Software Engineering for Students A Programming Approach

SELF-TEST QUESTION
17.5 Exceptions and exception handlers 245
17.6 You are driving in your car when you get a flat tire. You change the tire
and continue. What strategy are you adopting – forward or backward
error recovery?
Now that we have identified two strategies for error recovery, we return to our analysis
of the two main types of error. Anticipated faults can be analyzed and predicted.
Their effects are known and treatment can be planned in detail. Therefore forward
error recovery is not only possible but most appropriate. On the other hand, the effects
of unanticipated faults are largely unpredictable and therefore backward error recovery
is probably the only possible technique. But we shall also see how a forward error recovery
scheme can be used to cope with design faults.
17.5 ● Exceptions and exception handlers
We have already seen that we can define a class of faults that arise only occasionally,
but are easily predicted. The trouble with occasional error situations is that, once
detected, it is sometimes difficult to cope with them in an organized way. Suppose,
for example, we want a user to enter a number, an integer, into a text field, see
Figure 17.1.
The number represents an age, which the program uses to see whether the person
can vote or note. First, we look at a fragment of this Java program without exception
handling. When a number has been entered into the text field, the event causes a
method called actionPerformed to be called. This method extracts the text from
the text field called ageField by calling the library method getText. It then calls
the library function parseInt to convert the text into an integer and places it in the
integer variable age. Finally the value of age is tested and the appropriate message
displayed:
Figure 17.1 Program showing normal behavior