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Since the object was created from a class that implements the Display interface, it is guaranteed to have the functionality of a display. The object has an is-a relationship with Display. This object can now masquerade as an object of the Display type. An object can polymorphically act as any interface that its class or any superclass implements. Polymorphism and interfaces are very powerful tools.They are used extensively on large projects. As a professional developer, it is a good idea to study design patterns.This will provide common reusable software designs that make use of the concepts in this chapter. A good developer not only understands all of the basic concepts, but also knows how to best use them.
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When one specific object can be used as another general object polymorphically, the specific object can be used in place of the more general one without being cast. For example (see Figure 8-2), if class TypeC extends TypeB, and TypeB extends
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TypeA
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TypeA, TypeB, and TypeC
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TypeB
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TypeB extends TypeA
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TypeC
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TypeC extends TypeB
8:
Understanding Polymorphism
TypeA, anytime an object type of TypeA or TypeB is needed, TypeC can be used.
The following code segment shows an example of this:
TypeA var1 = new TypeA(); TypeA var2 = new TypeB(); TypeA var3 = new TypeC(); TypeB var4 = new TypeB(); TypeB var5 = new TypeC(); TypeC var6 = new TypeC();
In this example any subclass can be used interchangeably with its superclass. The variable var3 is declared as a TypeA object but is initialized with a new TypeC object. Even though var3 is really a TypeC object, it will be treated as a TypeA object anywhere var3 is referenced. This is okay because the TypeC object has inherited all of the functionality of the TypeA and TypeB objects. However, since var3 was declared as TypeA, it can now only be treated as an object of this type. If TypeC objects have additional methods that are not part of the TypeA class, these methods would be unavailable. More commonly, polymorphism will be used for method arguments. This allows a method to be written more abstractly and therefore be more flexible. For instance, a method may be required to accept a type of animal object as its argument and use it to determine if the animal is hungry. In this scenario, there is no benefit in creating a method that would accept a Penguin object and another that accepts a PolarBear object. Instead, it would be a better design to create one single method that accepts an Animal class. The state of hunger is general to the Animal class. The Animal class is a superclass for both the Penguin class and PolarBear class. These are basic examples to help give more meaning to the concepts of polymorphism. Keep in mind that they are described at a very high level. This chapter will conclude with examples that show polymorphism in greater depth.
Programming to an Interface
Programming to an interface is the concept that code should interact based on a defined set of functionality instead of an explicitly defined object type. In other words, it is better for the public interfaces of objects to use data types that are defined as interfaces as opposed to a particular class when possible. When an object is implementing an interface, it is declaring that it has a certain set of functionalities. Many different classes can implement the same interface and provide its functionality.
Practical Examples of Polymorphism
By a method using an interface as its argument type, it allows any object, regardless of its type, to be used as long as it implements the interface. This allows the code to be more abstract and flexible. It also promotes code reuse.
This chapter goes deeper into polymorphism than is required for the SCJA exam.This should better help you understand the test questions. Most of the questions on the test either will be a theory question regarding the definition of polymorphism, or will be a simple scenario that will require the SCJA candidate to choose a code segment that is correct.
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