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CS 535 Object-Oriented Programming
Spring Semester, 2003
Classes Part 1
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© 2003, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 27-Jan-03
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Contents of Doc 3, Classes Part 1
References
Ralph
Johnson's University of Illinois, Urbana-Champaign CS 497 lecture notes,
http://st-www.cs.uiuc.edu/users/cs497/
Reading
VisualWorks
Application Developer’s Guide, Chapter 1 pages 44-56. The VisualWorks
Application Developer’s Guide is the file
AppDevGuide.pdf
in the docs directory of the VisualWorks directory. On Rohan this file is
/opt/smalltalk/vw7nc/doc/AppDevGuide.pdf
Procedural
Paradigm
Programs
consists of data and procedures
Procedures
manipulate data passed to them
Programs
organized by
Object-Oriented
Paradigm
Program
consists of objects interacting
Objects:
- Know
things
- Do
things
- Collaborate
with other objects to perform task
Program
are organized by:
- Classes
- Inheritance
hierarchies
- Subsystems
Software
Development & Modeling
Each
stage of software development is modeling
- Modeling
the problem
- Modeling
the solution
- Making
the model run on a computer
- Fixing
and extending your model
Modeling
& Objects
Base
system design on structure of problem
Objects
are the bases of OO design
- Objects
considered more natural
- Match
way people think
Objects
come from the problem domain
Example
Problem
A
refrigerator has a motor, a temperature sensor, a light and a door. The motor
turns on and off primarily as prescribed by the temperature sensor. However,
the motor stops when the door is opened. The motor restarts when the door is
closed if the temperature is too high. The light is turned on when the door
opens and is turned off when the door is closed.
What
are some objects?
What
are some relationships between objects?
What
are some attributes of the objects?
What
are some behaviors of the objects?
Two
Views of Objects & Classes
Model
View verses Language Syntax View
Object
- Modeling Perspective
Abstraction
in the domain with both
- Attributes
(things it knows)
- Behavior
(things it can do)
Class
- Modeling Perspective
A
class is a specification for an arbitrary number of objects
Objects
that share the same behavior belong to the same class
An
object is an instance of a class
VisualWorks
Tools & Windows
Smalltalk
classes are created and viewed with a class browser. As we cover Smalltalk
class syntax it will be useful for you to work with a browser. So we will first
cover some aspects of the browser, then we will look at the language details.
There is a chicken-and-egg problem here. To understand what the browser is
showing, you need to know the language details. It helps to use the browser to
understand the language details.
Launcher
The
Launcher is the window (see below) in VisualWorks used to open (launch) tools.
Don’t close the Launcher window.
Viewing
a Class
In
most languages, source code is stored in a flat file and viewed with a text
editor. Smalltalk provides a browser to view and edit classes. We will look at
some of the features of the system browser using the Point class as an example.
System
Browser
The
System Browser is used to view, manage and create code in VisualWorks. Open a
System Browser by:
Selecting
the “System” item of the “Browse” menu in the Launcher or
Clicking
on the “System Browser” icon in the icon bar of the Launcher
We
will look at the Point class. In the Find window enter Point and hit the return
key. You should get something like:
Point
Definition Partially Explained
Smalltalk.Core defineClass: #Point
superclass: #{Core.ArithmeticValue}
indexedType: #none
private: false
instanceVariableNames: 'x y '
classInstanceVariableNames: ''
imports: ''
category: 'Graphics-Geometry'
Smalltalk.Core
- Namespace
in which Point class is defined
Core.ArithmeticValue
- Parent
class of Point
x
and y
- Points
instance variables
- All
methods in Point can access x & y
- No
direct access to x & y outside of Point
- Each
Point object has separate copy of x & y
Using
Point
Open
a workspace by clicking on the workspace in the launcher.
In
a workspace type:
| a b |
a := Point "x:y: is a class method that creates a point object"
x: 1
y: 1.
b := Point
x: 3
y: -1.
c := a + b.
Transcript
show: a x printString;
cr;
show: b x printString.
cr;
show: c printString.
Now
select the all the text and execute it. You execute text by:
Right
clicking in the workspace and selecting “Do it” in the popup menu or
Clicking
on the Do it icon or
Control-d
Viewing
Point Methods
If
you click on a category of methods the browser shows all methods in that
category. If you click on a method, the bottom pane will display the source
code of the method. Instance methods are methods that you send to an instance
of the class.
Using
Part of a Class Name
In
the find window you can type in only part of the class name. The browser will
find all matches for the partial name. If there is only one match the browser
will show that class. If there are multiple matches the browser will give a
list to select from. When I give the name: Poi
I
get the following list of classes:
Viewing
Inherited Methods
The
view of the Point class only shows the methods defined in the Point class.
However Point class inherits methods from its parent class. We can view these
methods by viewing the Parent classes or by setting the visibility for the
browser. To set the visibility of the browser select the
“Visibility” item in the “Method” menu of the browser.
The bottom of the popup menu shows the inheritance hierarchy of the class. You
can select how high in the hierarchy you are interested in. Below I select to
show all methods in the classes Magnitude, ArithmeticValue and Point.
Now
the name of Point methods will indicate if they are defined in a parent class.
Viewing
the Class Hierarchy
If
you select the “Hierarchy” tab in the browser you will see the
inheritance hierarchy. Now by selecting a class you can look at the methods and
instance variables defined in the class.
