Basic Random Rolling Dice Java

Question

I am trying to write a method rollDice(int number, int nSides) which returns the total result of rolling the number dice with nSides sides.

So for example rollDice(3

Answer 1

You only need to initialize Random r and int roll once each so I have removed them from the loop. The nextInt(int) method picks an integer from and including 0 to but not including the int. This is known as 0 (inclusive) to int (exclusive), so you have to add 1 to adjust the range to the die. You seem to have known that though I don't know why you used %. Using * to multiply would give you the same number for all the dice which I don't believe you mean to do. Here is one possible implementation of your class:

import java.util.Random;

public class Dice {

    public static  int rollDice(int number, int nSides)
    { 
        int num = 0;
        int roll = 0;
        Random  r = new Random(); 
        if(nSides >=3) 
        { 
            for(int i = 0; i < number; i++)
            { 
                roll = r.nextInt(nSides)+1;
                System.out.println("Roll is:  "+roll);
                num = num + roll; 
            } 
        } 
        else
        { 
            System.out.println("Error num needs to be from 3"); 
        } 
        return num;  
    } 

    public static void main(String[] args)
    {
        System.out.println("Total is: "+rollDice(3, 6));
    }
}
/*
Roll is:  4
Roll is:  1
Roll is:  2
Total is: 7
*/

Answer 2

Random.nextInt() has unpredicable behaviour - it can produce all values possible for an integer, including negative numbers. Use Random.nextInt(numSides) instead - it will return an integer from [0,numSides) i.e. including 0 and excluding numSides. To get your desired functionality [1,numSides] use

r.nextInt(numSides)+1;

See here for more information.

Answer 3

How about this as your rollDice method:

public static int rollDice(int number, int nSides) {
    int count = 0;
    for(int i = 0; i < number; i++) {
        count += (int)(Math.random() * nSides) + 1;
    }
    return count;
}

Answer 4

When you use % on a negative number you get a negative number.

In this case the solution is simple, use

int roll = r.nextInt(nSides) + 1; // 1 to nSizes
num += roll;

Answer 5

replace
int roll = r.nextInt();
with

int roll = r.nextInt(nSides);