3.3 Random Numbers

Key terms: pseudorandom number generator (PRNG)

What does it mean to say that a sequence of numbers is random? This is a surprisingly deep question, and attempts to answer it have engaged mathematicians, physicists, and philosophers alike. We all have strong intuition about what it should mean — unpredictable, patternless, lacking any hidden structure — but turning this intuition into a rigorous definition is remarkably challenging. For our purposes, it suffices to know that the concept is subtle and that practical programming relies on a more accessible idea.

A pseudorandom number generator (PRNG) is an algorithm that produces a long sequence of numbers that appear random and pass standard statistical tests, even though each number is completely determined by the preceding one. Using the same initial seed value reproduces the same sequence, which is useful for testing and debugging, while varying the seed—perhaps as a function of the current time—produces sequences that are effectively unpredictable. Hardware-based generators can produce sequences closer to truly random, but software PRNGs are fast, convenient, and sufficient for most applications. Going forward, pseudorandom numbers will be referred to as random for brevity.

Many of the programs in this book require random numbers to model events with unpredictable outcomes, such as games of chance. The standard library provides several classes with methods for this purpose, but ThreadLocalRandom is the modern, general-purpose choice: it is fast, easy to use, and avoids the limitations of older classes. An instance is obtained via the factory method current. Listing 3.3.1 uses it to roll a pair of virtual dice.

Listing 3.3.1 - DiceRoller.java

DiceRoller.java

package chap03.sect3;

import java.util.concurrent.ThreadLocalRandom;

/**
 * Rolls a pair of virtual dice and outputs the result.
 *
 * @author Drue Coles
 */
public class DiceRoller {

   public static void main(String[] args) {
      ThreadLocalRandom rand = ThreadLocalRandom.current();

      // roll dice and compute sum
      int die1 = rand.nextInt(1, 7);
      int die2 = rand.nextInt(1, 7);
      int sum = die1 + die2;

      System.out.println("First die: " + die1);
      System.out.println("Second die: " + die2);
      System.out.println("Sum: " + sum);
   }
}

Output 3.3.1

First die: 5
Second die: 3
Sum: 8

The variables die1 and die2 are initialized using nextInt, which returns an integer chosen uniformly at random from a specified range. The lower bound is inclusive and the upper bound exclusive, so nextInt(1, 7) simulates rolling a six-sided die.