Handling Exceptions with try-catch and throw in Java

Create QR Code 2d barcode in Java Handling Exceptions with try-catch and throw

<SCRIPT LANGUAGE= JavaScript TYPE= text/javascript > function getMonthName (monthNumber) { // JavaScript arrays begin with 0, not 1, so // subtract 1 monthNumber = monthNumber - 1 // Create an array and fill it with 12 values var months = new Array( Jan , Feb , Mar , Apr , May , Jun , Jul , Aug , Sep , Oct , Nov , Dec ) // If a month array element corresponds to the // number passed in, fine; return the array // element if (months[monthNumber] != null) {

return months[monthNumber] } // Otherwise, an exception occurred, so throw // an exception else { // This statement throws an error // directly to the catch block throw InvalidMonthNumber } } ////////////////////////////////////////////////////// // The try block wraps around the main JavaScript // processing code Any JavaScript statement inside // the try block that generates an exception will // automatically throw that exception to the // exception handling code in the catch block ////////////////////////////////////////////////////// // The try block try { // Call the getMonthName() function with an // invalid month # (there is no 13th month!) // and see what happens alert(getMonthName(13)) alert( We never get here if an exception is thrown ) } // The catch block catch (error) { alert( An + error + exception was encountered program vendor ) // // // // // } In a real-life situation, you might want to include error-handling code here that examines the exception and gives users specific information (or even tries to fix the problem, if possible) Please contact the

Take a look at Figure 17-4 to see the error that running the code in Listing 17-2 generates in Internet Explorer

The first code executed in Listing 17-2 is the code that you see defined in the try block:

alert(getMonthName(13))

Because only 12 months are defined in the months array, passing a value of 13 to getMonthName() causes an exception ( InvalidMonthNumber ) to be thrown, as shown here:

All thrown exceptions are processed automatically by whatever code exists in the catch block, so the message that you see in Figure 17-4 appears automatically when the exception is thrown If you want to write truly airtight JavaScript code, you need to identify all the events that could possibly cause an exception in your particular script (such as actions the user could take, error conditions the operating system could generate, and so on), and implement a try-catch block for each

Depending on your application, you might want to include more processing code in the catch block than the simple pop-up message shown in Figure 17-4 For example, you might want to include JavaScript statements that examine the caught exception, determine what kind of exception it is, and process it appropriately You aren t limited to a string literal when it comes to identifying a thrown exception Instead of InvalidMonthNumber, you can create and throw an elaborate custom exception object (by using the function and new operators that I describe in 3) For more information on how Netscape implements exception handling (including examples), visit

http://developernetscapecom/docs/manuals/js/core/jsguide/stmtsovhtm#1011537

To see how Microsoft does the same for Internet Explorer, check out this page:

Both Netscape and Microsoft offer free JavaScript debugging tools They include Netscape Navigator s built-in JavaScript console (available for use with Netscape Navigator 7x) Microsoft Internet Explorer s built-in error display In addition to the built-in debugging tools that I describe in this section, Netscape and Microsoft offer standalone script debuggers that you can download for free Venkman is the name of the free JavaScript debugger created for use with Netscape Navigator 7x Although support for this JavaScript debugger is spotty at best, you can get the latest documentation (and download your very own copy) from wwwhacksruscom/~ginda/venkman Microsoft offers a script debugger that you can use to debug JScript scripts (as well as scripts written in other scripting languages, such as Microsoft s own VBScript) To download a copy of Microsoft s script debugger, point your browser to