Object-oriented design patterns in UML [Software Modeling] [Computer Science] [Vrije Universiteit Amsterdam] [2016/2017]
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This presentation is about a lecture I gave within the "Software Modeling" course of the Computer Science bachelor program, of the Vrije Universiteit Amsterdam. http://www.ivanomalavolta.com
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Object-oriented design patterns in UML [Software Modeling] [Computer Science] [Vrije Universiteit Amsterdam] [2016/2017]
- 1. Software and Services research group (S2) Department of Computer Science, Faculty of Sciences Vrije Universiteit Amsterdam VRIJE UNIVERSITEIT AMSTERDAM Object-oriented design patterns in UML Software modeling (401016) – 2016/2017 Ivano Malavolta i.malavolta@vu.nl
- 2. VRIJE UNIVERSITEIT AMSTERDAM Roadmap • Object-oriented design patterns • Creational design patterns • Structural design patterns • Behavioural design patterns 2
- 3. VRIJE UNIVERSITEIT AMSTERDAM What is a design pattern? • a ”template” for how to solve a problem that can be used in many different situations • not a finished design • it cannot be transformed directly into source code • helps the designer in getting to the right design faster 3 A reusable form of a solution to a common design problem
- 4. VRIJE UNIVERSITEIT AMSTERDAM The “gang of four” • Erich Gamma • Richard Helm • Ralph Johnson • John Vlissides 1994: 4 IBM programmers observed and documented 23 common problems and their best accepted solutions 4
- 5. VRIJE UNIVERSITEIT AMSTERDAM • how objects can be created • maintainability • control • extensibility • how to form larger structures • management of complexity • efficiency • how responsibilities can be assigned to objects • objects decoupling • flexibility • better communication Types of design patterns 5 CREATIONAL STRUCTURAL BEHAVIOURAL
- 6. VRIJE UNIVERSITEIT AMSTERDAM Essential parts of a design patterns 6 Pattern name Provides a common vocabulary for software designers Intent What does the design pattern do? What is its rationale and intent? What particular design issue or problem does it address? Solution The basic elements providing the solution to the problem in terms of: structure, participants, collaborations Consequences What are the results and trade offs by applying the design pattern
- 7. VRIJE UNIVERSITEIT AMSTERDAM Example: the Singleton design pattern 7 Name Singleton Intent • To ensure that only one instance of a class is allowed within a system • Controlled access to a single object is necessary Solution Consequences • Controlled access to sole instance • Reduced name space • Permits a variable number of instances • … Examples • Logging utility • An entity representing the whole system • Central station within a robotic system • …
- 9. VRIJE UNIVERSITEIT AMSTERDAM Creational design patterns • Singleton • A class for which only a one single instance can exist in the system • Factory Method • Creates an object without hardcoding its type in the code • Abstract Factory • Creates groups of related objects without specifying the exact concrete classes • Object Pool • Allows to recycle objects that are no longer in use • Prototype • Allows to instantiate a class by copying or cloning the properties of an existing object 9 studied in this course
- 10. VRIJE UNIVERSITEIT AMSTERDAM Singleton 1010 Name Singleton Intent • To ensure that only one instance of a class is allowed within a system • Controlled access to a single object is necessary Solution Consequences • Controlled access to sole instance • Reduced name space • Permits a variable number of instances • … Examples • Logging utility • An entity representing the whole system • Central station within a robotic system • …
- 11. VRIJE UNIVERSITEIT AMSTERDAM Singleton – implementation (1/2) 11 https://www.tutorialspoint.com/design_pattern
- 12. VRIJE UNIVERSITEIT AMSTERDAM Singleton – implementation (2/2) 12 https://www.tutorialspoint.com/design_pattern
- 13. VRIJE UNIVERSITEIT AMSTERDAM Factory method 13 Name Factory method Intent • to abstract the process of object creation so that the type of the created object can be determined at run-time • to make a design more customizable in terms of which objects can be created • you want to avoid the new operator because you do not want to hard code which class you want to instantiate Solution [see next slide] Consequences • You have a dedicated class for creating instances of objects • You can pass arguments to that class for controlling the features of the objects you want to create Examples • A central entity for creating rovers, but you really do not want to know exactly how a rover is created • All the cases in which an object does not know what concrete classes it will be required to create at runtime, but just wants to get a class that will do the job
- 14. VRIJE UNIVERSITEIT AMSTERDAM Factory method – solution by example 14 <<abstract>>
- 15. VRIJE UNIVERSITEIT AMSTERDAM Factory method - implementation 15 https://www.tutorialspoint.com/design_pattern
- 16. VRIJE UNIVERSITEIT AMSTERDAM Factory method - implementation 16 https://www.tutorialspoint.com/design_pattern
- 17. VRIJE UNIVERSITEIT AMSTERDAM Factory method - implementation 17 https://www.tutorialspoint.com/design_pattern
