What Is an Object-Oriented System in Java

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What Is an Object-Oriented System
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Just what is an object-oriented system What makes an OO system different than other software systems One way to define an objectoriented system is to use a list of properties that characterize objectoriented systems A non-object-oriented system might share some properties such as using abstraction or encapsulation, but will not be built using objects or classes It is also possible to use an objectoriented language to implement a system using classes or objects, but the system must have all the following properties to be considered a true object-oriented system
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Any object-oriented software system will have the following properties:1 1 Abstraction with objects 2 Encapsulated classes 3 Communication via messages 4 Object lifetime 5 Class hierarchies 6 Polymorphism The next section gives a brief overview of each of these properties, while the following sections give more detailed explanations of each
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object orientation A method of developing software that uses
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abstraction with objects, encapsulated classes, communication via messages, object lifetime, class hierarchies, and polymorphism
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Fundamental Properties of an Object-Oriented System
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Abstraction with objects
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An abstraction is a mechanism that allows a complex, real-world situation to be represented using a simplified model Object orientation abstracts the real world based on objects and their interactions with other objects For example, one possible abstraction of a color is the RGB model
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abstraction A model of a real-world object or concept
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Encapsulated classes
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The abstractions of related ideas are encapsulated into a single unit The states and behaviors of the abstraction are incorporated into an encapsulated whole, called a class The actual internal implementation of the states and behaviors is hidden from the rest of the system While this not a new programming technique, in OO the encapsulation is an inherent and integral part of the system and design Earlier, we described a Color class that provided a way to change it red, green, or blue values In fact, as long as the outside world continues to see and use a Color object in a consistent way, it wouldn't matter just how color is represented internally by the Color object It could use either the HSV (hue, saturation, value) color model or the RGB model internally, and the outside world would be unaffected The state and behavior of objects are controlled by welldefined and restricted interfaces to the object Encapsulation ensures that the internal details of an object are hidden from the rest of the world, that each object maintains its own unique identity and state, and that the state can only be changed by well-defined messages
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encapsulation The process of hiding all the internal details of
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an object from the outside world In Java, encapsulation is enforced by having the definitions for attributes and methods inside a class definition
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Interaction via messages
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In order to accomplish useful tasks, objects need to interact with other objects The interaction can be between objects of the same class, or objects of different classes This interaction is handled by sending messages (in Java, this is done by calling methods) to other objects to pass information or request action For example, when a user selects a command button in a dialog box on the screen by clicking the mouse, a message is sent to the dialog object notifying it
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that the command button was pressed Messages can be used to change the state of an object or to request an action by the object
Object lifetime
All objects have a lifetime They are created and initialized as they are needed during program execution, exist and carry out their functions, and are eventually destroyed While objects exist, they maintain their own identity and state Many objects that are instances of the same class can exist at any given time Each object has a unique identity, and will have attributes that are different from other instances of objects in the same class