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The JSP Files (part 3): Black Light And White Rabbits By Vikram Vaswani and Harish Kamath This article copyright Melonfire 2000−2002. All rights reserved. Table of Contents Counting Down 1 Doing More With Loops .3 For−gone Conclusion 5 The Sound Of Breaking Loops 8 Paying The Piper .9 Screaming Out Loud .11 You Say Seven, I Say 7 .14 A Positive Response 17 The JSP Files (part 3): Black Light And White Rabbits i Counting Down Last time out, you learned a little bit about the various conditional statements and operators available in JSP. This week, we'll expand on those basics by teaching you a little bit about the different types of loops available in JSP, discuss a few more String object methods, and take a quick tour of the new Response object. First up, loops. As you may already know, a "loop" is a programming construct that allows you to execute a set of statements over and over again, until a pre−defined condition is met. The most basic loop available in JSP is the "while" loop, and it looks like this: while (condition) { do this! } Or, to make the concept clearer, while (temperature is below freezing) { wear a sweater } The "condition" here is a standard conditional expression, which evaluates to either true or false. So, were we to write the above example in JSP, it would look like this: while (temp <= 0) { sweater = true; } Here's an example: <html> <head> </head> <body> <%! int countdown=30; Counting Down 1 %> <% while (countdown > 0) { out.println(countdown + " "); countdown−−; } out.println("<b>Kaboom!</b>"); %> </body> </html> Here, the variable "countdown" is initialized to 30, and a "while" loop is used to decrement the value of the variable until it reaches 0. Once the value of the variable is 0, the conditional expression evaluates as false, and the lines following the loop are executed. Here's the output: 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Kaboom! The JSP Files (part 3): Black Light And White Rabbits Counting Down 2 Doing More With Loops There's one caveat with the "while" loop. If the conditional expression evaluates as false the first time, the code within the curly braces will never be executed. If this is not what you want, take a look at the "do−while" loop, which comes in handy in situations where you need to execute a set of statements *at least* once. Here's what it looks like: do { do this! } while (condition) For example, the following lines of code would generate no output whatsoever, since the conditional expression in the "while" loop would always evaluate as false. int bingo = 366; while (bingo == 699) { out.println ("Bingo!"); break; } %> However, the construction of the "do−while" loop is such that the statements within the loop are executed first, and the condition to be tested is checked after. Using a "do−while" loop implies that the code within the curly braces will be executed at least once − regardless of whether or not the conditional expression evaluates as true. Doing More With Loops 3 int bingo = 366; do { out.println ("Bingo!"); break; } while (bingo == 699); %> Try it yourself and see the difference. The JSP Files (part 3): Black Light And White Rabbits Doing More With Loops 4 For−gone Conclusion Both the "while" and "do−while" loops continue to iterate for so long as the specified conditional expression remains true. But there often arises a need to execute a certain set of statements a specific number of times − for example, printing a series of thirteen sequential numbers, or repeating a particular set of <TD> cells five times. In such cases, clever programmers reach for the "for" loop . The "for" loop typically looks like this: for (initial value of counter; condition; update counter) { do this! } Looks like gibberish? Well, hang in there a minute .the "counter" here is a JSP variable that is initialized to a numeric value, and keeps track of the number of times the loop is executed. Before each execution of the loop, the "condition" is tested − if it evaluates to true, the loop will execute once more and the counter will be appropriately incremented; if it evaluates to false, the loop will be broken and the lines following it will be executed instead. And here's a simple example that demonstrates how this loop can be used: <html> <head> <basefont face="Arial"> </head> <body> <center>Turning The Tables, JSP−Style!</center> <br> <%! // define the number int number = 7; int x; %> <% // use a for loop to calculate tables for that number For−gone Conclusion 5 for (x=1; x<=15; x++) { out.println(number + " X " + x + " = " + (number*x) + "<br>"); } %> </body> </html> And here's the output: Turning The Tables, JSP−Style! 