design patterns examples of smart use of inheritance and polymorphism: composite pattern state...

Design Patterns

Examples of smart use of inheritance and polymorphism:

Composite Pattern

State Pattern

FEN 2014 UCN Teknologi/act2learn 1

UCN Teknologi/act2learn 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


(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. 1995.)

FEN 2014


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


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


Example: Composite-pattern

1

Circle Rectangle Picture

Shape

0..*0..*

Position

1

Composite: Graphical editor, Word processor, Inventories etc..

(What will a Show()-method look like on Shape, Circle, Picture etc.)

FEN 2014


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


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 CircleFEN 2014


public class Picture : Shape{ List<Shape> 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 2014 10UCN Teknologi/act2learn

• Can be used to model general tree structures

General Tree

FEN 2014 11UCN Teknologi/act2learn

AbstractTreeElement

Leaf Tree

0..*

More Examples/Exercises

• Organisational Structures (example)• Project Management (exercise)

• Composite Pattern can be found in the .NET libraries: demos\RecursiveDirectoryTraverse

UCN Teknologi/act2learn 12FEN 2014

State Pattern

Models state machines

(DFAs: Deterministic Finite Automata).


Example: Controlling a printer(from: http://www.go4expert.com/forums/showthread.php?t=5127)


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.

http://www.go4expert.com/forums/showthread.php?t=5127

Implementations of DFAs• Choose an appropriate data structure to represent states:

– A List or– A Dictionary– A …?


event

state

2

states

n-1i0

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!


Class Diagram


Note how the design reflects the structure of the state machine


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


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.


design patterns examples of smart use of inheritance and polymorphism: composite pattern state...

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