ObjectOriented Software Engineering Practical Software Development using UML

Object-Oriented Software Engineering Practical Software Development using UML and Java Chapter 6: Using Design Patterns Part 1 © Lethbridge/Laganière 2001 Chapter 6: Using design patterns

Preview: • Want to look at more class diagrams – static. • But we want to look at recurring groupings of classes that are regularly used to address common problems. • Want to take advantage of experiences of others and create a better, more resilient design. • Want to use patterns that assist us in separating concerns (abstractionoccurrence, observer, player-role); patterns used to better create class hierarchies of instances; patterns in which one method simply calls another method in another class (have you seen this? ? ); patterns where you use delegation to gain access to facilites in one or more other classes (Adaptor, Façade, Proxy); patterns that help protect other objects from unanticipated access (immutable and read-only interfaces). © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 2

6. 1 Introduction to Patterns The recurring aspects of designs are called design patterns. • A pattern is the outline of a reusable solution to a general problem encountered in a particular context • Many of them have been systematically documented for all software developers to use • A good pattern should —Be as general as possible —Contain a solution that has been proven to effectively solve the problem in the indicated context. Studying patterns is an effective way to learn from the experience of others We will only look at a few. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 3

Pattern description Context: • The general situation in which the pattern applies Problem: —A short sentence or two raising the main difficulty. Forces: • The issues or concerns to consider when solving the problem Solution: • The recommended way to solve the problem in the given context. —‘to balance the forces’ Antipatterns: (Optional) • Solutions that are inferior or do not work in this context. Related patterns: (Optional) • Patterns that are similar to this pattern. References: • Who developed or inspired the pattern. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 4

Remember: our patterns define a relation between a certain context a problem a solution Patterns represent well-known knowledge Really documents common practice Patterns should be in the public domain Patterns need to be written for the public good. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 5

6. 2 The Abstraction-Occurrence Pattern • Context: — Often found in class diagrams that form part of the system domain model. —Often in a domain model you find a set of related objects (occurrences). —The members of such a set share common information but also differ from each other in important ways. (Sports cars …. ) • Problem: —What is the best way to represent such sets of occurrences in a class diagram? Use the commonality, yet represent the differences! • Forces: —You want to represent the members of each set of occurrences without duplicating the common information. - Cannot have this! —Want to maximize the flexibility of the system too. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 6

Abstraction-Occurrence • Solution: «Abstraction» Remember (Java): an Interface can have NO implementations; may at most have constants; Abstract Class has at least one abstract method; can have declarations. TVSeries * * series. Name producer Title name author isbn publication. Date lib. Of. Congress «Occurrence» Episode number title story. Synopsis * Library. Item bar. Code. Number Note: create an abstraction containing common data to all occurrences. This is the “abstraction”. Then create the “occurrence” class that represents instances (occurrences) of the abstraction. Realtionship is 1: * Foreign. Sports. Car { Car. Make; Countryof. Origin…} Auto { Model, Style, Cost. . } Note (as we shall see) this is not inheritance. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 7

Abstraction-Occurrence Antipatterns: Single class: Bad because info would have to be duplicated in each occurrence – and same info would be in each occurrence! © Lethbridge/Laganière 2001 Here, separate subclass for each title. All other information would have to be duplicated in each instance. Also, want to be able to add new books without programming new classes! Problem here is making the abstract class a super class of the occurrence. Attributes would be inherited, of course, but data would be lost! We’d have to fill in name, author … for each occurrence! Chapter 6: Using design patterns 8

Abstraction-Occurrence Square variant Scheduled. Train * number date * * Scheduled. Leg * scheduled. Dep. Time scheduled. Arr. Time origin * Specific. Train * Specific. Leg actual. Dep. Time actual. Arr. Time destination Station All we are saying here is that if the abstraction itself is an aggregate (note Scheduled. Train and Scheduled. Leg) the occurrences also are usually aggregates (Specific. Train; Specific. Leg). Read more on this on your own. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 9

6. 3 The General Hierarchy Pattern • Context: —Occurs in MANY class diagrams. —Objects in a hierarchy can have one or more objects above them (superiors), and one or more objects below them (subordinates). —Some objects cannot have any subordinates • Problem: —How do you represent a hierarchy of objects, in which some objects cannot have subordinates? • Forces: —You want a flexible way of representing the hierarchy - that prevents certain objects from having subordinates - yet, where all the objects have many common properties and operations • Main Thought: All hierarchies are NOT necessarily inheritance hierarchies!!!! © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 10

