More-sophisticated behavior - Objects First with Java: A Practical Introduction Using BlueJ - page 167

To generate a random number, we have to:
■ create an instance of class Random and
■ make a call to a method of that instance to get a number.

Looking at the documentation, we see that there are various methods called next Something for
generating random values of various types. The one that generates a random integer number is
called nextInt .

The following illustrates the code needed to generate and print a random integer number:

Random randomGenerator;

randomGenerator = new Random();
int index = randomGenerator.nextInt();
System.out.println(index);

This code fragment creates a new instance of the Random class and stores it in the randomGen-
erator variable. It then calls the nextInt method to receive a random number, stores it in the
index variable, and eventually prints it out.

Exercise 5.14 Write some code (in BlueJ) to test the generation of random numbers. To do
this, create a new class called RandomTester . You can create this class in the tech-support1
project, or you can create a new project for it—it doesn't matter. In class RandomTester ,
implement two methods: printOneRandom (which prints out one random number) and
printMultiRandom(int howMany) (which has a parameter to specify how many num-
bers you want, and then prints out the appropriate number of random numbers).

Your class should create only a single instance of class Random (in its constructor) and store it
in a field. Do not create a new Random instance every time you want a new number.

5.4.2 Random numbers with limited range

The random numbers we have seen so far were generated from the whole range of Java integers
(-2147483648 to 2147483647). That is okay as an experiment, but seldom useful. More often,
we want random numbers within a given limited range.

The Random class also offers a method to support this. It is again called nextInt , but it has a
parameter to specify the range of numbers that we would like to use.

Exercise 5.15 Find the nextInt method in class Random that allows the target range of
random numbers to be specified. What are the possible random numbers that are generated
when you call this method with 100 as its parameter?

Exercise 5.16 Write a method in your RandomTester class called throwDice that re-
turns a random number between 1 and 6 (inclusive).