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Class Inheritance

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Right, let us now try to add the Animal class to our image. Before we do this, we need to know a little bit about class inheritance. Open a Class  Browser and select Object in the class hierarchy pane at the top left. If Object is the only class that is visible in this pane then double-click it to expand the display.


You'll then see that the browser is displaying some of the many classes that already exist in the image. These classes exist in a hierarchy where each class (apart from Object) is a child of some parent. This hierarchy is important since it governs how any particular class can "inherit" some of its parent's functionality. For example, with Object selected, the browser will be displaying all of the methods defined for this class in the method list pane at top right. Now click on Boolean and you'll see a list of its methods instead. As you can see, the number of methods that appear to be available for Boolean is much smaller than for Object. However, this is not the definitive list. Since Boolean is a "subclass" of Object it inherits all of the latter's methods, which means that Boolean instances can respond to all of these messages too.

Tip: Actually, this is rather a false example since instances of Boolean can never exist. It is what is called an abstract class. If you double-click on Boolean to expand it and display its subclasses you'll see that it has two; True and False. Instances of these classes do exist and they can respond to any of the messages defined for Object and Boolean as well as those defined for themselves.

As you can no doubt see, this inheritance of behaviour can be a great time saver. By choosing an appropriate place in the hierarchy to define a new class you can save yourself having to rewrite, and duplicate, code which has already been written and tested previously.