【正文】 to anyone else. The hidden portion usually represents the tender insides of an object that could easily be corrupted by a careless or uninformed client programmer, so hiding the implementation reduces program bugs. The concept of implementation hiding cannot be overemphasized. In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is also a programmer, but one who is putting together an application by using your library, possibly to build a bigger all the members of a class are available to everyone, then the client programmer can do anything with that class and there’s no way to enforce rules. Even though you might really prefer that the client programmer not directly manipulate some of the members of your class, without access control there’s no way to prevent it. Everything’s naked to the the first reason for access control is to keep client programmers’ hands off portions they shouldn’t touch—parts that are necessary for the internal machinations of the data type but not part of the interface that users need in order to solve their particular problems. This is actually a service to users because they can easily see what’s important to them and what they can ignore. The second reason for access control is to allow the library designer to change the internal workings of the class without worrying about how itwill affect the client programmer. For example, you might implement a particular class in a simple fashion to ease development, and then later discover that you need to rewrite it in order to 大連交通大學(xué) 2020 屆本科生畢業(yè)設(shè)計（論文）外文翻譯 6 make it run faster. If the interface and implementation are clearly separated and protected, you can acplish this easily. Java uses three explicit keywords to set the boundaries in a class: public,private, and protected. Their use and meaning are quite straightforward. These access specifiers determine who can use the definitions that follow. public means the following definitions are available to everyone. The private keyword, on the other hand, means that no one can access those definitions except you, the creator of the type, inside member functions of that type. private is a brick wall between you and the client programmer. If someone tries to access a private member, they’ll get a piletime error. protected acts like private, with the exception that an inheriting class has access to protected members, but not private members. Inheritance will be introduced shortly. 大連交通大學(xué) 2020 屆本科生畢業(yè)設(shè)計（論文）外文翻譯 7 抽象過程 所有編程語言都提供抽象（ abstraction）機(jī)制。 it stores data, but you can “make requests” to that object, asking it to perform operations on itself. In theory, you can take any conceptual ponent in the problem you’re trying to solve (dogs, buildings, services, etc.) and represent it as an object in your program. 2. A program is a bunch of objects telling each other what to do by sending messages. To make a request of an object, you “send a message” to that object. More concretely, you can think of a message as a request to call a function that belongs to a particular object. 3. Each object has its own memory made up of other objects. Put another way, you create a new kind of object by making a package containing existing objects. Thus, you can build plexity in a program while hiding it behind the simplicity of objects. 4. Every object has a type. Using the parlance, each object is an instance of a class, in which “class” is synonymous with “type.” The most important distinguishing characteristic of a class is “What messages can you send to it?” 5. All objects of a particular type can receive the same messages. This is actually a loaded statement, as you will see later. Because an object of type “circle” is also an object of type “shape,” a circle is guaranteed to accept shape messages. This means you can write 大連交通大學(xué) 2020 屆本科生畢業(yè)設(shè)計（論文）外文翻譯 3 code that talks to shapes and automatically handle anything that fits the description of a shape. This substitutability is