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One of the most interesting object-oriented programming concepts is inheritance, as it allows you to generate a taxonomy of classes and objects. If you consider all living things as a class called LivingThing (see Figure 6-1), under that class you could have (and let s keep this simple, biologists!) Plant and Animal classes. Under Animal you d have Mammal, Fish, Amphibian, and so forth. Digging into Mammal, you could work through Primate and Human. A Human is a living thing, a Human is an Animal, a Human is a Mammal, and so forth, but each level down is more specific and targeted than that above it. This is class inheritance in action! The same system applied to the Shape example where Triangle and Square inherited directly from Shape.
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Figure 6-1. An example of a hierarchy of living things
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Ch aPt er 6 CLa S S eS , OB J eC tS , a ND MOD U Le S
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The benefit of inheritance is that classes lower down the hierarchy get the features of those higher up, but can also add specific features of their own. The basic all living things class is so generic that the only functionality you could give to it is a basic living or dead method. However, at the animal level, you could add methods such as eat, excrete, or breathe. At the human level you d inherit all this functionality but be able to add human methods and qualities such as sing, dance, and love. Ruby s inheritance features are similarly simple. Any class can inherit the features and functionality of another class, but a class can inherit only from a single other class. Some other languages support multiple inheritance, a feature that allows classes to inherit features from multiple classes, but Ruby doesn t support this. Multiple inheritance can cause some confusing situations for instance, classes could inherit from one another in an endless loop and the efficacy of multiple inheritance is debatable. Let s look at how inheritance works in code form: class ParentClass def method1 puts "Hello from method1 in the parent class" end def method2 puts "Hello from method2 in the parent class" end end class ChildClass < ParentClass def method2 puts "Hello from method2 in the child class" end end my_object = ChildClass.new my_object.method1
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Hello from method1 in the parent class
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my_object.method2
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Hello from method2 in the child class First you create the ParentClass with two methods, method1 and method2. Then you create ChildClass and make it inherit from ParentClass using the ChildClass < ParentClass notation. Last, you create an object instance of ChildClass and call its method1 and method2 methods.
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The first result demonstrates inheritance perfectly. ChildClass has no method1 of its own, but because it has inherited from ParentClass, and ParentClass has a method1, it uses it. However, in the second case, ChildClass already has a method2 method, so the method2 method supplied by the parent class is ignored. In many cases, this is ideal behavior, as it allows your more specific classes to override behavior provided by more general classes. However, in some situations you might want a child method to call an inherited method and do something with the result. Consider some basic classes that represent different types of people: class Person def initialize(name) @name = name end def name return @name end end class Doctor < Person def name "Dr. " + super end end In this case you have a Person class that implements the basic functionality of storing and returning a person s name. The Doctor class inherits from Person and overrides the name method. Within the name method for doctors, it returns a string starting with Dr., appended with the name as usual. This occurs by using super, which looks up the inheritance chain and calls the method of the same name on the next highest class. In this example, you only have two tiers, so using super within the name method in Doctor then uses the name method in Person. The benefit of using inheritance in this way is that you can implement generic functionality in generic classes, and then implement only the specific functionality that more specific child classes require. This saves a lot of repetition and means that if you make changes to the parent classes, child classes will inherit these changes too. A good example of this might be if you changed Person to take two arguments, firstname and lastname. The Doctor class wouldn t need to be changed at all to support this change. With one child class this doesn t seem too important, but when you have hundreds of different classes in an application, it pays to cut down on repetition!
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