UML实例--HELL_WORLD(二).doc

Key Abstractions In Java, the applet for printing “Hello, World!” in a Web browser is quite simple: import java.awt.Graphics; class HelloWorld extends java.applet.Applet { public void paint (Graphics g) { g.drawString (“Hello, World!”, 10, 10); } } The first line of code: import java.awt.Graphics; makes the class Graphics directly available to the code that follows. The java.awt prefix specifies the Java package in which the class Graphics lives. The second line of code: class HelloWorld extends java.applet.Applet { introduces a new class named HelloWorld and specifies that it is a kind of class just like Applet , which lives in the package java.applet. The next three lines of code: public void paint (Graphics g) { g.drawString (“Hello, World!”, 10, 10); } declare an operation named paint , whose implementation invokes another operation, named drawString , responsible for printing the string “Hello, World!” at the given coordinates. In the usual object-oriented fashion, drawString is an operation on a parameter named g , whose type is the class Graphics . Modeling this application in the UML is straightforward. As Figure 3-1 shows, you can represent the class HelloWorld graphically as a rectangular icon. Its paint operation is shown here as well, with all its formal parameters elided and its implementation specified in the attached note. Note: The UML is not a visual programming language, although, as the figure shows, the UMLdoes allow -- but does not require -- a tight coupling to a variety of programming languages, such as Java. The UML is designed to allow models to be transformed into code and to allow code to be reengineered back into models. Some things are best written in the syntax of a textual programming language (for example, mathematical expressions), whereas other things are best visualized graphically in the UML (for example, hierarchies of classes). 关键抽象概念 Java中，在网络浏览器中打印”你好，世界！”这类的小应用程序是非常简单的，如下所示： import java.awt.Graphics; class HelloWorl