Slot machine simulation

The problem

A slot machine is a gambling device that the user inserts money into and then pulls a lever (or presses a button). The slot machine then displays a set of random images. If two or more of the images match, the user wins an amount of money that the slot machine dispenses back to the user.

Create a program that simulates a slot machine. When the program runs, it should do the following:

  • Ask the user to enter the amount of money he or she wants to enter into the slot machine.
  • Instead of displaying images, the program will randomly select a word from the following list: Cherries, Oranges, Plums, Bells, Melons, Bars
  • To select a word, the program can generate a random number in the range of 0 through 5. If the number is 0, the selected word is Cherries; if the number is 1, the selected word is Oranges; and so forth. The program should randomly select a word from the list three times and display all three of the words.
  • If none of the randomly selected words match, the program will inform the user that he or she has won $0.
  • If two of the words match, the program will inform the user that he or she was won two times the amount entered.
  • If three of the words match, the program will inform the user that he or she has won three times the amount entered.
  • The program will ask whether the user wants to play again. If so, these steps are repeated. If not, the program displays the total amount of money entered into the slot machine and the total amount won.

Breaking it down

Initialize constants

static String[] reels = { "Cherries", "Oranges", "Plums", "Bells", "Melons", "Bars" };
static int numberOfReels = 3;

Declare variables

char playAgain;
double amountBet = 0;
double totalWon = 0;
DecimalFormat dollar = new DecimalFormat("#,##0.00");

// Create a Scanner object for keyboard input.
Scanner keyboard = new Scanner(;

Create supporting methods

 * Method should return the number of times an occurrence of a reel
 * @param reels
 * @return
static int determinePayOutPercentage(List<String> reels) {

    Multiset<String> reelCount = HashMultiset.create();

    // order the number of elements by the higest
    ImmutableMultiset<String> highestCountFirst = Multisets.copyHighestCountFirst(reelCount);

    int count = 0;
    for (Entry<String> entry : highestCountFirst.entrySet()) {
        count = entry.getCount();
    return count;

 * Method should get a reel randomly
 * @return string representing reel
static String getRandomReel() {

    // Create a Random object, set seed to the number of elements in array
    Random randomNumber = new Random();
    return getReel(randomNumber.nextInt(reels.length));

 * Method should return reel based on index of array 0 - cherries 1 -
 * oranges 2 - plums 3 - bells 4 - melons 5 - bars
 * @param element
 * @throws IndexOutOfBoundsException
 *             if {@code index} is negative or is not less than 6
 * @return reel
static String getReel(int index) {

    checkElementIndex(index, reels.length);

    return reels[index];


Play slot machine, record results

// Play the slot machine once, and repeat the game as long
// as the user confirms they want to continue to play.
do {
    System.out.println("Welcome to the Slot Machine Simulation.");

    // Get the amount the user wants to bet.
    System.out.print("\nEnter the amount you would like to bet: $");
    amountBet = keyboard.nextDouble();

    // Skip a line.

    // generate random reels
    List<String> reels = Lists.newArrayList();
    for (int x = 0; x < numberOfReels; x++) {

    // display reels

    // determine the payout percentage
    int payoutPercentage = determinePayOutPercentage(reels);

    // calcuate the amount won
    double amountWon = payoutPercentage * amountBet;

    // keep running total of amount won
    totalWon += amountWon;

    // Display the amount won.
    System.out.println("\nYou win $" + dollar.format(amountWon));

    // Prompt the user to play again.
    System.out.println("\nWould you like to play again?");
    System.out.print("Enter Y for yes or N for no: ");
    String input =; // Read a string.
    playAgain = input.charAt(0); // Get the first character.

} while (playAgain == 'Y' || playAgain == 'y');

Display totals

System.out.println("You won a total of $" + dollar.format(totalWon));


Welcome to the Slot Machine Simulation.

Enter the amount you would like to bet: $250


You win $500.00

Would you like to play again?
Enter Y for yes or N for no: y
Welcome to the Slot Machine Simulation.

Enter the amount you would like to bet: $500


You win $1,000.00

Would you like to play again?
Enter Y for yes or N for no: n
You won a total of $1,500.00

Unit tests

public void get_reel_negative () {

public void get_reel_greater_than_6 () {

public void get_reel_cherries () {
    assertEquals("Cherries", SlotMachineSimulation.getReel(0));

public void get_random_reel () {

public void determine_pay_out_percentage_two () {

    List<String> randomStrings = Lists.newArrayList("one", "one", "two", "three");

    assertEquals(2, SlotMachineSimulation.determinePayOutPercentage(randomStrings));;

public void determine_pay_out_percentage_one () {

    List<String> randomStrings = Lists.newArrayList("one", "two", "three");

    assertEquals(1, SlotMachineSimulation.determinePayOutPercentage(randomStrings));;

public void determine_pay_out_percentage_zero () {

    List<String> randomStrings = Lists.newArrayList();

    assertEquals(0, SlotMachineSimulation.determinePayOutPercentage(randomStrings));;

Level Up

  • Randomly allocate the number a pool of $.
  • Enhance output to look like a real slot machine.
  • Enhance getReal to be more efficient.
  • Create an ENUM to represent reel type and change parameter type to method getReel.
  • What changes would need to be made to play a slot machine with 4 reels?
  • Determine the algorithim flaw with the output.