Section 10.2. Handling Exceptional Conditions

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10.2. Handling Exceptional Conditions

To introduce you to handling exceptional conditions, Figure 10.1 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.

Figure 10.1. Poor design. No attempt is made to guard against a divide-by-zero error.

  /**    * Precondition: N > 0    * Postcondition: avgFirstN() = (1+2+... +N)/N    */ public double avgFirstN(int N) {     int sum = 0;     for (int k = 1; k <= N; k++)         sum += k;     return sum/N;         // What if N is 0? } // avgFirstN() 

10.2.1. Traditional Error Handling

Divide-by-zero error

Obviously, the method in Figure 10.1 should not simply ignore the possibility that N might be 0. Figure 10.2 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.

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Figure 10.2. One way to handle a divide-by-zero error might be to terminate the program if there is an attempt to divide by 0, assuming that it is the kind of program that can be safely aborted. This version does not use exception handling.

  /**    * Precondition:  N > 0    * Postcondition: avgFirstN() equals (1+2+... +N) divided by N    */ public double avgFirstN(int N) {     int sum = 0;     if (N <= 0) {       System.out.println("ERROR avgFirstN: N <= 0. Program terminating.");       System.exit(0);     }     for (int k = 1; k <= N; k++)         sum += k;     return sum/N;         // What if N is 0? } // avgFirstN() 

Effective Design: Unflxable 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.

The revised avgFirstN() method takes the traditional approach to error handling. The 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. 

10.2.2. Java's Default Exception Handling

To help detect and handle common runtime errors, Java's creators incorporated an exception-handling model into the language. 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 Figure 10.3. 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) 

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Figure 10.3. The two public classes defined in this figure would be saved in separate Java files.

public class CalcAverage {     public double avgFirstN(int N) {         int sum = 0;         for (int k = 1; k <= N; k++)             sum += k;         return sum/N;                                       // What if N is 0?     } // avgFirstN() } // CalcAverage class public class CalcAvgTest {     public static void main(String args[]) {         CalcAverage ca = new CalcAverage();         System.out.println( "AVG + " + ca.avgFirstN(0) );     } // main() } // CalcAvgTest class 

The error message describes the error and provides a trace of the method calls, from last to first, that led to the error. The trace shows that the error occurred in the CalcAverage.avgFirstN() method, which was called by the CalcAvgTest.main() method.

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 can be handled by the JVM.