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Now that you’ve read about what exceptions are and how to use them, it’s time to learn the advantages of using exceptions in your programs.Advantage 1: Separating Error-Handling Code from “Regular” Code
Exceptions provide the means to separate the details of what to do when something out of the ordinary happens from the main logic of a program. In traditional programming, error detection, reporting, and handling often lead to confusing spaghetti code. For example, consider the following pseudocode method that reads an entire file into memory:At first glance, this function seems simple enough, but it ignores all these potential errors.readFile { open the file; determine its size; allocate that much memory; read the file into memory; close the file; }To handle these cases, the
- What happens if the file can’t be opened?
- What happens if the length of the file can’t be determined?
- What happens if enough memory can’t be allocated?
- What happens if the read fails?
- What happens if the file can’t be closed?
readFile
function must have more code to do error detection, reporting, and handling. The function might look like this:There’s so much error detection, reporting, and returning here that the original seven lines of code are lost in the clutter. Worse yet, the logical flow of the code also has been lost, thus making it difficult to tell whether the code is doing the right thing: Is the file really being closed if the function fails to allocate enough memory? It’s even more difficult to ensure that the code continues to do the right thing after you modify the method three months after writing it. Many programmers “solve” this problem by simply ignoring it—errors are “reported” when their programs crash.errorCodeType readFile { initialize errorCode = 0; open the file; if (theFileIsOpen) { determine the length of the file; if (gotTheFileLength) { allocate that much memory; if (gotEnoughMemory) { read the file into memory; if (readFailed) { errorCode = -1; } } else { errorCode = -2; } } else { errorCode = -3; } close the file; if (theFileDidntClose && errorCode == 0) { errorCode = -4; } else { errorCode = errorCode and -4; } } else { errorCode = -5; } return errorCode; }Exceptions enable you to write the main flow of your code and to deal with the exceptional cases elsewhere. If the
readFile
function used exceptions instead of traditional error-management techniques, it would look more like this:Note that exceptions don’t spare you the effort of doing the work of detecting, reporting, and handling errors, but they do help you organize the work more effectively.readFile { try { open the file; determine its size; allocate that much memory; read the file into memory; close the file; } catch (fileOpenFailed) { doSomething; } catch (sizeDeterminationFailed) { doSomething; } catch (memoryAllocationFailed) { doSomething; } catch (readFailed) { doSomething; } catch (fileCloseFailed) { doSomething; } }Advantage 2: Propagating Errors Up the Call Stack
A second advantage of exceptions is the ability to propagate error reporting up the call stack of methods. Suppose that thereadFile
method is the fourth method in a series of nested method calls made by the main program:method1
callsmethod2
, which callsmethod3
, which finally callsreadFile
:Suppose also thatmethod1 { call method2; } method2 { call method3; } method3 { call readFile; }method1
is the only method interested in the errors that might occur withinreadFile
. Traditional error-notification techniques forcemethod2
andmethod3
to propagate the error codes returned byreadFile
up the call stack until the error codes finally reachmethod1
—the only method that is interested in them:Recall that the Java runtime environment searches backward through the call stack to find any methods that are interested in handling a particular exception. A method can “duck” any exceptions thrown within it, thereby allowing a method farther up the call stack to catch it. Hence, only the methods that care about errors have to worry about detecting errors:method1 { errorCodeType error; error = call method2; if (error) doErrorProcessing; else proceed; } errorCodeType method2 { errorCodeType error; error = call method3; if (error) return error; else proceed; } errorCodeType method3 { errorCodeType error; error = call readFile; if (error) return error; else proceed; }However, as the pseudocode shows, ducking an exception requires some effort on the part of the middleman methods. Any checked exceptions that can be thrown within a method must be specified in themethod1 { try { call method2; } catch (exception e) { doErrorProcessing; } } method2 throws exception { call method3; } method3 throws exception { call readFile; }throws
clause of the method.Advantage 3: Grouping and Differentiating Error Types
Because all exceptions thrown within a program are objects, grouping or categorizing of exceptions is a natural outcome of the class hierarchy. An example of a group of related exception classes in the Java platform are those defined injava.io
:IOException
and its descendants.IOException
is the most general and represents any type of error that can occur when performing I/O. Its descendants represent more specific errors. For example,FileNotFoundException
means that a file could not be located on disk.A method can write specific handlers that can handle a very specific exception. The
FileNotFoundException
class has no descendants, so the following handler can handle only one type of exception:A method can catch an exception based on its group or general type by specifying any of the exception’s superclasses in thecatch (FileNotFoundException e) { ... }catch
statement. For example, to catch all I/O exceptions, regardless of their specific type, an exception handler specifies anIOException
argument:This handler will catch all I/O exceptions, includingcatch (IOException e) { ... }FileNotFoundException
,EOFException
, and so on. You can find the details on what occurred by querying the argument passed to the exception handler. For example, to print the stack trace:You could even set up an exception handler that handles anycatch (IOException e) { e.printStackTrace(); // output goes to Sytem.err e.printStackTrace(System.out); // send trace to stdout }Exception
with this handler:Thecatch (Exception e) { // a (too) general exception handler ... }Exception
class is close to the top of theThrowable
class hierarchy. Therefore, this handler will catch many other exceptions in addition to those that the handler is intended to catch. You may want to handle exceptions this way if all you want your program to do, for example, is print out an error message for the user and exit.However, in most situations, you want exception handlers to be as specific as possible. The reason is that the first thing a handler must do is determine what type of exception occurred before it can decide on the best recovery strategy. In effect, by not catching specific errors, the handler must accommodate any possibility. Exception handlers that are too general can make code more error prone by catching and handling exceptions that weren’t anticipated by the programmer and for which the handler was not intended.
As we’ve shown, you can create groups of exceptions and handle exceptions in a general fashion, or you can use the specific exception type to differentiate exceptions and handle exceptions in an exact fashion.
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