Java array problem regarding Birthday Paradox - details inside post. I have been unable to figure out what is wrong with my code and need assistance

Question

Problem prompt

Birthday problem. Suppose that people enter a room one at a time. How people must enter until two share a birthday? Counterintuitively, after 23 people enter the room, there is approximately a 50–50 chance that two share a birthday. This phenomenon is known as the birthday problem or birthday paradox. Write a program Birthday.java that takes two integer command-line arguments n and trials and performs the following experiment, trials times:

Choose a birthday for the next person, uniformly at random between 0 and n−1. Have that person enter the room. If that person shares a birthday with someone else in the room, stop; otherwise repeat. In each experiment, count the number of people that enter the room. Print a table that summarizes the results (the count i, the number of times that exactly i people enter the room, and the fraction of times that i or fewer people enter the room) for each possible value of i from 1 until the fraction reaches (or exceeds) 50%. sampling from a discrete distribution

Example of how output should look

And here is my code. I keep outputting only one line, (1 999999.0 0.999999) and CANNOT figure out what is wrong.

public class Birthday {
    
    public static void main (String[]args)
    {
        int n = Integer.parseInt(args[0]);
        int trials = Integer.parseInt(args[1]);
        
        int[] times = new int[n];
        boolean [] found = new boolean[n-1];
        
        for (int i=0; i<trials; i++)
        {
            for (int j=0; j<n; j++)
            {
                int rand = (int) Math.random() * n;
                
                if (!found[rand])
                {
                    found[rand] = true;
                }
                else 
                {
                    times[j]++;
                    break;
                }
            }
        }
        
        int m = 0;
        double frac = 0;
        double count=0;
        
        while (frac < 0.5)
        {
            count += times[m];
            frac = count/trials;
            System.out.println(m+1 + "   " +count + "   "+ frac);
            m++;
        }
                
                
        
    }
    
}

Please help me, I am getting desperate lol! Happy MLK day everyone.

Answer 1

This line is always zero because the type casting operator has higher precedence than multiplication:

int rand = (int) Math.random() * n;

You need to use parentheses:

int rand = (int) (Math.random() * n);