Program Logic and Indefinite Loops - Building Java Programs: A Back to Basics Approach - page 334

Table 5.5 De Morgan's Laws

Original expression Negated expression Simplified negation

p || q !(p || q) !p && !q

p && q !(p && q) !p || !q

We can use the first De Morgan's law for our two-digit number program because
we are trying to find the negation of an expression that involves the logical OR opera-
tor. Instead of writing:

while (!(number >= 10 && number <= 99))

we can say:

while (number < 10 || number > 99)

Each individual test has been negated and the AND operator has been replaced
with an OR operator.
Let's look at a second example that involves the other De Morgan's law. Suppose
that you want to ask the user a question and you want to force the user to answer
either “yes” or “no”. If you have a String variable called response , you can use the
following test to describe what you want to be true:

response.equals("yes") || response.equals("no")

If we're writing a loop to keep reading a response until this expression evaluates to
true , then we want to write the loop so that it uses the negation of this test. So, once
again, we could use the NOT operator for the entire expression:

while (!(response.equals("yes") || response.equals("no")))

Once again it is best to simplify the expression using De Morgan's law:

while (!response.equals("yes") && !response.equals("no"))

5.4 User Errors

In the previous chapter, you learned that it is good programming practice to think
about the preconditions of a method and to mention them in the comments for the
method. You also learned that in some cases your code can throw exceptions if pre-
conditions are violated.