1. OOP: Creating a Class Topics More OOP concepts
An example that creates a ASSET class and shows how it might be used
Extend the ASSET class to subclasses of STOCKS, BONDS and SAVINGS.

2. Matlab’s Built-in Classes
others in additional Toolboxes
You can define your own Matlab classes:
data structure (the fields) is a Matlab struct
methods (functions) are stored in special directory that is a subdirectory of a directory in the Matlab path
a constructor is a method that is used to create an object (an instance of a class)
ARRAY
char
NUMERIC
cell
structure
java
class
function
handle
user class
int8, uint8,
int16, uint16,
int32, uint32
single
double
sparse

3. Let’s create a Matlab class called ASSET
What is an ASSET?
it is an item with a monetary value
examples could be a stock, or a bond, or a savings account
it could also be a physical item with a monetary value (car, house)
each asset has a different way of defining its monetary value
ASSET class
Superclass (Parent)
Properties:
descriptor
date
current_value
Subclass (Child)
Inherited Fields:
descriptor
date
current_value
STOCK Fields:
num_shares
share_price
asset
STOCK class
Inherited Fields:
descriptor
date
current_value
BOND Fields:
int_rate
asset
BOND class
Inherited Fields:
descriptor
date
current_value
SAVINGS Fields:
int_rate
asset
SAVINGS class
Inherited Fields:
descriptor
date
current_value
PROP Fields:
mort_rate
asset
PROP class

4. Creating the Object: Constructors
An object must first be created using the “definition” provided in the Class (this is called instantiation)
this is like defining a new double variable (e.g., A(3,1)=5.4)
the created object now has access to all the methods that are defined in the class from which it was created
OOP languages have sophisticated ways to keep these methods and data known only inside the object or shared with one or more other classes of objects (Matlab is very simple as we’ll see later)
we’ll need a special method called a constructor to create an object (all classes must define a constructor method)
in some OOP languages, you must also provide a destructor method to remove an object when you’re done

5. ASSET Constructor
function a=asset(varargin)
% ASSET constructor function for asset object
% Use: a = asset(descriptor, current_value)
% Fields:
% descriptor = string
% date = date string (automatically set to current)
% current_value = asset value ($)
switch nargin
case 0 % no arguments: create a default object
a.descriptor = 'none';
a.date = date;
a.current_value = 0;
a = class(a,'asset'); % this actually creates the object
case 1 % one arg means return same
if (isa(varargin{1},'asset'))
a = varargin{1};
else
error ('ASSET: single argument must be of asset class.')
end
case 2 % create object using specified values
a.descriptor = varargin{1};
a.current_value = varargin{2};
a = class(a,'asset'); % create the object
otherwise
error('ASSET: wrong number of arguments.')
varargin is a cell array with all the nargin arguments
a is a structure where:
a.date=date string
All constructors must handle these 3 cases
Polymorphism: different use of class() in constructor

6. Displaying the Object…
We need to add a method to display the object
Matlab always calls display for this purpose
we must add a method in our ASSET class…
There are lots of possible variations depending on how you want the display to look…
function display(a)
% DISPLAY(a) displays an ASSET object
str = sprintf('Descriptor: %s\nDate: %s\nCurrent Value:%9.2f',...
a.descriptor, a.date, a.current_value);
disp(str)
- Example showing how display() is used if the semicolon is omitted in a command line
>> a=asset('My asset', 1000)
Descriptor: My asset
Date: 29-Nov-2001
Current Value:

7. Comments
Every class must define a constructor and a display method (or it must be inherited from a superclass)
by convention, the constructor should handle at least the following 3 situations:
create an empty object if no arguments are supplied, or
make a copy of the object if one is supplied as the only argument. or
create a new object with specified properties.
The class() function (method) is polymorphic:
class(A) if used anywhere will return the type of variable A
class(a,’asset’) is only allowed inside a constructor and it tags a as being of class (type) ‘asset’
class(a,’asset’,parent1) tags a as above and also it inherits methods and fields from a parent1 object (class).

