Java™ Application Development on Linux - Dator

18.6 Matters of State: Cookies, Hidden Variables, and the Dreaded “Back” Button413in form fields before sending them to a Web server. You can see that in someURLs—space gets replaced with a “+” character, and special characters (suchas the plus sign) get replaced with a character sequence for hexadecimal values(for example, “+” becomes %2B ). The getParameter() method will automaticallydecode those. But we need to remember this if we want to generate anyliteral URLs in the HTML that we produce. (See the URLEncoder class in theJavadoc documentation for servlets.)One more annoyance that must be dealt with: What if the URL containsthe same argument twice—for example, www.google.com/search?cmd=search&cmd=bogus?If you make the call to getParameter() you will get the first value(search). If you want to handle such a situation differently, you can callgetParameterValues() which will return an array of Strings for all thedifferent values. In our example,String [] allofem = getParameterValues("cmd");will return an array such that:allofem[0] = "search"allofem[1] = "bogus"If there was only one value, then you get an array of one element. If theparameter wasn’t used in the URL, getParameterValues() returns null.18.6MATTERS OF STATE: COOKIES, HIDDEN VARIABLES,AND THE DREADED “BACK” BUTTONThe toughest part about working with HTML is, perhaps, its statelessness.HTML and browsers were not designed to keep a connection going. It’s not aphone call type of connection, where the line is kept open between the browserand the Web server. Rather, it’s a one-shot, send-me-what-you’ve-got mechanismmore like postal mail (but without the stamp). Here’s the rub: Just becauseyou mail a letter, you can’t assume that you’ll get an answer back. Thereis no on-going connection between browser and server, except for the durationof the data transfer. Once you’ve got your complete page displayed, the