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.

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 functions that run on
  a server or cluster of servers.

  For more information, see Part 6, “Distributed Application
  Techniques.”