Object orientation in Java

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Object orientation
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Packages are not a heirarchy Despite the misleading impression their names often suggest, Java and JavaFX packages are not arranged in a hierarchy. The package java.awt.event is actually a sibling of java.awt, not a child. If you import all the classes from the latter, you do not automatically get the former. This is a common newbie mistake.
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Once we ve compiled our code, we can bundle the package into a JAR file to make it easy to distribute. A JAR file is a zip archive with a standardized/recognized layout and content. By convention all of the classes in a given package are bundled inside a single JAR. It s possible to split a package over multiple JARs, but the practice is rarely used. However, JAR archives frequently contains multiple related packages.
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Object orientation
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Classes are an integral part of object orientation, encapsulating state and behavior for each component in a larger system, thereby allowing us to express our software in terms of the structures and relationships that link its autonomous component parts. Object orientation has become an incredibly popular way of constructing software in recent years: both Java and its underlying JVM environment are heavily object-centric. But what is object orientation The following sections describe object orientation from a JVM point of view, although I ve stuck with JavaFX Script terminology (functions, not methods). This is only a whistle-stop tour through OO; consult a book on the topic if you want to know more.
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C.4.1
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Modeling the world with classes At the sharp end of object-oriented software everything tends to boil down to types. What type is an object For example, is it a plane, a train, or an automobile Of course, all three are types of vehicle and share common properties and functionality. They all move and therefore have a speedometer and an odometer (mileage) and consume power. They all carry passengers as well. They all need some form of engine to drive them forward and a braking system to slow them down. But, of course, they also have a lot of differences. Trains cannot arbitrarily turn left or right because they are bound by the constraints of a track (or at least they shouldn t be able to under normal operating conditions). Cars cannot fly through the air like a plane (again, under normal operating conditions!), but they can move in reverse, which is something a plane in flight cannot do. (Unless it s a Harrier Jump Jet!) We build up object-oriented software by modeling these relationships. Classes are the nodes we link together to create such models. If we were building a transport simulator, we might start with a Vehicle class that contains all the data and functionality we know is common to all vehicles in our system. The odometer, for example, could be included in this top-level class, because all vehicles have a mileage. We could also define a few functions, perhaps speedUp() and slowDown(), because increasing and decreasing speed is common to all vehicles.
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APPENDIX C
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C.4.2
Classes from classes: subclassing and overriding Once we have a generic Vehicle class, we can define more specific vehicles based on it. We might define a Plane class, which adds an altitude attribute. We might also define an Automobile class, which adds turn-left and turn-right functions, and so on. The process of creating a more specific class in terms of a more general one is known as subclassing. Java and JavaFX Script also use the synonym extending. When a class subclasses another, it can replace the implementations of variables and functions in its parent (super) class with its own. This is known as overriding. It cannot change their type an integer variable must remain an integer but it can change their default value or (in the case of a function) their code body. Each Vehicle subclass, for example, could define its own implementation of speedUp() and slowDown(), simulating the specific mechanics of the given type of vehicle they represent. An object can be referenced in different ways: polymorphism An object on our simulation may be created as a type of the HarrierJumpJet class, which in turn is a type of Plane, which in turn is a type of Vehicle. Because we know the HarrierJumpJet inherited all the functionality of Plane (even if it did override some of it with its own implementation), and by proxy inherited all the functionality of Vehicle (again, even if it replaced some of it), the HarrierJumpJet object can be treated as being of type HarrierJumpJet, Plane, or Vehicle. This ability to treat objects by way of their superclass types (parents in the class hierarchy) is known as polymorphism. It means we can create a variable of Vehicle type and store any subclass from Vehicle in it, including SteamTrain, FordModelT, and ApolloSpaceCapsule. Likewise, a variable of type Plane can hold any object that is of type Plane or a subclass of Plane (HarrierJumpJet, Boeing747, Spitfire, etc.) A variable of type Plane could not hold a BatMobile, however, because BatMobile is a subclass of Automobile, not Plane. Partial implementation: abstract functions and interfaces Sometimes we want to create classes that are intended only for subclassing. For example, we probably do not want to create objects of type Plane directly, because the class is too generic; instead we want to create objects of specific plane types (HarrierJumpJet, Boeing747, etc.) that subclass Plane. By marking a class as abstract we can prevent it from being used to create objects directly. An abstract class must be subclassed before it can be used. Abstract classes can contain abstract functions. These are functions that have no functionality (no code body) and must be overridden before being used. For example, the speedUp() and slowDown() functions in Vehicle would likely be abstract, with each subclass overriding them to simulate precisely how its given type of vehicle accelerates or decelerates. If a class contains abstract functions, the class itself must be abstract.
C.4.3
C.4.4
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