When someone says his or her program is reliable, what do we expect from the
program? The majority of people would probably reply correctness as the most
important criterion in determining the reliability of a program. When a program is
claimed to be reliable, we certainly expect the program will produce correct results
for all valid input. It is hardly a reliable program if it produces correct results only
for some input values. As we all know by now, writing a correct program is easier
said than done. If we are not diligent and careful enough, we can easily introduce
bugs in our programs. And often we fail to eradicate them. A mechanism called an
assertion can be used to improve the likelihood of catching logical errors during the
development. We will introduce assertions in this chapter and show how to use them
effectively in our programs.
Program correctness guarantees correct results for all valid input. But
what happens when the input is invalid? Another important criterion of program
reliability is the robustness, which measures how well the program runs under
various conditions. If a program crashes too easily when a wrong type of argument
is passed to a method or an invalid input value is entered, we cannot say
the program is very reliable. A mechanism called exception handling can be
used to improve the program’s robustness. In this chapter, we will describe
how to code this exception-handling mechanism in Java to improve the program’s
robustness.
