I could not find a solution from my book's website (student-level access) but I feel that I am missing something that I'm supposed to learn from this exercise.

Instructions: Balls are dropped from the opening of the board. Every time a ball hits a nail, it has a 50% chance of falling to the left or to the right. The piles of balls are accumulated in the slots at the bottom of the board.

Book's example:

Enter the number of balls to drop: 5
Enter the number of slots in the bean machine: 7







My attempt:

import java.util.*;

public class BeanMachine {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter number of balls to drop: ");
        int numberOfBalls = input.nextInt();
        System.out.print("Enter the number of slots in the bean machine: ");
        int numberOfSlots = input.nextInt();

        //Simulate drops
        int[] slots = new int[numberOfSlots];
        for (int i = 0; i < numberOfBalls; i++) {

        //Display histogram
        for (int i = numberOfBalls; i > 0; i--) {
            for (int j = 0; j < slots.length; j++) {
                if (slots[j] >= i) {
                else {
                    System.out.print(" ");

    public static void simulateDrop(int[] slots) {
        StringBuilder path = new StringBuilder(slots.length);
        int slotIndex = 0;
        int numberOfNails = slots.length - 1;
        for (int i = 0; i < numberOfNails; i++) {
            int direction = (int)(Math.random() * 2);

            if (direction == 0) {
            else {

  • 1
    \$\begingroup\$ What is the goal here? How do you know if your program works correctly? (Does it?) \$\endgroup\$
    – janos
    Aug 23, 2015 at 6:04
  • \$\begingroup\$ @janos The question seems clear enough. The code appears to work as advertised. \$\endgroup\$ Aug 23, 2015 at 8:08
  • \$\begingroup\$ @Legate have you read the Wikipedia article? I'm not sure if your program simulates one of those (it may, but I'm not convinced). \$\endgroup\$
    – mkrieger1
    Aug 23, 2015 at 8:24
  • 1
    \$\begingroup\$ I thing you shouldn't try to find an easy solution by writing two methods but a clean solution with OOP. Separate input, presentation and business logic. \$\endgroup\$
    – Obenland
    Aug 23, 2015 at 22:23
  • \$\begingroup\$ There are two things that seem to be off here: 1. you never touch the slotIndex in your simulateDrop method 2. you initialize your slotIndex with 0 does a ball really drop into the 0th slot if it would drop straight down passing through each nail miracoulusly? \$\endgroup\$
    – Aron_dc
    Aug 28, 2015 at 7:45

1 Answer 1


Java has good support for OOP techniques - Using these will allow you to create a more elegant solution. Your code could be refactored with the correct use of these techniques to produce something of the form:

PegMachine sampleTest = PegMachine(500,100);

This approach hides the complexities of the program and allows you to conduct tests by simply creating a PegMachine specifying the number of pegs and balls for each sample.

I noticed Aron_dc pointed out the slotIndex variable in your simulateDrop method is never used - This can be removed as it does not contribute anything to your program.

Out of curiosity I conducted a larger sample using your code to see if it produced the correct normal-distribution curve. The results were as expected; confirming your codes correctness.

Besides suggesting the use of object orientation your code seems well written and correct.


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