Workshop 3. JSP Directives

Workshop 4. JSP Directives

Objective

There are two objectives to this workshop: first is to format a number for display and the second objective is to convert a String to double so that some basic maths can be carried out.

Time Available

This workshop should take no longer than 60 minutes.

Directives Workshop

The following demonstrates the objective of this workshop.

Demo

In the "JSP Module" folder create a new JSP named "directives.jsp" and add the following HTML.

<html>
<head>
<title>Directives</title>
</head>

<body>

<%
//Declare number and string
  double dblSalary = 5000;
  String strRent = "400";
%>


<p>
  Number formatted as string:
  <b><%= makeString(dblSalary) %></b>
</p>

<%
//Increase rent by 10%
  double dblNewRent = makeDouble(strRent);
  dblNewRent = dblNewRent + (dblNewRent * 0.1);
%>

<p>
  String converted to number and back again:
  <b><%= makeString(dblNewRent) %></b>
</p>


</BODY>
</HTML>

In the above code we can see two local methods being used makeString() and makeDouble(). Add a declaration section to your JSP and that includes the declaration of these two methods:

<%!
//Method for formatting a double as a string
  private String makeString(double n)
  {

  }


//Method for formatting a string to a double 
  private double makeDouble(String s)
  {

  }
%>

Now add the code to makeString() that will convert a double to a String. Java has some clever methods that offer great flexibility and the following code is one such example,
```
NumberFormat nf = NumberFormat.getCurrencyInstance(Locale.UK);
return nf.format(n);
```
Now the NumberFormat class is defined in the package java.text and the Locale class is defined in the package java.util. Add a directive that references both these classes.
Similarly add code to makeDouble() that will convert a double to a String:
```
Double d = new Double(s);
return d.doubleValue();
```
The Double class is defined in the package java.lang and this package must also be referenced in a directive.
Save and test your work so far.
Extra. Return to your JSP file and change the locale used in the makeString() method to another region, Locale.US for example, and access the JSP again to see the difference.

Now these methods for formatting numbers as currencies and converting Strings to doubles are pretty useful and it is likely that we'll want to use them again in other JSPs. This is an ideal scenario for exploiting the include directive.

Return to your "directives.jsp" file cut the lines of code for the page import directive and the declarations and paste these into a new file.
Save this file as "formatting.jsp".
Return to "directives.jsp" and add a directive to include the text from "formatting.jsp".
Save and test these changes.

By now your code should look like this:

Tech Tips

It is a good practice to work with include files in this way. First build the JSP as a single file and then cut and paste the text into the file that is being included.

An common error that can be made with include files is placing the Java code in the include file but forgetting the <%...%> tags. Other errors include declaring a variable both in the include file and in the parent JSP.

A good method for debugging is to copy and paste the code from the include file to the parent JSP at the exact point of the include directive and then look for problems such as above.

Quiz !

Now that you are an expert in JSP Syntax go for top marks in the following quiz.

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