CMSC 132 Object Oriented Programming II Design Patterns II Department of Computer Science University of Maryland College Park 1 More Design Patterns Marker interface Label semantic attributes of a class Observer A way of notifying change to a number of classes State Alter an object s behavior when its state changes Visitor Defines a new operation to a class without changing class 2 Marker Interface Pattern Definition Label semantic attributes of a class Where to use benefits Need to indicate attribute s of a class Allows identification of attributes of objects without assuming they are instances of any particular class 3 Marker Interface Pattern Example Classes with desirable property GoodProperty Original Store flag for GoodProperty in each class Using pattern Label class using GoodProperty interface Examples from Java Cloneable Serializable 4 Marker Interface Example public interface GoodProperty no methods class A implements GoodProperty class B class A goodObj new A class B badObj new B if goodObj instanceof GoodProperty True if badObj instanceof GoodProperty False 5 Observer Pattern Definition Updates all dependents of object automatically once object changes state Where to use benefits One change affects one or many objects Many object behavior depends on one object state Need broadcast communication Maintain consistency between objects Observers do not need to constantly check for changes 6 Observer Pattern Example Multiple windows views for single document Original Each window checks document Window updates image if document changes Using pattern Each window registers as observer for document Window Window Window Window Doc Doc Document notifies all of its observers when it changes 7 Observer Example public interface Observer public void update Observable o Object a called when observed object o changes public class Observable protected void setChanged changed protected void clearChanged no change boolean hasChanged return changed void addObserver Observer o track observers void notifyObservers notify if changed void notifyObservers Object a then clear change 8 Observer Example public class MyWindow implements Observer public openDoc Observable doc doc addObservers this add window to list public void update Observable doc Object arg redraw doc display updated document public class MyDoc extends Observable public void edit edit document setChanged mark change notifyObservers arg invokes update 9 State Pattern Definition Represent change in an object s behavior using its member classes Where to use benefits Control states without many if else statements Represent states using classes Every state has to act in a similar manner Simplify and clarify the program 10 State Pattern Example States representing finite state machine FSM Original Each method chooses action depending on state Behavior may be confusing state is implicit Using pattern State interface defines list of actions for state Define inner classes implementing State interface Finite state machine instantiates each state and tracks its current state Current state used to choose action 11 State Example Original Code public class FickleFruitVendor boolean wearingHat boolean isHatOn return wearingHat String requestFruit if wearingHat wearingHat false Apple return Banana Wearing Hat else Banana wearingHat true return Apple Not Wearing Hat 12 State Example public interface State boolean isHatOn String requestFruit public class WearingHat implements State public class NotWearingHat implements State 13 State Example public class FickleFruitVendor State wearingHat new WearingHat State notWearingHat new NotWearingHat explicitly track current state of Vendor State currentState wearingHat behavior of Vendor depends on current state public boolean isHatOn return currentState isHatOn public String requestFruit return currentState requestFruit 14 State Example class WearingHat implements State boolean isHatOn return true String requestFruit currentState notWearingHat change state return Banana class NotWearingHat implements State boolean isHatOn return false String requestFruit currentState wearingHat change state return Apple Apple Not Wearing Wearing Hat Hat end class Banana 15 Visitor Pattern Definition Define operations on elements of data structures without changing their classes Where to use benefits Add operations to classes with different interfaces Can modify operations on data structure easily Encapsulate operations on elements of data structure Decouples classes for data structure and algorithms Crossing class hierarchies may break encapsulation 16 Visitor Pattern Example Print elements in collection of objects Original Iterator chooses action based on type of object Many if else statements Using pattern Visitor interface defines actions during visit Visitable interface allow objects to accept visit Action automatically selected by polymorphic functions through double dispatch 17 Visitor Example Original Code public void messyPrintCollection Collection c for Object o c if o instanceof String System out println o toString add else if o instanceof Float System out println o toString f add f else System out println o toString 18 Visitor Example public interface Visitor public void visit VisitableString s public void visit VisitableFloat f public interface Visitable public void accept Visitor v 19 Visitor Example public class VisitableString implements Visitable private String value public VisitableString String s value s public String toString return value toString public void accept Visitor v v visit this public class VisitableFloat implements Visitable private Float value public VisitableFloat Float f value f public String toString return value toString public void accept Visitor v v visit this Double dispatch 20 Visitor Example public class PrintVisitor implements Visitor public void visitCollection Collection c for Object o c if o instanceof Visitable Visitable o accept this else System out println o toString public void visit VisitableString s System out println s toString public void visit VisitableFloat f System out println f toString f 21 UML Class Diagram of Abstract Visitor 22 Callback Definition Executable code passed as argument to other code Approach 1 Higher level code passes function as argument to lower level code In Java pass object implementing interface In C C pass pointer to function 2 Lower level code invokes callback function to perform desired task 23 Callback cont Motivation Keeps code separate
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