Programming terminology: objects, properties and classes

This user interface of this program has four objects:

• A Form which contains the other three objects
• A Button for displaying a message
• A Button for clearing the message area
• A TextBox in which the message is displayed

• The value of the Text property of top button is "Display a message"
• The value of the Text property of the second button is "Clear the message area"
• The value of the Text property of the form is "Message output program"
• The value of the BackColor (background color) property of the TextBox is white
• The value of the ForeColor (foreground color) property of the TextBox is red
• The value of the WordWrap property of the TextBox is true
• The value of the BackColor property of both buttons is grey
• The value of the ForeColor property of both buttons is black

In this example, we have used objects that appear in a user interface. Not all objects appear in the user interface. For example, in writing a personal scheduling program, you might define an object that was an appointment with properties like date, time, and purpose.

We will focus on user interface objects for now. There are many others beside Forms, Buttons and TextBoxes, for example:

• Labels, which display text on a form
• PictureBoxes, which display images
• CheckBoxes, where the user can select one or more options
• RadioButtons, where the user can select only one of several options
• Frames, which can group related controls
• ComboBoxes, in which the user can select from a list of items

Every object is an instance of a class. For example, Buttons are a class of object, and two of the objects in the user interface of our message output program are instances of the class Button. The other object in the form is an instance of the class TextBox.

You can think of a class as a template for creating objects. The computer uses that template to create new instances of the class. When a new object is created, the class template tells the computer how much memory to set aside, how many properties the object should have, what their default values are, etc.

Every object has a name so the program can refer to it. Do not confuse the name of an object with the value of its Text property. You must see the program to know the name of an object -- it is not shown in the user interface.

Note that this terminology can be used to describe real world objects as well as objects in computer programs. For example, Dog is a class of object. I have two particular dogs, Maggie and Minnie. Maggie and Minnie are instances of the class dog. One of the properties of dogs is color," and the value of Maggie's color property is white. The value of Minnie's color property is brown.