64 CHAPTER5. OBJECTSANDGRAPHICS
Figure5.2:Graphicswindow withtwo pointsdrawn.polygonsandtext.Eachoftheseobjectsis createdanddrawnin a similarfashion.Hereis a sampleinteraction
todraw variousshapesintoaGraphWin.
Open a graphicswindow
win = GraphWin(’Shapes’)
Draw a red circlecentered at point (100,100) with radius 30
center = Point(100,100)
circ = Circle(center,30)
circ.setFill(’red’)
circ.draw(win)
Put a textuallabel in the center of the circle
label = Text(center,"Red Circle")
label.draw(win)
Draw a squareusing a Rectangle object
rect = Rectangle(Point(30,30), Point(70,70))
rect.draw(win)
Draw a linesegment using a Line object
line = Line(Point(20,30),Point(180, 165))
line.draw(win)
Draw an ovalusing the Oval object
oval = Oval(Point(20,150),Point(180,199))
oval.draw(win)
Trytofigureoutwhateachofthesestatementsdoes.If youtypetheminasshown,thefinalresultwilllook
like Figure5.3.
5.3 UsingGraphicalObjects
Someoftheexamplesintheabove interactionsmaylooka bitstrangetoyou. To reallyunderstandthe
graphicsmodule,weneedtotake anobject-orientedpointofview. Recall,objectscombinedatawith
operations.Computationis performedbyaskinganobjectto carryoutoneofitsoperations.Inordertomake
useofobjects,youneedtoknow how tocreatethemandhow torequestoperations.
Intheinteractive examplesabove, wemanipulatedseveraldifferentkindsofobjects:GraphWin,Point,
Circle,Oval,Line,Text, andRectangle. Theseareexamplesofclasses. Everyobjectis aninstance
ofsomeclass,andtheclassdescribesthepropertiestheinstancewillhave.
Borrowinga biologicalmetaphor, whenwesaythatFidois a dog,weareactuallysayingthatFidois a
specificindividualinthelargerclassofalldogs. InOOterminology, Fidois aninstanceofthedogclass.