Section 10.2 Handling Exceptional Conditions
Subsection 10.2.1 Introducing an Exception
To introduce you to handling exceptional conditions, Listing 10.2.1 shows a method that computes the average of the first N integers, an admittedly contrived example. We use it mainly to illustrate the basic concepts involved in exception handling. As its precondition suggests, the
avgFirstN() method expects that N will be greater than 0. If N happens to be 0, an error will occur in the expression
sum/N, because you cannot divide an integer by 0. Try it to see this error.
Try the program below to see the error when N is 0.
Subsection 10.2.2 Traditional Error Handling
Obviously, the method in Listing 10.2.1 should not simply ignore the possibility that N might be 0. Listing 10.2.3 shows a revised version of the method, which includes code that takes action if the method's precondition fails. Because there is no way to compute an average of 0 elements, the revised method decides to abort the program. Aborting the program appears to be a better alternative than returning 0 or some other default value (like \(-1\)) as the method's result and thereby allowing an erroneous value to spread throughout the program. That would just compound the error.
Principle 10.2.2. EFFECTIVE DESIGN:Unfixable Error.
If an unfixable error is detected, it is far better to terminate the program abnormally than to allow the error to propagate throughout the program.
avgFirstN() method takes the traditional approach to error handling: Error-handling code is built right into the algorithm. If N happens to be 0 when
avgFirstN() is called, the following output will be generated:
ERROR avgFirstN: N <= 0. Program terminating.
Try the revised program below to see the error when N is 0.
Subsection 10.2.3 Java's Default Exception Handling
To help detect and handle common runtime errors, Java's creators incorporated an exception-handling model into the language itself. In the case of our divide-by-zero error, the Java Virtual Machine (JVM) would detect the error and abort the program. To see this, consider the program in Listing 10.2.4.
Note that the
avgFirstN() method is passed an argument of 0 in the
CalcAvgTest.main(). When the JVM detects the error, it will abort the program and print the following message:
Exception in thread "main" java.lang.ArithmeticException: / by zero at CalcAverage.avgFirstN(Compiled Code) at CalcAvgTest.main(CalcAvgTest.java:5)
The error message describes the error and provides a trace of the method calls, from last to first, that led to the error. This trace shows that the error occurred in the
CalcAverage.avgFirstN() method, which was called by the
As this example suggests, Java's default exception handling is able to detect and handle certain kinds of errors and exceptional conditions. In the next section, we will identify what kinds of conditions are handled by the JVM.