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Classifying an Error and an Exception
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When implementing an error- and exception-handling infrastructure, you need to define the difference between an error and exception. An error occurs when something happens that shouldn t have happened, but is predictable. An exception occurs when something happens
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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that shouldn t have happened, and isn t predictable. An exception should never happen, whereas an error could happen. The best way to illustrate the difference is the following bad example of using exceptions, in which an error handler would be the correct solution. (Note that the example could have been written more succinctly using other keywords, but that s not the point.) int[] args = new int[ 4]; try { for( int c1 = 0; ; c1 ++) { args[ c1] = 1; Tracer.output( 10, "Counter " + c1); } } catch( IndexOutOfRangeException excep) { Tracer.output( 10, "You've hit the end of the array"); } In the example, a for loop is created that incrementally counts and assigns the array args with a value of 1. In the definition of the for statement, there s no upper limit. The for loop will continue indefinitely, at least in theory. What happens in reality is once the boundary of the array has been exceeded, an exception is generated. An exception is always generated, so you need an exception handler that encapsulates the for loop. Looking at the solution, everyone will say that the problem is that there s no test for the upper limit in the for loop. Of course that s the problem, but I illustrated how it s possible to use exceptions when an upper limit test isn t used. This structure of putting together an application does happen albeit not with loops, but with other algorithms. This structure is wrong because an error handler, which is the upper limit, should have been used. Running the example code without the exception block generates the following exception: Unhandled Exception: System.IndexOutOfRangeException: Array index is out of range. in <0x00098> Chap02SectImplError.BadExceptionUsage:ExampleMethod () in <0x0001c> MainApp:DifferenceErrorAndException () in <0x00018> MainApp:Main (string[]) The exception that s generated is the IndexOutOfRangeException. Using the try and catch block catches the exception and ends the loop. Following is the generated output when the exception is caught: Counter 0 Counter 1 Counter 2 Counter 3 You've hit the end of the array Not all of the example is bad, because it illustrates how to deal with an exception when it does happen. The try and catch block is strategically added to encapsulate the potentially problematic code, and not all the code in general. A good exception-handling strategy does that because it allows you to focus on specific exceptions. Focusing on the exception makes it
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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possible to develop an exit strategy that may involve keeping the application running, and not arbitrarily exiting the application. When an exception is generated, many applications will exit, which isn t the best strategy as a general approach. Often the reason why applications exit is that it s the simplest method to implement. The clever reader will say that using an exception to indicate an end-of-array condition isn t a bad idea, and can even be efficient. Think of it as follows. To implement an error handler or upper limit test requires one clock cycle. To catch an exception requires 20 clock cycles. Therefore, if the array is 21 elements long, throwing an exception is more efficient than testing for an end of the array. However, even with a performance improvement, using exceptions in lieu of good error-handler functionality is wrong from a software engineering perspective. One of the main reasons not to use exceptions instead of error handlers is that code can become unpredictable, because there may be too many exception handlers. Consider the following modified source code based on the original example, which could be potentially unpredictable: int[] args = new int[ 10]; try { for( int c1 = 0; ; c1 ++) { int calculation = 1; try { Tracer.Output( "Counter " + c1); calculation = 23; } catch( Exception ex) { ; } args[ c1] = calculation; } } catch( IndexOutOfRangeException excep) { Tracer.output( 10, "You've hit the end of the array"); } In the modified source code, two pieces of functionality have been added. The first addition is a nested exception handler that catches the exception Exception. The second source code addition is the variable calculation. Going through the logic of the code, the variable calculation is defined and assigned a value of 1. Then, within the nested exception handler, the variable calculation is assigned a value of 23. Now imagine if the method Tracer.Output were to generate an error. The assignment of the calculation variable to 23 would be ignored, and the loop would continue processing the data without further consideration. What has happened is that the nested exception handler irons out any problems that result in allowing the application to continue. Yet the result is an invalid state as the assigning of calculation = 23 has been skipped. The assignment is an invalid assignment, and the result is an inconsistent application that appears to function properly. Focusing on the consistency problem, what s happening is the hiding of problems. Because there are a larger number of exception blocks, real exceptions are masked and captured by
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