16.10 Evaluating Expressions Conditionally

The JSP 2.0 expression language does not, in itself, provide a rich conditional evaluation facility. That capability is provided by the c:if and c:choose tags of the JSP Standard Tag Library (JSTL) or by another custom tag library. (JSTL and the creation of custom tag libraries are covered in Volume 2 of this book.)

However, the expression language supports the rudimentary ?: operator as in the Java, C, and C++ languages. For example, if test evaluates to true ,

 
 ${ test ? expression1 : expression2 } 

returns the value of expression1 ; otherwise , it returns the value of expression2 . Just remember that the main purpose of the expression language is to simplify presentation logic; avoid using this technique for business logic.

An Example

The servlet of Listing 16.12 creates two SalesBean objects (Listing 16.13) and forwards the request to a JSP page (Listing 16.14) for presentation (Figure 16-7). However, if the total sales number is negative, the JSP page wants to use a table cell with a red background. If it is negative, the page wants to use a white background. Implementing this behavior is a presentation task, so the ?: operator is appropriate.

Listing 16.12 Conditionals.java
 package coreservlets; /** Servlet that creates scoped variables that will be used  *  to illustrate the EL conditional operator (xxx ? xxx : xxx).  */ import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Conditionals extends HttpServlet {   public void doGet(HttpServletRequest request,                     HttpServletResponse response)       throws ServletException, IOException {  SalesBean  apples =       new SalesBean(150.25, -75.25, 22.25, -33.57);  SalesBean  oranges =       new SalesBean(-220.25, -49.57, 138.25, 12.25);     request.setAttribute("apples", apples);     request.setAttribute("oranges", oranges);     RequestDispatcher dispatcher =       request.getRequestDispatcher("/el/conditionals.jsp");     dispatcher.forward(request, response);   } } 
Listing 16.13 SalesBean.java
 package coreservlets; public class SalesBean {   private double q1, q2, q3, q4;   public SalesBean(double q1Sales,                    double q2Sales,                    double q3Sales,                    double q4Sales) {     q1 = q1Sales;     q2 = q2Sales;     q3 = q3Sales;     q4 = q4Sales;   }   public double  getQ1  () { return(q1); }   public double  getQ2  () { return(q2); }   public double  getQ3  () { return(q3); }   public double  getQ4  () { return(q4); }   public double  getTotal  () { return(q1 + q2 + q3 + q4); } } 
Listing 16.14 conditionals.jsp
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD><TITLE>Conditional Evaluation</TITLE> <LINK REL=STYLESHEET       HREF="/el/JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">   Conditional Evaluation </TABLE> <P> <TABLE BORDER=1 ALIGN="CENTER">   <TR><TH>       <TH CLASS="COLORED">Apples       <TH CLASS="COLORED">Oranges   <TR><TH CLASS="COLORED">First Quarter       <TD ALIGN="RIGHT">${apples.q1}       <TD ALIGN="RIGHT">${oranges.q1}   <TR><TH CLASS="COLORED">Second Quarter       <TD ALIGN="RIGHT">${apples.q2}       <TD ALIGN="RIGHT">${oranges.q2}   <TR><TH CLASS="COLORED">Third Quarter       <TD ALIGN="RIGHT">${apples.q3}       <TD ALIGN="RIGHT">${oranges.q3}   <TR><TH CLASS="COLORED">Fourth Quarter       <TD ALIGN="RIGHT">${apples.q4}       <TD ALIGN="RIGHT">${oranges.q4}   <TR><TH CLASS="COLORED">Total       <TD ALIGN="RIGHT"           BGCOLOR="  ${(apples.total < 0) ? "RED" : "WHITE" }  ">       ${apples.total}       <TD ALIGN="RIGHT"           BGCOLOR="  ${(oranges.total < 0) ? "RED" : "WHITE" }  ">       ${oranges.total} </TABLE> </BODY></HTML> 
Figure 16-7. You can use the C-style ?: operator to conditionally output different elements. However, the JSP Standard Tag Library (JSTL) is often a better alternative for this kind of conditional operation.

graphics/16fig07.jpg



Core Servlets and JavaServer Pages (Vol. 1.Core Technologies)
Core Servlets and Javaserver Pages: Core Technologies, Vol. 1 (2nd Edition)
ISBN: 0130092290
EAN: 2147483647
Year: 2002
Pages: 194

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