General Hierarchy «Node» • Solution: * «subordinate» 0. . 1 «Non. Superior. Node» Employee * supervises «Superior. Node» File. System. Item * 0. . 1 Secretary Technician Manager contains 0. . 1 File Directory Create an abstract <<Node>> class that represents features possessed by all – like on that each node can have a superior class. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 11

General Hierarchy «Node» • Solution: * «subordinate» 0. . 1 «Non. Superior. Node» Employee * supervises «Superior. Node» File. System. Item * 0. . 1 Secretary Technician Manager contains 0. . 1 File Directory Then create at least two subclasses of the <<Node>> class. One of the subclasses <<Superior. Node>> must be linked by a <<subordinates>> association to the superclass; whereas at least one subclass <<Non. Superior. Node>> must not be. The subordinates of <<Superior. Node>> can thus be instances of either Superior. Node or Non. Superior. Node. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 12

General Hierarchy «Node» • Solution: * «subordinate» 0. . 1 «Non. Superior. Node» Employee * supervises «Superior. Node» File. System. Item * 0. . 1 Secretary Technician Manager contains 0. . 1 File Directory The multiplicity of the <<subordinates>> association can be optional-to-many or many-to-many. If many-to-many, then the hierarchy of instances becomes a lattice, in which a node can have many superiors. The ‘optional’ allows for the case of the top node in the hierarchy, which has no superiors. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 13

General Hierarchy • Examples: Employee * supervises File. System. Item * 0. . 1 Secretary Technician Manager contains 0. . 1 File Directory Here, we have three types of employees in the organization: only managers can supervise subordinates. So, All are employees and inherit from Employee class. An employee has zero or one managers (manager - as an employee - may not have a manager). Manager may supervise many employees. Secretaries and Technicians cannot have subordinates; manager can. In the second examle, File. System. Item is the superiorclass. File and Directory inherit from File. System. Item. Only Directories can contain other file system objects, and this is described by the * relationship – a directory may contain any number of file system items, but a file system is contained in 0 or 1 directory. Where this is powerful is that both the File and the Directory inherit from File. System. Item, yet one of the subordinates can contain instances of the superclass. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 14

General Hierarchy Antipattern: Recording Video. Recoding Music. Video Audio. Recording Jazz. Recording Classical. Recording Blues. Recording Rock. Recording Don’t fall into the trap of thinking a hierarchy of categories is necessarily a hierarchy of classes! © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 15

6. 4 The Player-Role Pattern • Context: (More issues with Generalization…) — An object may play different roles in different contexts. Want to model class diagrams for this! —Pattern used to solving modelling problems when you are drawing many different types of class diagrams. —A role is a particular set of properties associated with an object in a particular context. • Problem: —How do you best model players and roles so that a player can change roles or possess multiple roles? © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 16

Player-Role • Forces: —It is desirable to improve encapsulation by capturing the information associated with each separate role in a class. —You want to avoid multiple inheritance. —You cannot allow an instance to change class • Solution: «Player» 1 * «Abstract. Role» Create a <<Player>> class to represent the object that plays different roles. Create an association from this class to an abstract <<Role>> class, which is a superset of all «Role 1» possible roles. The subclasses of this <<Role>> class encapsulate all the properties and behaviors associated with the different roles. (Recall abstract classes can have ‘some’ behaviors and can have declarations). © Lethbridge/Laganière 2001 «Role 2» Chapter 6: Using design patterns 17

Player-Role «Player» 1 * «Abstract. Role» «Role 1» «Role 2» If the <<Player>> can only play one role at a time, the multiplicity between <<Player>> and <<Role>> is one-to-one; otherwise it will be one-to-many. <<Role>> can be an Interface – but normally a Role contains a mechanism inherited by its subclasses, allowing them to access information about the <<Player>>. So make <<Role>> an interface only if this mechanism is not needed (cannot implement methods or have declarations in an interface (other than constants). © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 18

