Java Language Tutorial => Pseudo Random Numbers in Specific Range

Example

The method nextInt(int bound) of Random accepts an upper exclusive boundary, i.e. a number that the returned random value must be less than. However, only the nextInt method accepts a bound; nextLong, nextDouble etc. do not.

Random random = new Random();
random.nextInt(1000); // 0 - 999

int number = 10 + random.nextInt(100); // number is in the range of 10 to 109

Starting in Java 1.7, you may also use ThreadLocalRandom (source). This class provides a thread-safe PRNG (pseudo-random number generator). Note that the nextInt method of this class accepts both an upper and lower bound.

import java.util.concurrent.ThreadLocalRandom;

// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
ThreadLocalRandom.current().nextInt(min, max + 1);

Note that the official documentation states that nextInt(int bound) can do weird things when bound is near 2³⁰+1 (emphasis added):

The algorithm is slightly tricky. It rejects values that would result in an uneven distribution (due to the fact that 2^31 is not divisible by n). The probability of a value being rejected depends on n. The worst case is n=2^30+1, for which the probability of a reject is 1/2, and the expected number of iterations before the loop terminates is 2.

In other words, specifying a bound will (slightly) decrease the performance of the nextInt method, and this performance decrease will become more pronounced as the bound approaches half the max int value.

PDF - Download Java Language for free

Previous Next

Java Language

Fastest Entity Framework Extensions

Example

Got any Java Language Question?

Java Language

Java Language Random Number Generation Pseudo Random Numbers in Specific Range

Fastest Entity Framework Extensions

Example

Got any Java Language Question?