Terminology review
Classes, properties, and methods
In object-oriented programming, you create reusable classes to
perform application processing. These classes include properties and methods that define
the class’s behavior. To perform application processing,
you create instances of these classes. PowerBuilder implements
these concepts as follows:
- Classes PowerBuilder objects (such as windows, menus, window controls,
and user objects) - Properties Object variables and instance variables
- Methods Events and functions
The remaining discussions in this chapter use this PowerBuilder terminology.
Fundamental principles
Object-oriented programming tools support three fundamental
principles: inheritance, encapsulation, and polymorphism.
Inheritance Objects can be derived from existing objects, with access
to their visual component, data, and code. Inheritance saves coding
time, maximizes code reuse, and enhances consistency. A descendent
object is also called a subclass.
Encapsulation An object contains its own data and code, allowing outside access
as appropriate. This principle is also called information
hiding
. PowerBuilder enables and supports encapsulation
by giving you tools that can enforce it, such as access and scope.
However, PowerBuilder itself does not require or automatically enforce
encapsulation.
Polymorphism Functions with the same name behave differently, depending on
the referenced object. Polymorphism enables you to provide a consistent interface
throughout the application and within all objects.
Visual objects
Many current applications make heavy use of object-oriented
features for visual objects such as windows, menus, and visual user
objects. This allows an application to present a consistent, unified
look and feel.
Nonvisual objects
To fully benefit from PowerBuilder’s object-oriented
capabilities, consider implementing class user objects, also known
as nonvisual user objects:
Standard class user objects Inherit their definitions from built-in PowerBuilder system
objects, such as Transaction, Message, or Error. The nvo_transaction
Transaction object in the Code Examples sample application is an
example of a subclassed standard class user object. Creating customized standard
class user objects allows you to provide powerful extensions to
built-in PowerBuilder system objects.
Custom class user objects Inherit
their definitions from the PowerBuilder NonVisualObject class. Custom
class user objects encapsulate data and code. This type of class
user object allows you to define an object class from scratch. The
u_business_object user object in the Code Examples
sample application is an example of a custom class user object.
To make the most of PowerBuilder’s object-oriented capabilities,
you must use custom class user objects. Typical uses include:
- Global variable
container The custom class user object contains variables and functions
for use across your application. You encapsulate these variables
as appropriate for your application, allowing access directly or
through object functions. - Service object The custom class user object contains functions and variables
that are useful either in a specific context (such as a DataWindow)
or globally (such as a collection of string-handling functions). - Business rules The custom class user object contains functions and variables
that implement business rules. You can either create one object for
all business rules or create multiple objects for related groups
of business rules. - Distributed computing The custom class user object contains the functions that run
on the server.
For more information, see Part 6, “Distributed Application Techniques”.