Software Engineering for Students A Programming Approach

256 Chapter 17 ■ Software robustness
17.3 Consider a programming language with which you are familiar. In what ways can you
deliberately (or inadvertently) write a program that will:
1. crash
2. access main memory in an undisciplined way
3. access a file protected from you.
What damage is caused by these actions? How much damage is possible?
Assuming you didn’t already know it, is it easy to diagnose the cause of the problem?
Contemplate that if it is possible deliberately to penetrate a system, then it is
certainly possible to do it by accident, thus jeopardizing the reliability and security
of the system.
17.4 “Compile-time checking is better than run-time checking.” Discuss.
17.5 Compare and contrast exception handling with assertions.
17.6 The Java system throws an IndexOutOfBoundsException exception if a program
attempts to access elements of an array that lie outside the valid range of
subscripts. Write a method that calculates the total weekly rainfall, given an array
of floating point numbers (values of the rainfall for each of seven days of the
week) as its single parameter. The method should throw an exception of the same
type if an array is too short. Write code to catch the exception.
17.7 Outline the structure of recovery block software to cope with the following situation.
A fly-by-wire aircraft is controlled by software. A normal algorithm calculates the optimal
speed and the appropriate control surface and engine settings. A safety module
checks that the calculated values are within safe limits. If they are not, it invokes an
alternative module that calculates some safe values for the settings. If, again, this
module fails to suggest safe values, the pilots are alerted and the aircraft reverts to
manual control.
17.8 Compare and contrast the recovery block scheme with the n-programming scheme
for fault tolerance. Include in your review an assessment of the development times
and performance overheads associated with each scheme.
17.9 Searching a table for a desired object is a simple example of a situation in which it
can be tempting to use a goto to escape from an unusual situation. Write a piece
of program to search a table three ways:
1. using goto
2. using exceptions
3. avoiding both of these.
Compare and contrast the three solutions.