Program Logic and Indefinite Loops - Building Java Programs: A Back to Basics Approach - page 307

Table 5.2 Useful Methods of Random Objects

Method Description

Random integer between -2 31 and (2 31 -1) nextInt()

Random integer between 0 and (max - 1)

nextInt(max)

Random real number between 0.0 (inclusive) and 1.0 (exclusive) nextDouble()

Random logical value of true or false

nextBoolean()

To create random numbers, you first construct a Random object:

Random r = new Random();

You can then call its nextInt method, passing it a maximum integer. The number
returned will be between 0 (inclusive) and the maximum (exclusive). For example, if
you call nextInt(100) , you will get a number between 0 and 99. You can add 1 to
the number to have a range between 1 and 100.
Let's look at a simple program that picks numbers between 1 and 10 until a particu-
lar number comes up. We'll use the Random class to construct an object for generating
our pseudorandom numbers.
Our loop should look something like this (where number is the value the user has
asked us to generate):

int result;
while (result != number) {
result = r.nextInt(10) + 1; // random number from 1-10
System.out.println("next number = " + result);

}

Notice that we have to declare the variable result outside the while loop,
because result appears in the while loop test. The preceding code has the right
approach, but Java won't accept it. The code generates an error message that the vari-
able result might not be initialized. This is an example of a loop that needs
priming.

Priming a Loop
Initializing variables before a loop to “prime the pump” and guarantee that
the loop is entered.

We want to set the variable result to something that will cause the loop to be
entered, but the value isn't important as long as it gets us into the loop. We do want to
be careful not to set it to a value the user wants us to generate, though. We are dealing
with values between 1 and 10 in this program, so we could set result to a value
such as -1 that is clearly outside this range of numbers. We sometimes refer to this as