Thinking in Java Fourth Edition Bruce Eckel Chapter 3 Exercise 10

Exercise 10: A vampire number has an even number of digits and is formed by multiplying a pair of numbers containing half the number of digits of the result. The digits are taken from the original number in any order. Pairs of trailing zeroes are not allowed.
Examples include:
1260 = 21 * 60
1827 = 21 * 87
2187 = 27 * 81
Write a program that finds all the 4-digit vampire numbers. (Suggested by Dan Forhan.)

public class VampireNumbers
{  
        static int a(int i)
        {
            return i/1000;
        }
        static int b(int i)
        {
            return (i%1000)/100;
        }
        static int c(int i)
        {
            return ((i%1000)%100)/10;
        }
        static int d(int i)
        {
            return ((i%1000)%100)%10;
        }
        static int com(int i, int j)
        {
            return (i * 10) + j;
        }
        static void productTest (int i, int m, int n)
        {
            if(m * n == i)
            {
                System.out.println(i + " = " + m + " * " + n);
            }
        }  
    public static void main(String[] args)
    {      
        for(int i = 1001; i < 9999; i++)
        {          
            productTest(i, com(a(i), b(i)), com(c(i), d(i)));
            productTest(i, com(a(i), b(i)), com(d(i), c(i)));
            productTest(i, com(a(i), c(i)), com(b(i), d(i)));
            productTest(i, com(a(i), c(i)), com(d(i), b(i)));
            productTest(i, com(a(i), d(i)), com(b(i), c(i)));
            productTest(i, com(a(i), d(i)), com(c(i), b(i)));
            productTest(i, com(b(i), a(i)), com(c(i), d(i)));
            productTest(i, com(b(i), a(i)), com(d(i), c(i)));
            productTest(i, com(b(i), c(i)), com(d(i), a(i)));
            productTest(i, com(b(i), d(i)), com(c(i), a(i)));
            productTest(i, com(c(i), a(i)), com(d(i), b(i)));
            productTest(i, com(c(i), b(i)), com(d(i), a(i)));
        }          
    }
}

Output:

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