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> ## Functions

> Table 17-8 lists the JSTL functions

> Table 17-8 Functions
> Area | Function | Tags | Prefix
> --- | --- | --- | ---
> Functions | Collection length | length | fn
> String manipulation | `toUpperCase, toLowerCase` `substring, substringAfter, substringBefore` `trim` `replace` `indexOf, startsWith, endsWith, contains, containsIgnoreCase` `split, join` `escapeXml`

> While the `java.util.Collection` interface defines a `size` method, it does not conform to the JavaBeans design pattern for properties and cannot be accessed via the JSP expression language. The `length` function can be applied to any collection supported by the c`:forEach` and returns the length of the collection. When applied to a `String`, it returns the number of characters in the string.

> For example, the `greeting.jsp` page of the `hello2` application introduced in Updating Web Applications uses the `fn:length` function and `c:if` tag to determine whether to include a response page:

>     <%@ taglib uri="http://java.sun.com/jsp/jstl/core"
>       prefix="c" %>
>     <%@ taglib uri="http://java.sun.com/jsp/jstl/functions"
>       prefix="fn" %>
>     <html>
>     <head><title>Hello</title></head>
>     ...
>     <input type="text" name="username" size="25">
>     <p></p>
>     <input type="submit" value="Submit">
>     <input type="reset" value="Reset">
>     </form>

>     <c:if test="${fn:length(param.username) > 0}" >
>       <%@include file="response.jsp" %>
>     </c:if>
>     </body>
>     </html>

> The rest of the JSTL functions are concerned with string manipulation:

> - `toUpperCase`, `toLowerCase` - Changes the capitalization of a string.
> - `substring`, `substringBefore`, `substringAfter` - Gets a subset of a string.
> - `trim` - Trims whitespace from a string.
> - `replace` - Replaces characters in a string.
> - `indexOf`, `startsWith`, `endsWith`, `contains`, `containsIgnoreCase` - Checks if a string contains another string.
> - `split` - Splits a string into an array.
> - `join` - Joins a collection into a string.
> - `escapeXml` - Escapes XML characters in a string.

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