The Composite PatternAll References and Material From: Design Patterns,by (GOF!) E.Gamma, R.Helm, R.Johnson, J.Vlissides, Addison-Wesley, 2001

BTS530: Major Project Planning and Design

CompositeThe Problem:How do you treat a group or composition

structure of objects the same way (polymorphically) as a non-composite object?

Composite

A Suggestion/Solution:Define classes for composite and

atomic object so that they implement the same interface.

CompositeClassic example: Graphics applications

users can group components into larger components which can be grouped to form larger components…and so on.

problem: the code must treat primitive and container objects differently even though the user might treat them the same

Composite pattern uses recursive composition so clients don’t have to make a distinction

Design Patterns, p.163

Graphic

draw()add(Graphic g)

remove(Graphic)getChild(int)

Line

draw()

Picture

draw()add(Graphic g)remove(Graphic)getChild(int)

Rectangle

draw()

Text

draw()

1…n

forall g in graphics g.draw()

add g to list of graphics

graphics

Design Patterns, p.163

aPicture

aPicture aLine aRectangle

aText aLine aRectangle

Design Patterns, p.164

Component

operation()add(Component)

remove(Component)getChild(int)

Composite

operation()add(Component)remove(Component)getChild(int)

Leaf

operation()

1…n

forall c in children c.operation()

Client

children

Design Patterns, p.164

aComposite

aComposite aLeaf aLeaf

aLeaf aLeaf aLeaf

Design Patterns, p.165

CompositeComponent (Graphic):

declares the interface for objects in the compositionimplements common default behaviourdeclares an interface for managing child components

Leaf (Rectangle, Line, Text)has no childrendefines behaviour for primitives

Composite (Picture)defines behaviour for components having childrenstores child componentsimplements child-related operations through the

component interface

Design Patterns, p.165

CompositeThe Client:

uses the Component class interface to interact with objects in the composite structure

If recipient is Leafrequest handled directly

If recipient is Compositerequest is usually forwarded to child components,

possibly performing additional operations before and/or after forwarding

Design Patterns, p.165

Composite Exercise

draw a sequence diagram to illustrate: draw line l draw circle c group l and c into graphicA draw box b1 draw box b2 group b1 and b2 into graphicB group graphicB into graphicA select c in graphicA(assume lines, circles, boxes exist but not graphics)

: DrawingHandler

l : Line c : Circle A : Graphic b1 : Box B : Graphicb2 : Box

draw()

draw()

create()

add(l)

add(c)

draw()

draw()

create()

add(b1)

add(b2)

add(B)

getChild(c)