Documentation

Procedural Program Design

In procedural programming, your design focuses on the steps that must execute to achieve a desired state. Typically, you represent data as individual variables or fields of a structure. You implement operations as functions that take the variables as arguments. Programs usually call a sequence of functions, each one of which is passed data, and then returns modified data. Each function performs an operation or many operations on the data.

Object-Oriented Program Design

The object-oriented program design involves:

After performing this analysis, you define classes that describe the objects your application uses.

Classes and Objects

A class describes a set of objects with common characteristics. Objects are specific instances of a class. The values contained in an object's properties are what make an object different from other objects of the same class. The functions defined by the class (called methods) are what implement object behaviors that are common to all objects of a class.

Using Objects in MATLAB Programs

The MATLAB language defines objects that are designed for use in any MATLAB code. For example, consider the try/catch programming construct.

If the code executed in the try block generates an error, program control passes to the code in the catch block. This behavior enables your program to provide special error handling that is more appropriate to your particular application. However, you must have enough information about the error to take the appropriate action.

MATLAB provides detailed information about the error by passing an MException object to functions executing the try/catch blocks.

The following try/catch blocks display the error message stored in an MException object when you call a function (surf in this case) without the necessary arguments:

try
   surf
catch ME
   disp(ME.message)
end

Not enough input arguments.

In this code, ME is an object of the MException class, which the catch statement creates to capture information about the error. Displaying the value of the object message property returns the error message (Not enough input arguments). Your program can access other properties to get information about the error.

List all the public properties of an object with the properties function:

properties(ME);

Properties for class MException:

    identifier
    message
    cause
    stack

Objects Organize Data

Properties store the information returned in MException objects. Reference a property using dot notation, as in ME.message. This reference returns the value of the property. For example, assign the property value to a variable.

msg = ME.message;

The stack property contains a MATLAB struct:

s = ME.stack

s = 

  struct with fields: 

    file: D:\myMATLAB\matlab\toolbox\matlab\graph3d\surf.m
    name: 'surf'
    line: 49

You can treat ME.stack as a structure and reference its fields without assigning the value:

f = ME.stack.file;
disp(f)

D:\myMATLAB\matlab\toolbox\matlab\graph3d\surf.m

The file field of the struct contained in the stack property is a character array:

c = class(ME.stack.file);
disp(c)

char

You could, for example, use a property reference in MATLAB expressions:

if ME.stack.name == 'surf'
   disp('Error in surf')
end

Objects Manage Their Own Data

You could write a function that generates a report from the data returned by MException object properties. This function could become complicated because it would have to be able to handle all possible errors. Perhaps you would use different functions for different try/catch blocks in your program. If the data returned by the error object must change, you would have to update the functions to use the new data.

Objects define their own operations as part of their interface. The MException object can generate its own report. The methods that implement an object's operations are part of the object definition (that is, specified by the class that defines the object). The object definition can be modified many times, but the interface your program use does not change. Objects isolate your code from the object's code.

To see what methods exist for MException objects, use the methods function:

methods(ME)

Methods for class MException:

addCause       getReport      ne             throw          
eq             isequal        rethrow        throwAsCaller          

Static methods:
last   

You can use these methods like any other MATLAB statement when there is an MException object in the workspace. For example:

rpt = ME.getReport;
disp(rpt)

Error using surf (line 49)
Not enough input arguments.

Objects often have methods that overload (redefined for the particular class of the object) MATLAB functions. Overloading enables you to use objects just like other values. For example, MException objects have an isequal method. This method enables you to compare these objects in the same way you would compare variables containing numeric values. If ME1 and ME2 are MException objects, you can compare them with this statement:

isequal(ME1,ME2)

However, what really happens in this case is MATLAB calls the MException isequal method because you passed MException objects to isequal.

Similarly, the eq method enables you to use the == operator with MException objects:

ME == ME2

Objects should support only those methods that make sense. For example, it would probably not make sense to multiply MException objects so the MException class does not implement methods to do so.

