The Exception Flow in Java

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The Exception Flow
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When you throw an exception, it follows a well-de ned ow, much like events do. Exceptions bubble up through the call stack until they nd themselves in a try block. For each one of these, the exception compares its type to the types of the errors the associated catch blocks are looking for, top to bottom. If no catch block catches that type of exception, it continues its way up the call stack. If it reaches the top of the call stack without being caught, it becomes an uncaught exception. This means that not only do you have a choice of where to catch exceptions, but that choice makes a difference in how your program behaves. Take, for example, the class shown in Example 24-1, which computes 10 square roots of whole numbers between 50 and 50.
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24: Errors and Exceptions
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EXAMPLE 24-1
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http://actionscriptbible.com/ch24/ex1
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Throwing and Catching an Exception
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package { import com.actionscriptbible.Example; public class ch24ex1 extends Example { public function ch24ex1() { try { for (var i:int = 0; i < 10; i++) { var n:Number = Math.round(Math.random() * 100 - 50); trace(" "+n+" = "+squareRoot(n)); } } catch (err:ArgumentError) { trace("ERROR: " + err.message); } } protected function squareRoot(n:Number):Number { if (n < 0) { throw new ArgumentError("squareRoot() doesn t support \ imaginary numbers."); } return Math.sqrt(n); } } }
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The squareRoot() method checks its input and considers an attempt to calculate the square root of a negative number an error. Checking your inputs for validity is one important part of an overall error-handling strategy. The virtual machine and compiler can ensure your arguments are of the right type, but sometimes your preconditions must be more speci c. In this case, the best the type system can do is ensure that a Number is passed to squareRoot(), but it can t verify that the number is positive with types alone. An ArgumentError is an Error subclass designated for exceptions relating to arguments passed to a method. In any case, this version of the example will terminate the loop the rst time a negative number is chosen. You might receive only three square roots before an error message, or seven, or none, depending on your luck. This says that the entire loop is predicated on all the iterations nishing successfully. However, you can change this behavior by simply moving the try block. By putting the try block inside the loop body, the program always prints out 10 results or errors rather than quitting after the rst error. Why don t you modify the code and try it (Groan.) The ow of control in your program is affected by exception handling, and changes in your exceptions can change the outcome of your program. The catch blocks specify what kind of error is being handled. Because all exceptions are either an instance of Error or a subclass thereof, catching an Error type handles any kind of exception. You can use this in the same way you would use default in a switch statement. Example 24-2 shows an error being handled further up the call stack, and with multitiered catch blocks.
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Part V: Error Handling
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EXAMPLE 24-2
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http://actionscriptbible.com/ch24/ex2
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Errors and the Call Stack
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package { import com.actionscriptbible.Example; public class ch24ex2 extends Example { public function ch24ex2() { try { var n:int = sumSomeNumbers(); trace(n); } catch (e:UnrelatedError) { trace("something unrelated happened."); } catch (e:DispleasingNumberError) { trace("somewhere, a number is displeasing."); } catch (e:Error) { trace("something bad happened"); } } protected function sumSomeNumbers():int { var sum:int = 0; for (var i:int = 0; i < 10; i++) { sum += someNumber(); } return sum; } protected function someNumber():int { var n:int = Math.round(Math.random() * 10); if (n < 2) { throw new PatheticallySmallNumberError(); } return n; } } } class DispleasingNumberError extends Error {} class PatheticallySmallNumberError extends DispleasingNumberError {} class UnrelatedError extends Error {}
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With any luck, this example will eventually throw a PatheticallySmallNumberError because it is displeased with encountering the number 0 or 1. This exception originates in someNumber(), goes unhandled to the code that called it in sumSomeNumbers(), which again is not in a try block, and nally goes up to where the call originated, when the constructor CatchBlocksExample() called sumSomeNumbers(). This is inside a try block, so the exception compares itself with the handled exception types in the associated catch blocks in order. Because the runtime is comparing the exception object to different classes to see if it is an instance of the class speci ed in the catch clause or its subclasses, you can think of this comparison as using the is operator. The PatheticallySmallNumberError is not an UnrelatedError, but it is a
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