Design patterns. Jef De Smedt Beta VZW

Transcription

1 Design patterns Jef De Smedt Beta VZW

2 Who Beta VZW Association founded in 1993 Computer training for the unemployed Computer training for employees (Cevora/Cefora) 9:00-12:30 13:00-16:00

3 You Name Function/Job description Experience with programming languages in general Object Oriented programming languages Expectations (or what is your excuse for being here)

4 Overview Introduction Creational Design patterns Structural Design patterns Behavorial Design patterns Enterprise Design patterns (Fowler) S.O.L.I.D. principles ( uncle Bob Martin)

5 Design pattherns: higher level of abstraction Classsse Procedure Design Patterns + Data Sequence Selection Iteration sum = sum + 5 MOV AL, 01h

6 Stages in design pattern usage 1. What is a design pattern? 2. Let s take a look at a catalog of design patterns and see which pattern I can use in my code 3. I realize I can use design pattern X in my code 4. I do not think anymore of design patterns. I use design principles intuitively

7 Design patterns are about communication SQLBuilder, FileVisitor, XmlParserFactory, ListIterator, InvoiceProxy Every 5 years the number of programmers doubles => half the programmers are inexperienced(write bad code) How do we communicate with so many new programmers?

8 Design pattern Name Description of the problem Description of the solution(pattern) Advantages/disadvantages

9 General principles Program to an interface, not to an implementation Declaration void ExecuteCommand(String cmd, IConnection c) Call ExecuteCommand(...,new OracleConnection()) ExecuteCommand(...,new MySQLConnection())

10 General principles Favor object composition over class inheritance class List{ public void add(object o){...} } class PersonList: List{ //inheritance } class PersonList{ //composition private List l = new List(); public void add(person p){ l.add(p);} }

11 Singleton Name: Singleton Description: I want one global object Solution: Define private static instance + static method Advantages/disadvantages: Can be used to manage a pool of objects

12 Creational patterns (Patterns to create an object)

13 Factory method Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

14 Abstract Factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

15 Builder Separate the construction of a complex object from its representation so that the same construction process can create different different representations.

16 Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

17 UML symbols Generalisation, inheritance Aggregation (filled diamond =composition) Association Dependency

18 Structural patterns Combine classes and object to form larger structures

19 Adapter Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn t otherwise because of incompatible interfaces

20 Bridge Decouple and abstractation from its implementation so that the two can vary independently.

21 Composite Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly

22 Decorator Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

23 Problem Purchase has to be approved < it can be approved by Director < it can be approved by vice president Else it must be approved by president For some purchases only vice president and president can approve For some purchases only director and president can approve

24 Facade Privide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

25 Flyweight Use sharing to support large numbers of fine-grained objects efficiently.

26 Proxy Provide a surrogate or placeholder for another object to control access to it.

27 Behavorial Patterns Algorithms and the assignment of responsibilities between objects

28 Chain of responsibility 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.

29 Command Encapsulate request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations

30 Interpreter Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

31 Iterator Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

32 Mediator Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

33 Memento Without violating encapsulation, capture and externalize an object s internal state so that the object can be restored to this state later.

34 Observer Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

35 State Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

36 Strategy Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

37 Template method Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template method lets subclasses redefine certain steps of an algorithm without changing the algorithm s structure.

38 Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Visitor

39 S.O.L.I.D. Single Responsibility Principle Open for extension, closed for modification Liskov Substitution Principle Interface Segregation Principle Dependency Inversion Principle

40 Grasp(Generalized Responsibility Assignment Software Patterns/Principles) Information Expert: Which class is responsible for what Creator: Who is responsible for the creation of an object Controller: communication between UI and service layer Low Coupling: classes are independent High Cohesion: class does one thing

41 Grasp Polymorfism: implementation belongs to a type Pure Fabrication: non-business domain class to implement a design principle Indirection: go-between class to connect two classes Protected Variations: Hide changes using interfaces or polymorfisme

42 Conclusion Design patterns are no silver bullet Design patterns is no cookbook Good developers use Design patterns intuitively Design patterns are a way of communicating