What Is an Object? - Java Tutorial 5.0 英文版

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Trail: Learning the Java Language
Lesson: Object-Oriented Programming Concepts

What Is an Object?

Objects are key to understanding object-oriented technology. You can look around you now and see many examples of real-world objects: your dog, your desk, your television set, your bicycle.
Real-world objects share two characteristics: They all have state and behavior. For example, dogs have state (name, color, breed, hungry) and behavior (barking, fetching, wagging tail). Bicycles have state (current gear, current pedal cadence, two wheels, number of gears) and behavior (braking, accelerating, slowing down, changing gears).
Software objects are modeled after real-world objects in that they too have state and behavior. A software object maintains its state in one or more variables. A variable is an item of data named by an identifier. A software object implements its behavior with methods. A method is a function (subroutine) associated with an object.
Definition: An object is a software bundle of variables and related methods.
You can represent real-world objects by using software objects. You might want to represent real-world dogs as software objects in an animation program or a real-world bicycle as a software object in the program that controls an electronic exercise bike. You can also use software objects to model abstract concepts. For example, an event is a common object used in window systems to represent the action of a user pressing a mouse button or a key on the keyboard. The following illustration is a common visual representation of a software object.
A software object.
Everything the software object knows (state) and can do (behavior) is expressed by the variables and the methods within that object. A software object that models your real-world bicycle would have variables that indicate the bicycle's current state: Its speed is 18 mph, its pedal cadence is 90 rpm, and its current gear is 5th. These variables are formally known as instance variables because they contain the state for a particular bicycle object; in object-oriented terminology, a particular object is called an instance. The following figure illustrates a bicycle modeled as a software object.
A bicycle modeled as a software object.

In addition to its variables, the software bicycle would also have methods to brake, change the pedal cadence, and change gears. (It would not have a method for changing its speed because the bike's speed is just a side effect of which gear it's in and how fast the rider is pedaling.) These methods are known formally as instance methods because they inspect or change the state of a particular bicycle instance.
Object diagrams show that an object's variables make up the center, or nucleus, of the object. Methods surround and hide the object's nucleus from other objects in the program. Packaging an object's variables within the protective custody of its methods is called encapsulation. This conceptual picture of an object — a nucleus of variables packaged within a protective membrane of methods — is an ideal representation of an object and is the ideal that designers of object-oriented systems strive for. However, it's not the whole story.

Often, for practical reasons, an object may expose some of its variables or hide some of its methods. In the Java programming language, an object can specify one of four access levels for each of its variables and methods. The access level determines which other objects and classes can access that variable or method. Refer to the Controlling Access to Members of a Class section for details.

Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two primary benefits to software developers:

Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Also, an object can be easily passed around in the system. You can give your bicycle to someone else, and it will still work.
Information-hiding: An object has a public interface that other objects can use to communicate with it. The object can maintain private information and methods that can be changed at any time without affecting other objects that depend on it. You don't need to understand a bike's gear mechanism to use it.

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