In this lesson you will learn how to use the Class browser. As
you progress through the Open University M206 course LearningBooks
you will find that this browser becomes more sophisticated, and
includes more classes. Most of these changes are listed in the
table below. John McGuinn
The Classes from previous LBs will also be present. I.e.
LB-13 will have Account, Frog, HoverFrog, Toad classes and
additional class Object.
The majority of these classes are in normal font weight.
Do NOT attempt to modify any of these classes, methods or
instance variables.
Classes, methods, instance variables that are in a bold
font. Are provided by the course team for you to amend,
or have been added by yourself.
These classes usually only appear in a single LearningBook. When you start adding or amending classes, methods
etc., keep a record of which book you use or you will
waste a lot of time searching through different books
looking for your work. See tip below.
Tip
Copy this table to a Word processor. Use the 2 blank columns to
insert the names of Classes and methods that you write or amend
Add other LearningBooks as required. A few minutes work now could
save you hours later in the course.
Magnitude and 11 subclasses.
Discusses the introduction of Amphibian class. Abstract class
LB-22.
Amphibian, Behaviour & 2
subclasses, CharTable, Dialog, Character, Date,
Time.
Note the introduction of the Amphibian class, totally changes
the Frog etc. Hierarchy.
Beverage Customer Customer StaffMember
VatLANAdmin VendingMachine VMAccount. Note Some of this LB may be optional.
Find, Edit
LB-31
Stream +9 Subclasses. Note Some of this LB may be optional.
File
LB-39
Collection-(List, Interval, and
Text) AddressBook AddressBookFixedFormat Model +2 Subclasses.
Using the GUI
LB-45
Initially no change. A set of executable
files can be downloaded nearer to the end of the course, that
will incrementally add additional classes, methods, instance variables
and class variables for the final Hospital, MECEBSC systems.
What
is the Class browser used for.
Writing amending removing allowable (The ones in bold text):-
Methods - Instance and Class.
Variables - Instance and Class.
A source of information:-
See what methods and variables a class has.
View the comments for classes, methods and variables.
See the hierarchy of classes.
Opening the Class Browser
For the exercises you are going to use LB-27. You will be able
to confirm what you have learnt so far in Tutorials 1-3.
Do NOT attempt at this stage to alter any of the code in the Class
Browser.
Exercise 1. Opening the Class Browser
Start the M206 LearningWorks Application.
Select LB-27 Developing Applications.
Click on the Open button.
In the new LB-27 Developing Applications: Practicals and
Notes window.
Click and hold down the left mouse button on Section
button. (Top right).
Slide the mouse pointer over Vending machine simulation.
Release the mouse button.
In the new LB-27 Developing Applications: Vending machine
simulation.
Click on the Class Browser Tab.
In the Class pane click on Account
Class Browser
Diagram Description
The Class Browser window for LB-27 should now look like the following.
The LB-27 Class browser, showing the Account class selected.
Tip. When working
in the class browser it helps to enlarge the size of the
window, a little bit on the width, and a lot on the length.
Towards the end of the course you will be designing windows,
with widgets similar to those shown
in the Class Browser diagram above.
Starting from the top you have 2 Tab widgets, with the Class
Browser tab selected. Note that this widget is not available
when you start designing your own windows.
The rest of the window is described in the table below.
Widget
Qty
Description
Labels
3
The labels state what the widget below the label
contains.
Class
Instance / Class variables. Depending on selected
radio button below.
Instance / Class methods. Depending on selected radio
button below.
List
1
Class. List all the Classes that are visible
at that stage of the course.
The Account class has been selected and is highlighted.
The list is indented, this indentation shoes the class hierarchy.
Selecting a class will alter what appears in the large pane
below.
1
Instance / Class variables. Depending on
selected radio button below.
Lists either the Instance or Class variables.
Clicking on a variable in this list will display a comment
in the bottom pane.
1
Instance / Class methods. Depending on selected
radio button below.
Lists either the Instance or Class methods.
Clicking on a method in this list will display the method
in the bottom pane.
Radio buttons
2
Instance and Class. Only one of these 2 buttons
can be selected. They control the labels and contents of the 2
panes above.
Action button
1
Find. Brings up a pop up menu.
Class
References to selected class
References to selected variable
References to methods...
Implementers of method...
