Question:
How do I write a Java program that generates random numbers withinlimits, such as 1 to 100, or 1 to 1000?
Answer:
The java.util package includes a Random class which will generate asequence of pseudo-random numbers. Pseudo-random number sequencesappear to have a random distribution, but have a definite order.Given the same seed, a pseudo-random number generator will alwaysproduce the same sequence of numbers. Therefore, you shouldinitialize the Random class with as random a seed as possible. Usingthe current time as a seed is often sufficient.
However, when apseudo-random number sequence is exhausted, it wraps around and startsagain from the beginning. Consequently, for long-lived applicationsthat generate many random numbers over time, you will want toperiodically reseed your random generator.
The Random class does not provide a generic means of generating randomintegers within a specific range. Rather, it generates a uniformlydistributed set of values between 0 and some upper bound. For doublesand floats it generates a value between 0 and 1. You can use this togenerate arbitrary ranges of random integers. The following exampleprogram provides a random integer generating class that takes care ofconverting a random double into a random integer within a specificrange. It also takes care of reseeding the random number generatorafter the sequence has been in use for a long time.
import java.util.*;public final class RandomIntGenerator { public static final int DEFAULT_MIN_RANGE = 1; public static final int DEFAULT_MAX_RANGE = 100; int _minRange, _maxRange, _range; long _numCalls; Random _random; public RandomIntGenerator() { this(DEFAULT_MIN_RANGE, DEFAULT_MAX_RANGE); } public RandomIntGenerator(int minRange, int maxRange) { _random = new Random(System.currentTimeMillis()); setRange(minRange, maxRange); _numCalls = 0; } public void setRange(int minRange, int maxRange) { _minRange = minRange; _maxRange = maxRange; _range = maxRange - minRange + 1; } public int nextInt() { double d; d = ((double)_range)*_random.nextDouble(); if(++_numCalls == Integer.MAX_VALUE) { // The pseudo-random number sequence is sufficiently // exhausted that it is time to switch to a new seed. _numCalls = 0; _random.setSeed(System.currentTimeMillis()); } return (_minRange + (int)d); } public static final void main(String[] args) { RandomIntGenerator randomInt; randomInt = new RandomIntGenerator(); for(int i = RandomIntGenerator.DEFAULT_MIN_RANGE; i <= RandomIntGenerator.DEFAULT_MAX_RANGE; ++i) System.out.println(randomInt.nextInt()); }}