7 X 1 = 7 7 X 2 = 14 7 X 3 = 21 7 X 4 = 28 7 X 5 = 35 7 X 6 = 42 7 X 7 = 49 7 X 8 = 56 7 X 9 = 63 7 X 10 = 70 7 X 11 = 77 7 X 12 = 84 7 X 13 = 91 7 X 14 = 98 7 X 15 = 105 Let's dissect this a little bit: Right up front, a variable is defined, containing the number to be used for the multiplication table; we've used 7 here − you might prefer to use another number. Next, a "for" loop has been constructed, with "x" as the counter variable. If you take a look at the first line of the loop, you'll see that "x" has been initialized to 1, and is set to run no more than 15 times. The JSP Files (part 3): Black Light And White Rabbits For−gone Conclusion 6 Finally, the println() function is used to take the specified number, multiply it by the current value of the counter, and display the result on the page. The JSP Files (part 3): Black Light And White Rabbits For−gone Conclusion 7 The Sound Of Breaking Loops When dealing with loops, there are two important keywords you should be aware of: "break" and "continue". The "break" keyword is used to exit a loop when it encounters an unexpected situation. A good example of this is the dreaded "division by zero" error − when dividing one number by another one (which keeps decreasing), it is advisable to check the divisor and use the "break" statement to exit the loop as soon as it becomes equal to zero. As you've already seen, the "continue" keyword is used to skip a particular iteration of the loop and move to the next iteration immediately − it's demonstrated in the following example: int x; for (x=1; x<=10; x++) { if (x == 7) { continue; } else { out.println(x + " "); } } %> In this case, JSP will print a string of numbers from 1 to 10 − however, when it hits 7, the "continue" statement will cause it to skip that particular iteration and go back to the top of the loop. So your string of numbers will not include 7 − try it and see for yourself. The Sound Of Breaking Loo . 8 [...]... out.println( "The string " + beta + " starts with " + prefix + " "); } if (alpha.endsWith(suffix)) { out.println( "The string " + alpha + " ends with " + suffix + " "); } if (beta.endsWith(suffix)) { out.println( "The string " + beta + " ends with " + suffix + " "); } %> Screaming Out Loud 12 The JSP Files (part 3): Black Light And White Rabbits And the output is: The string black light starts with bl The string white. .. equals 973 You Say Seven, I Say 7 15 The JSP Files (part 3): Black Light And White Rabbits The upshot? parseInt() and toString() are your best friends when converting between string and numeric data types in JSP You Say Seven, I Say 7 16 A Positive Response Another interesting object, and one that comes in handy in the oddest places, is the Response object As opposed to the Request object, which is used... out.println(someNumber + " plus " + aRandomNumber + " equals " + (someNumber+aRandomNumber)); %> And here's the output 97 plus 3 equals 973 97 plus 3 equals 100 [output] You Say Seven, I Say 7 14 The JSP Files (part 3): Black Light And White Rabbits If you'd prefer to do things the other way around, the next example bears careful consideration // define variables int someNumber = 97; int aRandomNumber = 3; // at... identify the last occurrence of a character or substring in a larger string Take a look: Paying The Piper 9 The JSP Files (part 3): Black Light And White Rabbits And the output is The string... character or sequence of characters The following example should illustrate this clearly Screaming Out Loud 11 The JSP Files (part 3): Black Light And White Rabbits // define variables String alpha = "black light" ; String beta = "white rabbit"; String prefix = "bl"; String suffix = "it"; // check each string for prefixes and suffixes if (alpha.startsWith(prefix)) { out.println( "The string " + alpha + " starts... of a document with the setContentType() method A Positive Response 17 The JSP Files (part 3): Black Light And White Rabbits response.setContentType("image/jpeg"); %> and even set a cookie with the addCookie() method − more on this as we progress through this series That's about all we have time for Next time, we'll be exploring the JSP Response object, used to process data from HTML forms so you don't... = "The Pied Piper Of Hamlin"; // find index int i = storyName.indexOf("i"); // print index out.println( "The letter i first occurs at " + i + " in the string " + storyName); %> this is what you would see: The letter i first occurs at 5 in the string The Pied Piper Of Hamlin Yes, the first character is treated as index 0, the second as index 1, and so on These programmers The opposite of this is the. .. 9 in the string The Pied Piper Of Hamlin In case the character or substring is not located, the function will return an error code of −1 Paying The Piper 10 Screaming Out Loud Next, the trim() function comes in handy when you need to remove white space from the ends of a string // define variable String whatIWant = " gimme my space "; // trim! // returns "gimme my space" whatIWant.trim(); %> The toUpperCase()... toUpperCase() and toLowerCase() methods come in handy to alter the case of a string // define variable String someString = "don't SCREam, help is oN the WAy!"; // uppercase − returns "DON'T SCREAM, HELP IS ON THE WAY!" someString.toUpperCase(); // lowercase − returns "don't scream, help is on the way!" someString.toLowerCase(); %> The startsWith() and endsWith() functions are used to verify whether a string... on this), the Response object is typically used to send information to the browser; this information could include HTTP headers, redirection URLs, cookies and a variety of other items Since the Response object is an "implicit" object (so called because you do not need to explicitly create an instance of the object when you want to use it), you can begin using it immediately in a JSP document The following . %> The JSP Files (part 3): Black Light And White Rabbits Screaming Out Loud 12 And the output is: The string black light starts with bl The string white. by the current value of the counter, and display the result on the page. The JSP Files (part 3): Black Light And White Rabbits For−gone Conclusion 7 The
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