Viewing
Methods that Use Instance Variables
There
are several ways to find methods that access an instance variable defined in a
class. The first method is to select the “Instance Variable” tab.
When
you do this the third list pane from the left contains a list of the instance
variables of the class, in this case x & y. When you select one of the
variables the fourth pane lists all the methods in the current class that
access that variable.
Viewing
Methods in all classes that Read or Write Instance Variables
Select
the “Instance Variables” item in the Class menu of the browser. The
submenu will allow you select all methods that access an instance variable,
read a variable or write to a variable. When you select an option you will be
asked to select one variable from a list of instance variables. You will then
get a list of methods in the class inheritance hierarchy that access, read or
write to the variable.
Finding
Methods
In
the find field of the System browser you can type the name of a method. If the
method is not a keyword method you will be asked if you wish to look for a
method. If there is more than one implementation of the method you will be give
a list to select from. This does not work well for non-keyword methods, as the
browser will first try to find a match with a class first.
Often
one is looking at a source code and comes across a method you would like to
look up. For example go to the Point class and in the arithmetic category (or
protocol) of methods find the abs method. The source of this method is:
abs
"Answer a new Point whose x and y are the absolute values of the receiver's
x and y."
^x abs @ y abs
What
is this @ message? To find out more about it we would like to see its
implementation. To see the implementations of the @ message select the
“@” item in the “Implementators” item of the
“Method” menu of the system browser. A new browser will open
listing all the classes that implement @.
Note
that right clicking in the forth list pane from the left will also produce the
Method menu.
Viewing
Class Methods
To
view the class methods click on the class radio button in the browser.
Class
Comments
When
you select a class in the browser (without selecting a method category). The
code pane at the bottom of the browser has five tabs. Select the
“Comment” tab to view the class comment.
Code
Critic
The
code pane at the bottom of the browser has a code critic tab. Select the types
of problems want the code critic to examine, then click the “Check”
button. The code critic will examine the currently selected code unit. That is
you just select a category of classes, all classes in that category are
checked. If you also select a class, only that class will be checked. If you
also select a category of methods, only the methods in that category will be
checked. By the way I do use the code critic to help me grade student
assignments!
Defining
a New Class
We
will go through the steps of creating a new class called SimpleCircle.
Creating
a Category
First
we will create a category to keep our classes separate from the system classes.
Open
a system browser by clicking on the "System Browser" icon in the Launcher.
Select the “New...” menu item from the Category menu in the System
Browser. (You could instead right-click in the category pane.)
You
get a dialog window, in which to enter the new category.
Creating
a Class in the new Category
Select
the CS535 category in the left most pane. Now select the "New" item in "Class"
menu of the browser.
In
the lower pane you will the template:
Core
defineClass: #NameOfClass
superclass:
#{NameOfSuperclass}
indexedType:
#none
private:
false
instanceVariableNames:
'instVarName1 instVarName2'
classInstanceVariableNames:
''
imports:
''
category:
'CS535'
Edit
the template to look like:
Core defineClass: #SimpleCircle
superclass: #{Object}
indexedType: #none
private: false
instanceVariableNames: 'origin radius '
classInstanceVariableNames: ''
imports: ''
category: 'Course-Examples'
Accept
the changes by either:
Right-clicking
in the lower pane and selecting the "Accept" item in the popup menu.
Or
use control-s keyboard short cut
When
you do this, you will see the class listed in one of the upper level panes.
Using
the Class
The
class SimpleCircle does not do much, but we can still create an instance. In a
workspace evaluate the following code with "Print it". What happens? Evaluate
the code with "Inspect it". What happens?
SimpleCircle new
Adding
Methods
Now
to add a method to the class. Make sure the class is selected in the browser.
Select the "New..." item in the Protocol menu.
You
will be prompted to enter a category name. Enter "accessing". Once this is done
the lower pane will contain the template:
message selector and argument names
"comment stating purpose of message"
| temporary variable names |
statements
Edit
this code to be:
radius: aNumber
radius := aNumber
Accept
these changes. (Right-click and select “Accept” or use control-s).
After accepting these changes select all the text in the lower pane and replace
it with:
area
^radius * radius * Float pi
Accept
these changes (right-click or control-s). Your class now has two methods: area
and radius.
In
a workspace evaluate the following code with "Print it". Is your result
reasonable?
| circle |
circle := SimpleCircle new.
circle radius: 2.0.
circle area
You
may wish to save your image so you do not lose these changes. To save your
image click on the save icon in the launcher
.
If you use a different name for the image, remember to start that image next
time you run VisualWorks.
Versions
of Methods
In
the browser select the area method in the SimpleCircle class. Change it to be:
area
^radius + radius + Float pi
Save
the changes and try running the above program to compute the area of a circle.
You should get the wrong answer.
Now
to browse the different versions of the method. With the area method selected
in the method list pane (the right most list pane in the browser) select the
“Browse Versions” item in the Method menu of the System Browser.
You
will get a new window that lists the versions of the method. In this window you
can view the source of each version. Select a version and use the
“Replay” menu to replay a version, making it the current version.
Select the “Show Conflicts” item in the “Show” menu
then select a version of the method. In two panes at the bottom of the window
you will see how the selected method differs from the current version of the
method.
Copyright ©, All rights reserved.
2003 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.
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