A class is a group of objects. Classes are used for:

•Performing one action for many objects, using just one action statement.

•Responding to many objects, using just one response statement.

•Dynamically changing the response definition for an object. During runtime, you can change the composition of a class by adding objects to the class and deleting objects from it. By moving an object from one class to another, you effectively change the response that is to be taken for that object.

As a general rule, it is inadvisable to group different types of objects as members of the same class. You should define classes that consist of only one type of object; for example, classes that consist of only keys or only graphical regions. This is especially true for textual regions, which have many unique action and drawing statements, and cannot use most of the action and drawing statements that are used by graphical objects.