zxing qr code reader java 14: Make Your Program Errorproof in Java

Maker QR Code ISO/IEC18004 in Java 14: Make Your Program Errorproof

CHAPTER 14: Make Your Program Errorproof
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Understand Exception Bubbling
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The try-catch statement can also capture errors generated inside function calls within the try clause For instance, say we were to move some of the code from the previous example into a function of its own
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var isIE = (navigatoruserAgentindexOf("MSIE")) > -1; function calculate() { var a = 1; return a / b; } try { var c = calculate(); alert("The result of a / b is " + c); } catch (e) { var msg = "JavaScript encountered the following error:\n\n"; // IE and Netscape pass errors differently to the catch clause if (isIE) { msg += edescription; } else { msg += e; } alert (msg); }
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The try-catch statement will still receive notification of the error, even though it occurs inside the calculate() function This is because exceptions, like events, bubble Exception bubbling is the term that is used to describe the process of how an exception will seek out the first try-catch statement eligible to handle it If there is no try-catch statement immediately surrounding the line of code with the error, control will jump back to the code that called the function containing the error If there is no try-catch there, control will jump back to the code that called it, and so on, until either a try-catch is found or the browser handles the exception by displaying it to the user
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For example, the following code generates an error after several levels of function calls have been made The exception still manages to make its way back to the try-catch statement surrounding the first function call
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var isIE = (navigatoruserAgentindexOf("MSIE")) > -1; function fn1 () { // The first function (fn1) calls the second (fn2) fn2(); } function fn2 () { // The second function (fn2) calls the third (fn3) fn3(); } function fn3 () { // The third function (fn3) calls the fourth (fn4) fn4(); } function fn4 () { // The fourth function (fn4) calls the fifth (fn5) fn5(); } function fn5 () { // The fifth function (fn5) contains an error return some_undefined_variable; } // MAIN program try { fn1(); } catch (e) { var msg = "JavaScript encountered the following error:\n\n"; // IE and Netscape pass errors differently to the catch clause if (isIE) { msg += edescription; } else { msg += e; } alert (msg); }
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CHAPTER 14: Make Your Program Errorproof
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As you can see from the following screenshot, both Netscape and IE are able to pass the exception properly to our catch clause
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You do not always have to respond to an exception by displaying a message to the user You could always ignore the error and continue
Use the IE Error Object
As we observed in the previous section, Internet Explorer passes an Error object to the parameter of the catch clause The Error object contains two properties:
number A numeric value related to the error description A string describing the error
The code in this section will work only in Internet Explorer IE automatically creates an Error object whenever an exception occurs, but you are also free to create your own error using the following new statement syntax:
var myerr = new Error (1000, "Value falls outside acceptable range of 0-81");
Using this object allows your program to easily distinguish between different types of errors while also retaining the plain-English description to display to the user
// Error 1000: Ignore, set return code to 0 if (enumber == 1000) { var returncode = 0; // Error 1001: Exit loop, set return code to 9999 } else if (enumber == 1001) { var returncode = 9999; // Other error code: Can't handle this, report to user } else { alert(edescription); }
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The only real problem with this method is that Netscape uses a different approach, which means you have to have browser-sniffing code in order to create cross-platform-compatible code
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