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Polymorphism via Class Inheritance
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Polymorphism happens when a certain object type is needed and an object of that type or another more specific object is accepted in its place. Remember, an object is
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Polymorphism
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a more specific type of another object when it extends that object. For example, a method may require a Human object. When the Child and Adult classes extend the Human class they would each possess all of the functionality of a Human, plus all the more specific functionality of their age. The Child and Adult objects are guaranteed to have all of the methods that a Human object has because they gain them through inheritance. Therefore both the Child and Adult objects would satisfy any operation that required a Human object. This could be continued further with the Shannon and Colleen classes, which each extend the Adult class. Each of the objects created from the Shannon and Colleen classes would have the functionality of the more general Adult and Human classes, and can be used anywhere a Human or Adult object is required. Polymorphism utilizes the is-a relationship. In Figure 8-1 the Child object is-a Human object, and the Adult object is-a Human object. Both Child and Adult are just specific types of a Human object. Furthermore, the Shannon object is-an Adult object and is-a Human object. This is also true for the Colleen object. The Shannon object is not only a more specific type of Adult object, but also a more specific type of Human object. Any object is a more specific type of an object that it is a subclass of. The is-a relationship is created when an object inherits, or extends, another. Any object that extends another object can be said to have an is-a relationship to the object that it extends. Any object that has an is-a relationship with another can polymorphically be used as that object. When an object is polymorphically acting as another object, the more specific object is restricted to only using the public interface of the more general object.
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Polymorphic objects
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Child extends Human
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Adult extends Human
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Adult
Shannon and Colleen extend Adult
Shannon
Colleen
8:
Understanding Polymorphism
In the above example, when the Adult object is used as a Human object, only the methods that are available in the Human class can be used. This is because the Java code that is using the Adult object as a Human object has no knowledge that this Human object is really an Adult object. This is the benefit of polymorphism. The Java code does not always have to be aware of the specifics of an object. If a general object meets the needs of a method, then that method does not care if it is a general object or more specific object. The only requirement is that the object has an is-a relationship with the object that the method requires. This relationship is unidirectional. Only the more specific object can take the place of a general object. For example, if an Adult object were needed, a more general Human object would not be able to provide all of the functionality of an Adult object.
Abstract classes and concrete classes behave the same way with polymorphism. Since an abstract class cannot be instantiated, the only way to assign an object to an abstract data type is by using polymorphism. Pay close attention to how abstract classes are initialized.
Polymorphism via Implementing Interfaces
The application of polymorphism is not limited to class inheritance. Polymorphism can also be applied to the objects of classes that implement interfaces. When a class implements an interface, it is then required to implement all of the methods that the interface contains. By doing this, the class is guaranteed to have the functionality that the interface defines. This allows the objects created from these classes to polymorphically behave as the data type of the interface. An interface called Display can be used for classes that have the ability to display text on a screen. This interface contains two methods. One method is used to display text, and the second is used to get the text that is currently being displayed. Any class that implements this interface is declaring to other objects that it has the functionality of a Display. By implementing this interface the class is required to then implement every method that the interface contains.
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