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Random.nextInt(number)함수는 0부터 number를 제외한 이하의 수치값을 반환한다
There are two principal means of generating random (really pseudo-random) numbers:
- the Random class generates random integers, doubles, longs and so on, in various ranges.
- the static method Math.random generates doubles between 0 (inclusive) and 1 (exclusive).
To generate random integers:
- do not use Math.random (it produces doubles, not integers)
- use the Random class to generate random integers between 0 and N.
To generate a series of random numbers as a unit, you need to use a single Random object - do not create a new Random object for each new random number.
Other alternatives are:
- SecureRandom, a cryptographically strong subclass of Random
- ThreadLocalRandom, intended for multi-threaded cases
Here are some examples using Random.
Example 1
import java.util.Random; /** Generate 10 random integers in the range 0..99. */ public final class RandomInteger { public static final void main(String... aArgs){ log("Generating 10 random integers in range 0..99."); //note a single Random object is reused here Random randomGenerator = new Random(); for (int idx = 1; idx <= 10; ++idx){ int randomInt = randomGenerator.nextInt(100); log("Generated : " + randomInt); } log("Done."); } private static void log(String aMessage){ System.out.println(aMessage); } }
Example run of this class:
Generating 10 random integers in range 0..99. Generated : 44 Generated : 81 Generated : 69 Generated : 31 Generated : 10 Generated : 64 Generated : 74 Generated : 57 Generated : 56 Generated : 93 Done.
Example 2
This example generates random integers in a specific range.
import java.util.Random; /** Generate random integers in a certain range. */ public final class RandomRange { public static final void main(String... aArgs){ log("Generating random integers in the range 1..10."); int START = 1; int END = 10; Random random = new Random(); for (int idx = 1; idx <= 10; ++idx){ showRandomInteger(START, END, random); } log("Done."); } private static void showRandomInteger(int aStart, int aEnd, Random aRandom){ if (aStart > aEnd) { throw new IllegalArgumentException("Start cannot exceed End."); } //get the range, casting to long to avoid overflow problems long range = (long)aEnd - (long)aStart + 1; // compute a fraction of the range, 0 <= frac < range long fraction = (long)(range * aRandom.nextDouble()); int randomNumber = (int)(fraction + aStart); log("Generated : " + randomNumber); } private static void log(String aMessage){ System.out.println(aMessage); } }
An example run of this class:
Generating random integers in the range 1..10. Generated : 9 Generated : 3 Generated : 3 Generated : 9 Generated : 4 Generated : 1 Generated : 3 Generated : 9 Generated : 10 Generated : 10 Done.
Example 3
This example generates random floating point numbers in a Gaussian (normal) distribution.
import java.util.Random; /** Generate pseudo-random floating point values, with an approximately Gaussian (normal) distribution. Many physical measurements have an approximately Gaussian distribution; this provides a way of simulating such values. */ public final class RandomGaussian { public static void main(String... aArgs){ RandomGaussian gaussian = new RandomGaussian(); double MEAN = 100.0f; double VARIANCE = 5.0f; for (int idx = 1; idx <= 10; ++idx){ log("Generated : " + gaussian.getGaussian(MEAN, VARIANCE)); } } private Random fRandom = new Random(); private double getGaussian(double aMean, double aVariance){ return aMean + fRandom.nextGaussian() * aVariance; } private static void log(Object aMsg){ System.out.println(String.valueOf(aMsg)); } }
An example run of this class:
Generated : 99.38221153454624 Generated : 100.95717075067498 Generated : 106.78740794978813 Generated : 105.57315286730545 Generated : 97.35077643206589 Generated : 92.56233774920052 Generated : 98.29311772993057 Generated : 102.04954815575822 Generated : 104.88458607780176 Generated : 97.11126014402141