Player-Role Example 1: Note the two roles of animals: role based on: type of food, habitat. Idea behind these roles is that an animal may have to switch from one role to another. We don’t want to have to model this situation by destroying one class and creating another class – or opposite. Could make habitat an attribute of Habitat. Role and omit two subclasses. But then we lose the advantage of polymorphism for any operations that would differ in Aquatic Animal and Land Animal. Here, an animal may have a varying number of roles: aquatic, land-based or both. Can also have used a role to capture whether animal is carnivore, herbivore or omnivore. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 19

Player-Role Example 2: Here, we have two separate <Role>> superclasses. In one, student is characterized by his/her attendance status and by whether or not s/he is a graduate student (or not). . Both of these statuses can changed during the life of the Student object. This pattern, therefore, makes it possible to represent a full or part time graduate or undergraduate student. Here, modeling Student in this manner is much better and flexible. It supports polymorphism and is responsive to any changes in role the Student might take, whether these changes be in attendance or in level (or both). © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 20

Player-Role Confusing in spots: All this can be confusing. For example, it appears that the playerrole could be an abstraction-observer pattern. Certainly has similar structure. Player has many roles associated with it just like the abstraction has many occurrences. But there is a major difference: In the Abstraction-Occurrence pattern, an abstraction is … abstract, while its occurrences ten to be real-work things, such as copies of books, or autos In the Player-Role pattern, just the opposite is true, where the player is normally the real-world entity (e. g. a person) while its roles are abstractions. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 21

6. 5 The Singleton Pattern • Context: —It is very common to find classes for which only one instance should exist (singleton) - Examples: a Main Window; Company or University class. • Problem: —How do you ensure that it is never possible to create more than one instance of a singleton class? • Forces: —The use of a public constructor cannot guarantee that no more than one instance will be created. —The singleton instance must also be accessible to all classes that require it © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 22

Singleton • Solution: «Singleton» the. Instance get. Instance Have a private class variable, possibly called, ‘the. Instance. ’ This stores the instance. Then have a public class method (static method) possibly called, ‘get. Instance. ’ Company the. Company if (the. Company==null) the. Company= new Company(); Company «private» get. Instance return the. Company; First time method is called, it creates a single instance and stores it in the. Instance. . Subsequent calls simply return the. Instance. A private Constructor, which ensures no other class will be able to create an instance of the singleton class is needed. © Lethbridge/Laganière 2001 Here, Company class may embody several important characteristics of the Company (operations and attributes). The public class method get. Instance() makes this instance globally accessible. Note: effectively, the Singleton instance is effectively a global variable. Minimize these. Chapter 6: Using design patterns 23

6. 6 The Observer Pattern • Second in the gang-of-four patterns (Singleton was the first) • This is another VERY popular one. • Context: —When you have a two-way association is created between two classes, the code for the classes becomes inseparable. —If you want to reuse one class, then you also have to reuse the other. There is a dependency. • Problem: —How do you reduce the interconnection between classes, especially between classes that belong to different modules or subsystems? • Forces: —You want to maximize the flexibility of the system to the greatest extent possible © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 24

Observer • Solution: So, what do we do? We create an abstract class we will call <<Observable>> that maintains a collection of <<Observer>> instances. «Observable» * add. Observer notify. Observers * «interface» «Observer» update <<Observable>> class is «Concrete. Observer» «Concrete. Observable» very simple; it merely has a mechanism to add and remove observers as well as a method, notify. Observers, that sends an update message to each <<Observer>>. * Any application class can declare itself to be a subclass of the <<Observable>> class. In Java, we call these ‘listeners. ’ © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 25

Observer • Solution: <<Observer>> is an interface, defining only an abstract update method. Any class can thus be made to observe an <<Observable>> by declaring that it implements the interface, and by asking to be a member of the observer list of the <<Observable>>. The <<Observer>> can then expect a call to its update method whenever the <<Observable>> changes. Using this pattern, the <<Observable>> neither has to know the nature of the number of classes that will be interested in receiving an update messages nor what they will do with this information. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 26

Observer • Example: Java has an Observer interface and an Observable class. This is a specific implementation of this pattern. Consider: a ‘forecast’ requires a lot of computations. Once done, it ‘notifies’ all interested instances. Just a class… Forecaster is thus an observable object. One observer object might be an interface object responsible for displaying weather forecast; another might be dependent on weather information to plan a schedule. . Observer pattern in widely used to structure software cleanly into relatively independent modules. It is the basis of the MVC architecture. © Lethbridge/Laganière 2001 Chapter 6: Using design patterns 27