8. Examples with our new class
Try the following examples:
Make sure you have created a subdirectory in the Matlab work directory (or a directory in the Matlab path)
copy the functions we’ve described into this directory
type in the examples at the Command prompt…
>> a=asset('My asset', 1000)
Descriptor: My asset
Date: 10-Oct-2002
Current Value:
>> b=asset(a);
>> whos
Name Size Bytes Class
a x asset object
b x asset object
Grand total is 92 elements using 2428 bytes
>> c=asset
Descriptor: none
Current Value:
Creates an object
Creates a copy of a
Creates an empty object

9. Our Class has no Methods yet…
Arithmetic operations are not defined
>> a+b
??? Error using ==> +
Function '+' not defined for variables of class 'asset'.
>> x=a(1);
>> x
Descriptor: My asset
Date: 29-Nov-2001
Current Value:
>> class(x)
ans =
asset
>> whos
Name Size Bytes Class
a x asset object
b x asset object
c x asset object
x x asset object
Grand total is 88 elements using 1664 bytes
Using an index simply references the entire object (x is a copy of a)

10. Listing (getting) Property Values from Object
We will want to extract property values from an object
GET is a “standard” name
function val = get(a, prop_name)
% GET: Get asset properties from specified object
if ~ischar(prop_name), error('GET: prop_name must be string.'), end
prop_name = lower(prop_name(isletter(prop_name))); %remove nonletters
if (length(prop_name) < 2)
error('GET: prop_name must be at least 2 chars.')
end
switch prop_name(1:2)
case 'de'
val = a.descriptor;
case 'da'
val = a.date;
case 'cu'
val = a.current_value;
otherwise
error(['GET: ',prop_name,' is not a valid asset property.']);
Must check for valid input
Note the tricky way to remove all nonletters in the string !
You can test for some or all of the property name string (shortens how much the user must type)

11. Changing Property Values in an Object
W must be able to change the values of some of the properties in the object:
we’ll create a method (function) called get() for this
it will be part of the asset class located in directory
function a = set(a,varargin)
% SET: set asset properties and return updated object. More than one
% property can be set but must provide: ‘prop_name’, value pairs
if rem(nargin,2) ~= 1
error('SET: prop_name, values must be provided in pairs.')
end
for k=2:2:nargin-1
prop_name = varargin{k-1};
if ~ischar(prop_name), error('SET: prop_name must be string.'), end
prop_name = lower(prop_name(isletter(prop_name))); %remove nonletters
value = varargin{k};
switch prop_name(1:2)
case 'de'
a.descriptor = value;
case 'da'
a.date = value;
case 'cu'
a.current_value = value;
otherwise
error('SET: invalid prop_name provided.')
NOTE: you should include code to check for proper values as well.

12. Example using get and set
Try the following examples…
Make sure you have created a subdirectory in the Matlab work directory (or a directory in the Matlab path)
copy the functions we’ve described into this directory
type in the examples at the Command prompt…
>> a
Descriptor: My asset
Date: 10-Oct-2002
Current Value:
>> get(a,'Descriptor')
ans =
My asset
>> a=set(a,'CurrentValue',2000)
Current Value:
>> a=set(a,'Date',datestr(datenum(date)+1))
Date: 11-Oct-2002
Get a property value
Set a property value
Change date…

13. Important Notes We didn’t need a return value using get, but…
We MUST show the same object we’re modifying as the return value for set
this is because Matlab does not allow us to modify the values of the arguments in a function and have them returned to the calling workspace!!!
We could use the assignin() function to avoid this problem (see also the evalin() function for another variation on this)
Other OOP languages don’t have this problem…
This is perhaps the most awkward, inelegant aspect of OOP in Matlab. It is VERY non-standard when compared to Java, C++ or SmallTalk, etc.
set() must return the same object as provided in the argument list
>> a=set(a,'CurrentValue',2000)
Descriptor: My asset
Date: 29-Nov-2001
Current Value:

14. Summary… OOP has let us create an entirely new data type in Matlab
this is done by creating a class to define this data type
we must also create all the methods (functions) that can be used with this special data type
the internal details are hidden inside the class
What else could we do with this new class?
create a subclass of assets called STOCK that will include fields for data specific to a stock asset (see next slide)
the methods for this new subclass will be in subdirectory
we could continue to create subclasses for BOND and PROPERTY types of assets, each with their own extended data values and perhaps new methods.
AE6382 Design Computing
Fall 2006

15. Where to from here?
With only a little modification we can create an entirely new subclass that is based on our STOCK class
We can do this by specifying that our new class will inherit the structure and methods from a parent or superclass (or from several parent classes).
This is called “inheritance” and is an important feature of OOP.
Consider a BOND class
It has an identical structure to a STOCK,
But there will be different properties
We will create new display, get and set methods
We will use the set and get methods from ASSET to modify properties in the parent

16. Summary Learning Objectives Action Items OOP concepts
ASSET & STOCK classes
Summary
Action Items
Review the lecture
Perform the exercises
See if you can create the BOND class and get it working like we did for the STOCK class.
Consider how this might be extended to other applications