Java's Random Class

Whether you're simulating a dice roll in a game or generating random data for testing, the java.util.Random class is a powerful tool for generating random numbers.

The Random class is part of the java.util package and provides methods for generating random values of various types, including integers, booleans, and floating-point numbers. You can use this class to generate random numbers in a range, or with specific properties.

To use the Random class, you first need to create an instance of it:

import java.util.Random;
public class RandomExample {
  public static void main(String[] args) {
    Random rand = new Random();  // Creating a Random object
    double myDouble = rand.nextDouble(); // Generate a random Double   
    System.out.println(myDouble);
  } //main
} //RandomExample

You can also create a Random object with a specific seed value for repeatable sequences of random numbers (useful for testing or debugging). Just supply the value when creating the random object.

Random rand = new Random(12345);  // A specific seed

To generate random integers use rand.nextInt() like this…

import java.util.Random;

public class RandomExample {

  public static void main(String[] args) {

    Random rand = new Random();  // Creating a Random object
        
    int myInt = rand.nextInt(); // Generate a random Integer  
    System.out.println(myInt);
  } //main
} //RandomExample

You can limit the range of values generated by placing a value in the round brackets like this…

int myInt = rand.nextInt(100);  // Random integer from 0 to 99

or add to the max value like this…

//simulate a dice roll

int myInt = rand.nextInt(6)+1;  // Random integer from 1 to 6

Generate a random float value.

float myFloat = rand.nextFloat();  // Random float between 0.0 and 1.0

Generate a random boolean value (either true or false).

boolean myBool = rand.nextBoolean();

If you need to generate an array of random bytes, you can use the nextBytes method:

import java.util.Random;

public class RandomExample {

  public static void main(String[] args) {

    Random rand = new Random();  // Creating a Random object
        
    byte[] myByteArray = new byte[5];
    rand.nextBytes(myByteArray); //fills myByteArray with random bytes

    for(int i = 0; i < myByteArray.length; i++) {
      System.out.println(myByteArray[i]);
    }
  } //main
} //RandomExample

If you need random numbers that follow a Gaussian (normal) distribution with mean 0 and standard deviation 1, use the nextGaussian() method:

double gaussian = rand.nextGaussian();  // Gaussian value with mean 0 and std dev 1

System.out.println(gaussian);

You can scale and shift this to generate random values with a different mean and standard deviation:

double customMean = 10;

double customStdDev = 2;

double customGaussian = customMean + customStdDev * rand.nextGaussian();

System.out.println(customGaussian);