一些JAVA的小程序

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																What Is an Object?												function openWin(term) {				  url="../../information/glossary.html#" + term;				  myWin= window.open(url, "Glossary",				              "width=400,height=150,scrollbars=yes,status=no,toolbar=no,menubar=no");				  myWin.focus();				}				//-->																.FigureCaption   { margin-left: 1in; margin-right: 1in; font-family: Arial, Helvetica, sans-serif; font-size: smaller; text-align: justify }				-->				 												The JavaTM Tutorial																																												Start of Tutorial				>				Start of Trail				>				Start of Lesson																								Search				Feedback Form																																								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.																																																																								Start of Tutorial				>				Start of Trail				>				Start of Lesson																								Search				Feedback Form																																Copyright				1995-2005 Sun Microsystems, Inc.  All rights reserved.																							

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