Part 12


Encryption algorithms, machine learning and making computer games less predictable all require randomness. We can model randomness using random numbers. Java offers ready-made Random class for creating random numbers. An instance of the Random class can be used as follows:

import java.util.Random;

public class Raffle {
    public static void main(String[] args) {
        Random ladyLuck = new Random(); // create Random object ladyLuck

        for (int i = 0; i < 10; i++) {
            // Draw and print a random number
            int randomNumber = ladyLuck.nextInt(10);

Above we create an instance of the Randomclass. It has nextInt method, which gets an integer as a parameter. The method returns a random number between [0, integer[ or 0..(integer -1).

The program output is not always the same. One possible output is the following:

Sample output

2 2 4 3 4 5 6 0 7 8


We can use the nextInt method to create diverse randomness. For example, we might need a program to give us a temperature between [-30,50]. We can do this by first creating random a number between 0 and 80 and then subtracting 30 from it.

Random weatherMan = new Random();
int temperature = weatherMan.nextInt(81) - 30;

A Random object can also be used to create random doubles. These can for example be used for calculating probabilities. Computers often simulate probabilities using doubles between [0..1].

The nextDouble method of the Random class creates random doubles. Let's assume the weather follows these probabilities:

  • There is 0.1 probability it rains (10%)
  • There is 0.3 probability it snows (30%)
  • There is 0.6 probability the sun shines (60%)

Let's create a weather forecast using these probabilities.

import java.util.ArrayList;
import java.util.Random;

public class WeatherMan {
    private Random random;

    public WeatherMan() {
        this.random = new Random();

    public String forecast() {
        double probability = this.random.nextDouble();

        if (probability <= 0.1) {
            return "It rains";
        } else if (probability <= 0.4) { // 0.1 + 0.3
            return "It snows";
        } else { // rest, 1.0 - 0.4 = 0.6
            return "The sun shines";

    public int makeAForecast() {
        return (int) (4 * this.random.nextGaussian() - 3);

The makeAForecast method is interesting in many ways. The this.random.nextGaussian() call is a regular method call. However what is interesting is that this method of the Random class returns a normally distributed number (normally distributed numbers can be used to for example model the heights and weights of people — if you are not interested in different kinds of randomness that is OK!).

public int makeAForecast() {
    return (int) (4 * this.random.nextGaussian() - 3);

In the previous example we use explicit type casting to convert doubles to integers (int). We can equally convert integers to doubles with (double) integer.

Let's now add a main which uses the WeatherMan class.

// imports

public class Program {

    public static void main(String[] args) {
        WeatherMan forecaster = new WeatherMan();

        // save days of the week to a list
        ArrayList<String> days = new ArrayList<>();

        System.out.println("Next week's weather forecast:");

        for (String day : days) {
            String weatherForecast = forecaster.forecast();
            int temperatureForecast = forecaster.makeAForecast();

            System.out.println(day + ": " + weatherForecast + " " + temperatureForecast + " degrees.");

The program output could be:

Sample output

Next week's weather forecast: Mon: It snows 1 degrees. Tue: It snows 1 degrees. Wed: The sun shines -2 degrees. Thu: The sun shines 0 degrees. Fri: It snows -3 degrees. Sat: It snows -3 degrees. Sun: The sun shines -5 degrees

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