Java How to Program Fourth Edition - DCC

Chapter 6 Methods 261
6.8 Random-Number Generation
We now take a brief and, hopefully, entertaining diversion into a popular programming application,
namely, simulation and game playing. In this section and the next section, we will
develop a nicely structured game-playing program that includes multiple methods. The
program uses most of the control structures we have studied to this point in the book and
introduces several new concepts.
There is something in the air of a gambling casino that invigorates people—from the
high rollers at the plush mahogany-and-felt craps tables to the quarter poppers at the onearmed
bandits. It is the element of chance, the possibility that luck will convert a pocketful
of money into a mountain of wealth. The element of chance can be introduced through the
random method from the Math class. [Note: Java also provides a Random class in
package java.util. Class Random is covered in Chapter 20.]
Consider the following statement:
double randomValue = Math.random();
The random method of class Math generates a random double value from 0.0 up to, but
not including, 1.0. If method random truly produces values at random, then every value
from 0.0 up to, but not including, 1.0 should have an equal chance (or probability) of being
chosen each time method random is called. Note that the values returned by random are
actually pseudo-random numbers—a sequence of values produced by a complex mathematical
calculation. That calculation uses the current time of day to seed the random number
generator, such that each execution of a program yields a different sequence of random
values.
The range of values produced directly by method random often is different from the
range of values required in a particular Java application. For example, a program that simulates
coin tossing might require only 0 for “heads” and 1 for “tails.” A program that simulates
rolling a six-sided die would require random integers in the range from 1 to 6. A
program that randomly predicts the next type of spaceship (out of four possibilities) that
will fly across the horizon in a video game would require random integers in the range from
1 to 4.
To demonstrate method random, let us develop a program that simulates 20 rolls of
a six-sided die and displays the value of each roll. We use the multiplication operator (*)
in conjunction with method random as follows to produce integers in the range from 0
to 5:
(int) ( Math.random() * 6 )
This manipulation is called scaling the range of values produced by Math method random.
The number 6 in the preceding expression is called the scaling factor. The integer cast
operator truncates the floating-point part (the part after the decimal point) of each value
produced by the expression. Next, we shift the range of numbers produced by adding 1 to
our previous result, as in
1 + (int) ( Math.random() * 6 )
Figure 6.7 confirms that the results of the preceding calculation are integers in the range
from 1 to 6.
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