • Factory: something that creates an instance • Advantages over a public constructor • They have names. Example:BigInteger(int, int, random) vs.BigInteger.probablePrime() • Might need more than one constructor with same/similar signatures • Can return objects of a subtype (if needed) • Wrapper class example: Double d1 = Double .valueOf(“3.14”); Double d2 = new Double (“3.14”); • More info: Bloch’s Effective Java

Examples from Java Library • In Swing, you can create a Border and add it to a component Border b = BorderFactory.createBevelBorder(3); • Need the exact same border again? • The above method doesn’t create a new one. Just gives you the same one • Note: works because the bevel is immutable

Example from the Java Library • Java can manipulate its own class definitions • Example: Class classObj = Class.forName(“edu.uva.cs441.MyClass”); • Then we can do things to the Class object • What if we call this again? • Only one Class object per Java-class

Example #2: PlayLists and Songs • We saw this in CS201 (maybe)

Could PlayLists “Nest”? • Could a PlayList contain Songs and other PlayLists? • This is the “Composite Design Pattern” • Make PlayList contain • A list of PlayableItems, not Songs • PlayList.play() calls play() on each item • Polymorphism at work

Class Diagram for This Composite

General Class Diagram • Could be many Leaf classes

Polymorphism in Action • Playlist contains a list of PlayableItems • PlayList.play() calls play() on each item for (int i=0; i<list.size(); ++i) { ( (PlayableItem) list.get(i) ).play(); } • Will call the Song.play() or PlayList.play() depending on what item i really is • Review question: why is the cast needed?

Façade Pattern • Intent: • Simplify interface to set of interfaces in an existing subsystem.Or, need to define new/variant interface • Problem: • Only need part of a complex system.Or, need to interact in a particular way. • Participants, collaborators: • Façade class, subsystem classes, client class

Façade Pattern (cont’d) • Consequences: • Certain subsystem operations may not be available in the façade • Clients may “go around” the facade

Note in textbook • Reduce coupling for client • Existing interfaces requires client to access many objects to carry out task • Façade object may be one object that’s a “gateway” to multiple objects • In other words: the Façade encapsulates the system • Another motto: find what varies and encapsulate it

Examples: • Java’s URL class • Really hides a set of objects and methods • But not easy to see that it does this IMHO • An example from our recent CS1110 textbook • Make I/O easier for beginners • Class StdIn: static methods to read • Class In: create object to read from keyboard, file or URL

Class In from Princeton course text See Javadoc here: http://www.cs.princeton.edu/introcs/stdlib/javadoc/In.html

Encapsulate Scanner, simplify methods public final class In { private Scanner scanner; ... public In() { scanner = new Scanner( new BufferedInputStream(System.in), charsetName); scanner.useLocale(usLocale); }... /** * Is the input stream empty? */ public boolean isEmpty() { return !scanner.hasNext(); }

Create “reader” from filename or URL public In(String s) { try { // first try to read file from local file system File file = new File(s); if (file.exists()) { scanner = new Scanner(file, charsetName); scanner.useLocale(usLocale); return; } // next try for files included in jar URL url = getClass().getResource(s); // or URL from web if (url == null) { url = new URL(s); } URLConnection site = url.openConnection(); InputStream is = site.getInputStream(); scanner = new Scanner( new BufferedInputStream(is), charsetName); scanner.useLocale(usLocale); } catch (IOException ioe) { System.err.println("Could not open " + s); } }

Think About and Discuss • Façade: any relation to the Law of Demeter? • “Principle of Least Knowledge” • Talk only your immediate friends

Adaptor Pattern • Intent: • Make an existing object match an existing interface that it doesn’t match • Problem: • Existing component has right functionality but doesn’t match the interface used • Interface here? Abstract class (or Java-interface) used polymorphically • Solution: • Wrapper class with delegation etc. • Two flavors: Object Adaptor vs. Class Adaptor

Adaptor Pattern (cont’d) • Participants and Collaborators • Adapter class: “new” interface for… • Adaptee class (existing class) • Target: the needed interface • Client who uses objects of type Target • Consequences: • The good: solves problem and meets intent above! • Some other wrinkles…

Class Diagram for Object Adaptor • Object Adaptor pattern • Note that Adapter is anexample of a “wrapper class” • A common implementation techniqueused in several design patterns

Book’s Example • Abstract class (or interface) Shape • Methods include: display(), fill() • Our code relies on Shape hierarchy and polymorphism • We need a Circle class • Sweet! Another one exists! XXCircle • Bummer! Method names are different • Solution: see book or notes on board

Class Adaptor Pattern • Don’t create a separate object that wraps the existing object • Instead, create new class that • Includes what existing class has (inherit) • Also meets Target interface • With a Java interface, or multiple inheritance in other languages

Class Adaptor example • Problem: • Existing code has a CommandBuffer class • Doesn’t have an Iterator • You have code that could do great things if only you could use CommandBuffer as if it were Iterable

Example of Class Adaptor pattern • The new class implements Iterator methods in terms of getCommand() etc defined in existing class • No pair of objects: just an instance of new class

Think About and Discuss • Class adaptors are different than object adaptors • Does this difference relate to a “rule” or “principle” of OO design we’ve seen?

Comparing Façade and Adaptor • Similarities: • Both designed to help us use existing classes better • Both can use wrapper objects • Similar implementation solves different problems • Adaptor: existing interface we must design to (not for Façade) • New object used polymorphically • Façade: need simpler interface (not a goal for Adaptor) • See table 7-1 on page 110

Summary of Intro to Design Patterns • Understand background, general principles of design patterns • Examples: • Singleton • Composite • Façade • Adaptor • Recognize them, apply them in simple situations

Exercise: Can you Adapt? • Deprecated Java interface Enumerator • hasMoreElements() • nextElement() • Replaced by interface Iterator • hasNext() • next() • remove()

Your task • Create a class: EnumerationIterator • This let us design and write new code that works with Iterators, andalso interact with legacy code that exposes Enumerators for use to use • First question: class or object adaptor?

Questions to Answer • Question: which classes play which roles in the design pattern? Write this down. • Final question: write Java class with constructor and next() and hasNext().

CS 4240: Introduction to Design Patterns