Understand a Problem in Terms of Its Objects

Thinking in terms of objects is simpler and more natural for some problems. Think of the nouns in your problem statement as the objects to define and the verbs as the operations to perform.

Consider the design of classes to represent money lending institutions (banks, mortgage companies, individual money lenders, and so on). It is difficult to represent the various types of lenders as procedures. However, you can represent each one as an object that performs certain actions and contains certain data. The process of designing the objects involves identifying the characteristics of a lender that are important to your application.

Identify Commonalities.  What do all money lenders have in common? All MoneyLender objects can have a loan method and an InterestRate property, for example.

Identify Differences.  How does each money lender differ? One can provide loans to businesses while another provides loans only to individuals. Therefore, the loan operation might need to be different for different types of lending institutions. Subclasses of a base MoneyLender class can specialize the subclass versions of the loan method. Each lender can have a different value for its InterestRate property.

Factor out commonalities into a superclass and implement what is specific to each type of lender in the subclass.

Add Only What Is Necessary.  These institutions might engage in activities that are not of interest to your application. During the design phase, determine what operations and data an object must contain based on your problem definition.

Objects Manage Internal State

Objects provide several useful features not available from structures and cell arrays. For example, objects can:

Reducing Redundancy

As the complexity of your program increases, the benefits of an object-oriented design become more apparent. For example, suppose that you implement the following procedure as part of your application:

You can implement this procedure as an ordinary function. But suppose that you use this procedure again somewhere in your application, except that step 2 must perform a different computation. You could copy and paste the first implementation, and then rewrite step 2. Or you could create a function that accepted an option indicating which computation to make, and so on. However, these options lead to more complicated code.

An object-oriented design can factor out the common code into what is called a base class. The base class would define the algorithm used and implement whatever is common to all cases that use this code. Step 2 could be defined syntactically, but not implemented, leaving the specialized implementation to the classes that you then derive from this base class.

Step 1
function checkInputs()
   % actual implementation
end

Step 2
function results = computeOnFirstArg()
   % specify syntax only
end

Step 3
function transformResults()
   % actual implementation
end
 
Step 4
function out = checkOutputs()
   % actual implementation
end

The code in the base class is not copied or modified. Classes you derive from the base class inherit this code. Inheritance reduces the amount of code to be tested, and isolates your program from changes to the basic procedure.

Defining Consistent Interfaces

The use of a class as the basis for similar, but more specialized classes is a useful technique in object-oriented programming. This class defines a common interface. Incorporating this kind of class into your program design enables you to:

Reducing Complexity

Objects reduce complexity by reducing what you must know to use a component or system:

To illustrate these advantages, consider the implementation of a data structure called a doubly linked list. See Class to Implement Linked Lists for the actual implementation.

Here is a diagram of a three-element list:

To add a node to the list, disconnect the existing nodes in the list, insert the new node, and reconnect the nodes appropriately. Here are the basic steps:

First disconnect the nodes:

Now create the new node, connect it, and renumber the original nodes:

The details of how methods perform these steps are encapsulated in the class design. Each node object contains the functionality to insert itself into or remove itself from the list.

For example, in this class, every node object has an insertAfter method. To add a node to a list, create the node object and then call its insertAfter method:

nnew = NodeConstructor;
nnew.insertAfter(n1)

Because the node class defines the code that implements these operations, this code is:

The object methods enforce the rules for how the nodes interact. This design removes the responsibility for enforcing rules from the applications that use the objects. It also means that the application is less likely to generate errors in its own implementation of the process.

Fostering Modularity

As you decompose a system into objects (car –> engine –> fuel system –> oxygen sensor), you form modules around natural boundaries. Classes provide three levels of control over code modularity:

Overloaded Functions and Operators

When you define a class, you can overload existing MATLAB functions to work with your new object. For example, the MATLAB serial port class overloads the fread function to read data from the device connected to the port represented by this object. You can define various operations, such as equality (eq) or addition (plus), for a class you have defined to represent your data.