1
Add. Brings up a radio button menu.
a class
an instance variable
an instance method
a class variable
a class method
and OK and Cancel buttons
1
Remove. Removes variables or methods that
are in bold text.
1
View. From LB-22 this button allows you to
look at metaclasses for methods like new.
1
Edit. I can't remember using this.
Label
1
A descriptive label for the edit box below.
Edit
1
The contents displayed in this edit box depend on
what has been selected in the top 3 list boxes.
Exercise 2a. Finding a Class.
Technique 1
You could just scroll up and down the list but as there are more
than 100 entries a better technique is required.
Click in the Class list pane, say on Object, and type in
the first letter or 2 of the required class, the list will
automatically start to home in.
Start to search for Toad, and type t. Toad is
found straight away.
Start another search by Clicking in the pane. Search
for ByteString by typing in by and ByteString is
found.
Start another search by Clicking in the pane. Search
for Character.
Type in c finds ClassDescription, continue
typing
h finds CharTable, continue typing
ara finds CharacterArray
you can complete the typing but this technique fails
to find Character.
Exercise 2b. Finding a Class. Technique 2
Click on the Find button and hold down the mouse button.
Move over Class... and release.
Read the text, note how we can use pattern matching.
Type in c*
Select Character from the list of about 8 classes.
Click OK
If you had typed in ch* or chara* etc. the list would
be shorter.
Exercise 3. Finding a Method
Hold down the left mouse button with the pointer over the
Find button.
Slide the pointer over Implementors of method...and
release.
Type in the method name, including the : (colon)
if a keyword method.
Click OK
Search for the following. plus:, holder
Note how you have a View window with a single class
in the upper pane, and the actual method in the lower
pane.
Search for the following. at:put:
Note how you have a View window with a multiple
classes in the upper pane, the method in the lower
pane displays the method for the selected class.
Exercise 4. Finding Examples
of a Method
This is useful to see how a method is used. You can find how the
method that you are interested in is used within other methods.
Hold down the left mouse button with the pointer over the
Find button.
Slide the pointer over References to method...and
release.
Type in the method name, including the : (colon)
if a keyword method.
Click OK
Exercise 5. Class
Hierarchy.
This can be seen in the Class pane. Object is at the top of the
list. All the other classes are indented.
The first indentation shows the direct subclasses of Object.
E.g.
Account
Amphibian. Note you are using the Class browser
in LB-27. There was no Amphibian class before LB-21,
Frog and Toad classes were direct subclasses
of Object class earlier in the course hierarchy
Behaviour
Beverage
Etc.
The second indentation shows the direct subclasses of one
of the direct subclasses of object.
Account does not have any subclasses. Note that
if you have created any subclasses for class Account,
they will only appear in the LB you used to create them.
Amphibian, has direct subclasses of Frog and
Toad. (See note above).
Behaviour, has a single direct subclass of ClassDescription.
Etc.
The third indentation, is similar to the second.
Frog has a direct subclass of HoverFrog.
Note that HoverFrog is NOT a subclass
of Toad
Now find your printout of the Collection class hierarchy. Then
in the Class browser.
Using any of the 3 techniques (Exercises 2a or b) locate
the Collection class.
Look at the 16 subclasses indented below the Collection
class.
Compare this to printed diagram that has 19 subclasses
of Collection class.
This is due to the fact that the printout was produced from
the larger list in LB-39, where 3 more classes were added.
If you have not spotted them they are listed in the table
above, under LB-39.
Get a feel and background understanding of this very important
class hierarchy, by reading the various comments. At this
stage you are not going into any detail. When you reach the
various chapters in the M206 course you can repeat these exercises
in more detail.
Find and verify the abstract classes.
Other classes that the course covers which you could look at are:-
Account
Boolean
Frog, HoverFrog, and Toad. Depending where
you are in the course, use a Class brower before LB-21 if
required.
Dialog- Look at the Class methods.
Magnitude and its subclasses, especially
Association
Character
Date Check out the Instance and class variables
and methods.
Time
Plane. The only class method, is very interesting,
pay particular attention to how it creates Student. The
only class method, is very interesting, pay particular
attention to how it creates a plane and sets the flight
identifier to aString.
Student. The only class method, is very interesting,
pay particular attention to how it creates and initializes
an instance of Student.
