JavaSercer Pages Chapter 6. Using Scripting Elements When (Guild web hosting)
JavaSercer Pages Chapter 6. Using Scripting Elements When you develop a JSP-based application, I recommend that you try to place all Java code in JavaBeans, in custom actions, or in regular Java classes. However, to tie all these components together, you sometimes need additional code embedded in the JSP pages themselves. Recall from Chapter 3, that JSP lets you put actual Java code in pages using a set of scripting elements. In this chapter we look at how you can use these scripting elements and when it makes sense to do so. We start with a brief introduction to the Java language constructs you’re likely to use in a JSP page. If you already know Java by heart you can safely skip the first section. But if you have never written a Java program, or are still a “newbie,” you should read it carefully. Don’t expect to become a Java guru after reading this introduction, of course. The Java language, combined with the standard libraries, provides many powerful features not covered here. To learn more about Java, I recommend that you read one of the many books dedicated to the language and its libraries, for instance Java in a Nutshell and Java Examples in a Nutshell, both by David Flanagan (O’Reilly). 6.1 Java Primer You don’t have to be a Java expert to develop JSP pages, but it helps to have an understanding of the basic concepts. This overview of the Java language and some of the standard classes should be enough to get you started. 6.1.1 Classes and Objects Java is an object-oriented language. This means that everything in Java is an object, except for a few primitive types such as numbers, characters, and Boolean values. An object is an instance of a class, which serves as a source code template describing how the object should behave. It’s helpful to think of a class as a blueprint from which identical copies (objects) are created. Example 6.1 shows a simple Java class. Example 6.1. Simple Java Class /** * This is just a simple example of a Java class * with two instance variables, a constructor, and * some methods. */ public class Rectangle { // Data private int width; private int height; // Constructor public Rectangle(int w, int h) { width = w; height = h; } // Methods public int getWidth( ) { return width; } public void setWidth(int w) { width = w; } public int getHeight( ) { return height; } public void setHeight(int h) { height = h; } public double getArea( ) { double area; area = width * height; return area; } } It’s important to remember that a class always defines two items: data : a collection of information in an object methods : a set of functions that act on that data page 55
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