Concepts of Programming Languages

logic expressions and conventional control statements. One of the Smalltalk

control constructs is illustrated in the example in the next subsection.

2.15.3 Evaluation

Smalltalk has done a great deal to promote two separate aspects of comput-

ing: graphical user interfaces and object-oriented programming. The window-

ing systems that are now the dominant method of user interfaces to software

systems grew out of Smalltalk. Today, the most significant software design

methodologies and programming languages are object oriented. Although the

origin of some of the ideas of object-oriented languages came from SIMULA

67, they reached maturation in Smalltalk. It is clear that Smalltalk’s impact on

the computing world is extensive and will be long-lived.

The following is an example of a Smalltalk class definition:

"Smalltalk Example Program"

"The following is a class definition, instantiations

of which can draw equilateral polygons of any number of

sides"

class name Polygon

superclass Object

instance variable names ourPen

numSides

sideLength

"Class methods"

"Create an instance"

new

^ super new getPen

"Get a pen for drawing polygons"

getPen

ourPen <- Pen new defaultNib: 2

"Instance methods"

"Draw a polygon"

draw

numSides timesRepeat: [ourPen go: sideLength;

turn: 360 // numSides]

"Set length of sides"

length: len

sideLength <- len

"Set number of sides"

sides: num

numSides <- num

2.15 Object-Oriented Programming: Smalltalk 87