Chair of Software Engineering 1 Introduction to Programming Bertrand Meyer Exercise Session 13 10....

Chair of Software Engineering

1

Introduction to Programming

Bertrand Meyer

Exercise Session 13

10. November 2008

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This week

Inheritance: Deferred classes & features Redefinition, renaming Observer Pattern

Genericity Hint for Assignment

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Let's play Lego!

BRICK

LEGO_BRICK

LEGO_BRICK_WITH_HOLE LEGO_BRICK_SLANTED

Inheritance Relation

of Lego bricks

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class BRICK

deferred class

BRICK

feature -- Access

width: INTEGER

depth: INTEGER

height: INTEGER

color: COLOR

volume: INTEGER

deferred

end

end

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Class LEGO_BRICK

class LEGO_BRICK

inherit BRICK

feature -- Access

number_of_nubs: INTEGER do Result := ... end volume: INTEGER do Result := ... endend

Inherit all features of class BRICK

New feature, calculateall nubs

Implementation of`volume'

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Class LEGO_BRICK_SLANTED

Feature `volume' is going to be redefined (=changed).

`volume' comes from LEGO_BRICK

class LEGO_BRICK_SLANTED

inherit LEGO_BRICK redefine volume end

feature -- Access

volume: INTEGER do Result := ... end end

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Class LEGO_BRICK_WITH_HOLE

class LEGO_BRICK_WITH_HOLE

inherit LEGO_BRICK redefine volume end

feature -- Access

volume: INTEGER do Result := ... end end

Feature `volume' is going to be redefined (=changed).

`volume' comes from LEGO_BRICK

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Notation

volume++

Notation:

Deferred *

Effective +

Redefinition ++

BRICK

LEGO_BRICK

LEGO_BRICK_WITH_HOLE LEGO_BRICK_SLANTED

+

++

volume*

volume+

*

volume++

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Deferred & effective

Deferred Deferred classes can possess deferred features A class with at least one deferred feature must be declared

as deferred A deferred feature does not have an implementation yet Attributes can't be deferred (because they don't need a

feature body anyways) Deferred classes cannot be instantiated and hence cannot

contain a create clause

Effective Effective classes do not possess deferred features (the

“standard case”) Effective features have an implementation of their feature

body (“do .. end”)

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Precursor

If a feature was redefined, but you still wish to call the old one, use the precursor keyword

volume: INTEGER do Result := Precursor - ... end

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Types in the source code

Expressions have a static type

For attributes and functions it is denoted after the declaration ':'

name: STRING

list: LINKED_LIST [INTEGER]

A class is the static part of a program, which contains only static types

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Type of an object

The type of an object is set upon creation

local

lego: LEGO_BRICK

brick: BRICK

create {LEGO_BRICK} brick -- explicit

-- same as

--create lego; brick := lego

create lego -- implicit Objects belong to the dynamic part of a program -

>dynamic types

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Dynamic types

local

a_bag: SOME_BAG

do

create {MONEY_BAG} a_bag.put (10)

What is the static type of `a_bag'?

What's the dynamic type of the object denoted by `a_bag'?

Hands-On

SOME_BAG

MONEY_BAG


The Observer Pattern

PUBLISHER*

GUI_CLASS

subscribedattachdetachtrigger

OBSERVER*

APP_CLASS

Deferred (abstract)

Effective (implemented)

*+

Inherits fromClient (uses)

update*subscribe

update+


Observer pattern

Publisher keeps a list of observers:

subscribed: LINKED_LIST [OBSERVER]

To register itself, an observer may execute

subscribe (some_publisher)

where subscribe is defined in OBSERVER:

subscribe (p: PUBLISHER)-- Make current object observe p.

requirepublisher_exists: p /= Void

dop.attach (Current)

end


Attaching an observer

In class PUBLISHER:

attach (s: OBSERVER)-- Register s as subscriber to current publisher.

requiresubscriber_exists: s /= Void

do

subscribed.extend (s)

end

The invariant of PUBLISHER includes the clause

subscribed /= Void

(List subscribed is created by creation procedures of PUBLISHER)


trigger -- Ask all observers to -- react to current event.

do from subscribed.start until subscribed.after loop subscribed.item. subscribed.forth end

end

Each descendant of OBSERVER defines its own versionof update

Triggering an event

update

PUBLISHER*

GUI_CLASS

attachdetach

OBSERVER*

APP_CLASS

update*

update+


Changing the implementation of the pattern

How would the implementation of the Observer pattern change if:

each observer subscribes not itself, but one of its routines to the publisher

when the event occurs, the publisher must call these subscribed routines with information about the event

Hands-On


Exercise: a news agency

Consider you must create an application which allows a news agency to dispatch news to various papers, radio and TV stations, etc.

How would you design and implement this application?

Hands-On


Genericity

But what if a certain box should only contain Lego bricks, i.e. all kinds of them (slanted, 2x4, etc.)?

classBOX [G]

featurewipe_out is

...

put (v: G) is...

end


For Assignment 7

Vector Geometry

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End exercise session 15

Chair of Software Engineering 1 Introduction to Programming Bertrand Meyer Exercise Session 13 10....

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