Design Patterns Examples of smart use of inheritance and polymorphism: Composite Pattern State Pattern FEN 2014UCN Teknologi/act2learn1

2 (OO) Design Patterns The concept of patterns originates from architecture (Christopher Alexander, 1977): “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice” (Christopher Alexander e. a.: “A Pattern Language”. Oxford University Press, New York, 1977.) FEN 2014

UCN Teknologi/act2learn3 (OO) Design Patterns A well known and widely accepted concept in software engineering Developed in the early 1990s and published by Gamma e.a. (“Gang of Four”, GoF) in 1995: “(…) design patterns (…) are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.” (Erich Gamma e.a.:”Design Patterns. Elements of Reusable Object-Oriented Software”. Addison-Wesley ) FEN 2014

UCN Teknologi/act2learn4 The benefits of patterns A pattern captures a proven good design: –A pattern is based on experience –A pattern is discovered – not invented It introduces a new (and higher) level of abstraction, which makes it easier: –to talk and reason about design on a higher level –to document and communicate design One doesn’t have to reinvent solutions over and over again Patterns facilitate reuse not only of code fragments, but of ideas. FEN 2014

UCN Teknologi/act2learn5 Patterns as a learning tool It is often said that good skills in software construction require experience and talent …and neither can be taught or learned at school Patterns capture experience (and talent) in a way that is communicable and comprehensible …and hence experience can be taught (and learned) So one should put a lot of effort in studying patterns FEN 2014

UCN Teknologi/act2learn6 Example: Composite-pattern Composite: Graphical editor, Word processor, Inventories etc.. (What will a Show()-method look like on Shape, Circle, Picture etc.) FEN 2014

UCN Teknologi/act2learn7 public abstract class Shape { private int id; private String color; private int xpos; private int ypos; public void MoveTo(Position newPos){ // PRE none // POST pos'=newPos } public abstract void Show(); // PRE none // POST the shape is drawn public abstract void Hide(); // PRE none // POST the shape is hidden public abstract float Area(); // PRE none // POST Computes the area with 4 digits. }//end Shape FEN 2014

UCN Teknologi/act2learn8 public class Circle: Shape{ private int r; //radius public void Show(){ //PRE none //POST the circle is drawn //Must be implemented using appropriate //graphical routines } public void Hide(){ //PRE none //POST the circle is hidden //Must be implemented using appropriate //graphical routines } public override float Area() { return (float)(r*r); } }//end Circle FEN 2014

UCN Teknologi/act2learn9 public class Picture : Shape{ List pictList; // operationer til at tilføje og slette figurer mv. public void Show(){ //PRE none //POST The composite shape is drawn foreach(Shape s in pictList) s.show(); } public override float Area() { float result=0; for(int i=0;i<pictList.Count;i++) { result= result+pictList[i].Area(); } return result; }//end Picture Dynamic type definesshow() Static type FEN 2014

Composite Pattern FEN UCN Teknologi/act2learn Can be used to model general tree structures

General Tree FEN UCN Teknologi/act2learn AbstractTreeElement Leaf Tree 0..*

More Examples/Exercises Organisational Structures (example)Organisational Structures Project Management (exercise)Project Management Composite Pattern can be found in the.NET libraries: demos\RecursiveDirectoryTraverse demos\RecursiveDirectoryTraverse UCN Teknologi/act2learn12FEN 2014

State Pattern Models state machines (DFAs: Deterministic Finite Automata). FEN 2014UCN Teknologi/act2learn13

Example: Controlling a printer (from: FEN 2014UCN Teknologi/act2learn14 To implement this, we need a set of events E, and a set of states S, and a transition function t: t(E, S) -> S that gives the next state.

Implementations of DFAs Choose an appropriate data structure to represent states: –A List or –A Dictionary –A …? FEN 2014UCN Teknologi/act2learn15 event state 2 states n-1 i 0 s1 Find a way to implement the transition function: –A matrix perhaps? Difficult to maintain.

OO Implementation State is an object State Pattern can be applied: –abstract class State specifies one or more abstract methods: transition(-) – returns state corresponding to next event action(-) – if any processing is to be done when a transition occurs (code generation, event handling etc.) each concrete state inherits from State and implements the abstract methods The controller loop uses references having the abstract class State as static type. Polymorphism and dynamic binding handles the rest! FEN 2014UCN Teknologi/act2learn16

Class Diagram FEN 2014UCN Teknologi/act2learn17

Note how the design reflects the structure of the state machine FEN 2014UCN Teknologi/act2learn18 View source Exercise: Add a new event: “Paper Jam”, which can occur in the state “Printing” and brings the printer in an error state where it remains stopped.

State Pattern FEN 2014UCN Teknologi/act2learn19

The Classes of the Pattern Context : Defines the objects that we want maintain state information about (for instance DialogBox). This class has a reference (static type: ContextState – the abstract super class) to some concrete state (that is an object of one of the sub classes – dynamic type). ContextState: The abstract super class defining a common interface to all the concrete states. ConcreteState1,...: The sub classes to ContextState. One sub class to each state in the DFA. The key to the design is in the processEvent-method, which takes an event as input and returns the next state. The method is abstract in ContextState and implemented (differently) in the subclasses. FEN 2014UCN Teknologi/act2learn20