FlipCoin.java

import java.util.Random;

public class FlipCoin
{
    public static void main(String[] args)
    {
        // Set up basic variables for use in the program.
        String a = "THH", b = "THT", flips;
        Random rand = new Random();
        double aAverage = 0, bAverage = 0;
        int count;

        // Run the program 10000 times to get a good sample size.
        for (count = 0; count < 10000; count++)
        {
            // Initialise flips with 3 (minimum amount for pattern).
            flips = "";
            for (int i = 0; i < 3; i++)
            {
                // If true, the coin lands heads up (H). If false, tails up (T).
                if (rand.nextBoolean())
                {
                    flips += "H";
                }
                else
                {
                    flips += "T";
                }
            }

            // Keep adding coin flips until both patterns are found.
            while (flips.indexOf(a) == -1 || flips.indexOf(b) == -1)
            {
                // If true, the coin lands heads up (H). If false, tails up (T).
                if (rand.nextBoolean())
                {
                    flips += "H";
                }
                else
                {
                    flips += "T";
                }
            }

            /*
             * Total up the number of flips needed until each pattern is found.
             * Since the indexOf() method returns the index of the first
             * character, we need to add the length of the pattern to get the
             * actual number of flips. So if the pattern is 3 characters long,
             * and is found at the first index (0) it must have been found in
             * 3 tosses (0, 1, 2).
             */
            aAverage += flips.indexOf(a) + a.length();
            bAverage += flips.indexOf(b) + b.length();
        }

        // Compute the averages by dividing the totals by the iteration count.
        aAverage /= count;
        bAverage /= count;

        // Print out the results.
        System.out.println(a + " found after an average of " + aAverage +
            " (~" + Math.round(aAverage) + ") flips.");
        System.out.println(b + " found after an average of " + bAverage +
            " (~" + Math.round(bAverage) + ") flips.");
    }
}