Lecture 26: Design Patterns(part 2)Kenneth M. AndersonObject-Oriented Analysis and DesignCSCI 6448 - Spring Semester, 2003April 10, 2003 © Univeristy of Colorado, 2003 2Last Lecture Design Patterns Background and Core Concepts Examples Singleton, Factory Method, and AdapterApril 10, 2003 © Univeristy of Colorado, 2003 3Goals of Lecture Cover Additional Design Patterns State Iterator Flyweight Decorator Observer CompositeApril 10, 2003 © Univeristy of Colorado, 2003 4State Intent Allow an object to alter its behavior whenits internal state changes Motivation TCPConnection example A TCPConnection class must respond toan open operation differently based on itscurrent state: established, closed, listening,etc.April 10, 2003 © Univeristy of Colorado, 2003 5State, continued Applicability Use State when an object’s behavior depends on its state operations have large, multipart conditional statements that depend onthe object’s state Participants Context defines the interface of interest to clients maintains an instance of a ConcreteState subclass State defines an interface for encapsulating the behavior associated with aparticular state of the Context ConcreteState each subclass of State implements a different behavior that implementsthe correct behavior for a particular stateApril 10, 2003 © Univeristy of Colorado, 2003 6State, continued Structure Page 306 of Design Patterns Collaborations Context delegates state-specific requests to thecurrent ConcreteState object A context may pass itself as an argument to theState object handling the request Context is the primary interface of clients Either Context or ConcreteState subclasses candecide which state succeeds another and underwhat circumstancesApril 10, 2003 © Univeristy of Colorado, 2003 7State, continued Consequences State localizes state-specific behavior andpartitions behavior for different states State makes state transitions explicit State objects can be shared Example We saw an example of the state patternback in Lecture 20April 10, 2003 © Univeristy of Colorado, 2003 8Iterator Intent Provide a way to access the elements of anaggregate object (e.g. a collection class)sequentially without exposing its underlyingrepresentation Also Known As Cursor Motivation A collection may have multiple ways of being“traversed”; Iterator lets you keep traversaloperations out of the core collection interfaceApril 10, 2003 © Univeristy of Colorado, 2003 9Iterator, continued Applicability Use the Iterator pattern to access an aggregate object’s contents without exposing its internalrepresentation to support multiple traversals of aggregate objects to provide a uniform interface for traversing different aggregate structures (that is,to support polymorphic iteration) Participants Iterator defines an interface for accessing and traversing elements ConcreteIterator implements Iterator interface and keeps track of current position within collection Aggregate defines an interface for creating an Iterator (factory method) ConcreteAggregate implements the factory methodApril 10, 2003 © Univeristy of Colorado, 2003 10Iterator, continued Structure page 259 of Design Patterns Collaborations A ConcreteIterator keeps track of the current object in theaggregate and can compute the next object in the traversal Consequences The Iterator pattern supports multiple traversals for eachcollection (e.g. inorder, preorder, postorder for trees) Iterators simplify Aggregate interface More than one traversal can occur on a single collection atonce; as long as the traversal is read-onlyApril 10, 2003 © Univeristy of Colorado, 2003 11Iterator, continued Implementation The Iterator interface in the Java Collectionclasses java.util.Iterator (interface) java.util.List (interface) java.util.LinkedList (class) java.util.ListIterator (interface) implementing subclass is private within List classApril 10, 2003 © Univeristy of Colorado, 2003 12Flyweight Intent Use sharing to support large numbers of fine-grained objects efficiently Motivation Imagine a text editor that creates one object percharacter in a document For large documents, that is a lot of objects! but for simple text documents, there are only 26 letters,10 digits, and a handful of punctuation marks beingreferenced by all of the individual character objectsApril 10, 2003 © Univeristy of Colorado, 2003 13Flyweight, continued Applicability Use flyweight when all of the following are true An application uses a large number of objects Storage costs are high because of the sheer quantity ofobjects Most object state can be made extrinsic Many groups of objects may be replaced by relatively fewshared objects once extrinsic state is removed The application does not depend on object identity. Sinceflyweight objects may be shared, identity tests will returntrue for conceptually distinct objectsApril 10, 2003 © Univeristy of Colorado, 2003 14Flyweight, continued Participants Flyweight declares an interface through which flyweights can receive andact on extrinsic state ConcreteFlyweight implements Flyweight interface and adds storage for intrinsicstate UnsharedConcreteFlyweight not all flyweights need to be shared; unshared flyweightstypically have children which are flyweights FlyweightFactory creates and manages flyweight objects Client maintains extrinsic state and stores references to flyweightsApril 10, 2003 © Univeristy of Colorado, 2003 15Flyweight, continued Collaborations Data that a flyweight needs to process must beclassified as intrinsic or extrinsic Intrinsic is stored with client; Extrinsic is stored with client Clients should not instantiate ConcreteFlyweightsdirectly Consequences Storage savings is a tradeoff between totalreduction in number of objects verses the amountof intrinsic state per flyweight and whether or notextrinsic state is computed or stored greatest savings occur when extrinsic state is computedApril 10, 2003 © Univeristy of Colorado, 2003 16Flyweight, continued See code example (available from classwebsite) Simple implementation of flyweightpattern Focus is on factory and flyweight ratherthan on client Demonstrates how to do simple sharing
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