- 19. VRIJE UNIVERSITEIT AMSTERDAM Structural design patterns • Adapter • For gluing together incompatible classes • Proxy • An object representing another object • Bridge • Separates an object’s interface from its implementation • Decorator • Add responsibilities to objects dynamically • Façade • A single class that represents an entire subsystem/library • Flyweight, Composite , Private Class Data • … 19 studied in this course
- 20. VRIJE UNIVERSITEIT AMSTERDAM Adapter 20 Name Adapter Intent • To convert the interface of a class into another interface • To let two or more classes with incompatible interfaces work together • To wrap an existing class with a new one • To have a sort of homogenous interface that masks the diversity of some set of various objects Solution [see next slide] Consequences • You have a single class which is responsible to join functionalities of independent or incompatible classes Examples • A wrapper of a complex Java library in order to expose only the APIs that you need in your system • A class uses a naïve coordinate reference system, but you want to mask it and show a more straightforward one to the rest of your system
- 22. VRIJE UNIVERSITEIT AMSTERDAM Adapter - example 22 https://sourcemaking.com/design_patterns/adapter
- 23. VRIJE UNIVERSITEIT AMSTERDAM Adapter - implementation 23 public class Wrapper { // this is the wrapped object private LegacyComponent legacyComponent; // constructor public Wrapper (LegacyComponent instance) { this.legacyComponent = instance; } // call to the wrapped method public int doThis() { int result = 0; float value = this.legacyComponent.doThat(); // ...here you transform value into an integer... return result; } }
- 25. VRIJE UNIVERSITEIT AMSTERDAM Behavioural design patterns (1/2) • Observer • A way of notifying change to a number of classes • Chain of responsibility • A way of passing a request between a chain of objects • Command • Encapsulate a command request as an object • Interpreter • A way to include language elements in a program • Iterator • Sequentially access the elements of a collection • Mediator • Defines simplified communication between classes 25 studied in this course
- 26. VRIJE UNIVERSITEIT AMSTERDAM Behavioural design patterns (2/2) • Memento • Capture and restore an object's internal state • Null Object • Designed to act as a default value of an object • State • Alter an object's behavior when its state changes • Strategy • Encapsulates an algorithm inside a class • Template method • Defer the exact steps of an algorithm to a subclass • Visitor • Defines a new operation to a class without change 26
- 27. VRIJE UNIVERSITEIT AMSTERDAM Observer 27 Name Observer Intent • To let one or more objects be notified of state changes in other objects within the system • When one object changes state, all its dependents are notified and updated automatically Solution [see next slide] Consequences • Support for broadcast communication • State changes in one or more objects should trigger behavior in other objects • You can reuse subjects without reusing their observers, and vice versa • You can add or remove observers without modifying the subjects Examples • A central station can issue commands to a subset of rovers moving in the environment • When a rover finishes its tasks it can notify the central station • etc.
- 34. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility 34 Name Chain of responsibility Intent • Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request • Chain the receiving objects and pass the request along the chain until an object handles it • à it is sequential (Observer is in parallel) Solution [see next slide] Consequences • Reduced coupling between objects • every objects just needs to know its successor • More than one object may handle a request, and the handler is not known a priori • You can change responsibilities by changing the chain at run-time • A request can also go unhandled Examples • A central station keeps a pool of idle robots and assign tasks without knowing the exact number of available robots
- 35. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility - solution 35this chain goes on until the request is handled
- 36. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility – implementation (1/4) 36
- 37. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility – implementation (2/4) 37
- 38. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility – implementation (3/4) 38
- 39. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility – implementation (3/4) 39
- 40. VRIJE UNIVERSITEIT AMSTERDAM Chain of responsibility – implementation (4/4) 40
- 41. VRIJE UNIVERSITEIT AMSTERDAM What this lecture means to you? • Design patterns = solutions to common software design problems • They are not prescriptive specifications for software • Do not memorize them à it is more important that you understand when they are needed and why • They are not always needed • Do not apply design patterns to a trivial solution à you will overcomplicate your code and lead to maintainability issues 41
- 43. VRIJE UNIVERSITEIT AMSTERDAM Exercise 1. Reconsider the FB class diagram 2. Identify a potential issue 3. Apply a design pattern for solving it 43 Exercise inspired by prof. Lago’s lecture at the VU
- 44. VRIJE UNIVERSITEIT AMSTERDAM Readings • https://en.wikipedia.org/wiki/Design_Patterns • https://sourcemaking.com/design_patterns • http://www.blackwasp.co.uk/gofpatterns.aspx • [optional] Detailed info on the GoF book 44