How Polymorphism, Encapsulation, and Inheritance Work Together In Java
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Polymorphism, Encapsulation, and Inheritance Work Together
When properly applied, polymorphism, encapsulation, and inheritance combine to produce
a programming environment that supports the development of far more robust and scalable
programs than does the process-oriented model. A well-designed hierarchy of classes is the
basis for reusing the code in which you have invested time and effort developing and testing.
Encapsulation allows you to migrate your implementations over time without breaking the
code that depends on the public interface of your classes. Polymorphism allows you to create
clean, sensible, readable, and resilient code.
Of the two real-world examples, the automobile more completely illustrates the power
of object-oriented design. Dogs are fun to think about from an inheritance standpoint, but
cars are more like programs. All drivers rely on inheritance to drive different types (subclasses)
of vehicles. Whether the vehicle is a school bus, a Mercedes sedan, a Porsche, or the family
minivan, drivers can all more or less find and operate the steering wheel, the brakes, and
the accelerator. After a bit of gear grinding, most people can even manage the difference
between a stick shift and an automatic, because they fundamentally understand their common
superclass, the transmission.
People interface with encapsulated features on cars all the time. The brake and gas pedals
hide an incredible array of complexity with an interface so simple you can operate them
with your feet! The implementation of the engine, the style of brakes, and the size of the
tires have no effect on how you interface with the class definition of the pedals.
The final attribute, polymorphism, is clearly reflected in the ability of car manufacturers
to offer a wide array of options on basically the same vehicle. For example, you can get an
antilock braking system or traditional brakes, power or rack-and-pinion steering, and 4-, 6-,
or 8-cylinder engines. Either way, you will still press the brake pedal to stop, turn the steering
wheel to change direction, and press the accelerator when you want to move. The same
interface can be used to control a number of different implementations.
As you can see, it is through the application of encapsulation, inheritance, and
polymorphism that the individual parts are transformed into the object known as a car.
The same is also true of computer programs. By the application of object-oriented principles,
the various parts of a complex program can be brought together to form a cohesive, robust,
maintainable whole.
As mentioned at the start of this section, every Java program is object-oriented. Or, put
more precisely, every Java program involves encapsulation, inheritance, and polymorphism.
Although the short example programs shown in the rest of this chapter and in the next few
chapters may not seem to exhibit all of these features, they are nevertheless present. As you
will see, many of the features supplied by Java are part of its built-in class libraries, which
do make extensive use of encapsulation, inheritance, and polymorphism.