Instead of defining the same five
attributes every time you need a button to close a frame, you can
create a button that has all the attributes that you need already
defined. You do this by creating a subclass.
A
subclass is derived from another class, called a parent class. The subclass
inherits all the attributes and methods of that parent. This means
that if you change the value of an attribute or the way a method works
on the parent, the same change is propagated to the subclass.
You can also define
on the subclass new values for attributes that the subclass inherits.
These new values override the values that were inherited from the
parent class. For example, if the parent class has a width attribute
that is set to 50, you can change that value to 100 on the subclass.
This appendix guides
you through the process of creating a subclass of the Push Button
class. The new subclass overrides the values for the following attributes:
After making the Close
Window Button subclass, you can drag and drop it onto a frame and
the button will function without any further configuration. Besides
eliminating repetitive work, subclassing helps to ensure conformity:
you won't be able to make the mistake of labeling one button “Close
Window” and another “Exit”.