Question

In the function fermatFactorization(), a and b are being passed as reference parameters, since I am using the Long Class. However, in function testFermatFactorization() when I pass a and b to fermatFactorization(), the values of a and b do not get changed, and so testFermatFactorization() prints (0)(0). I tested this by printing out a and b in fermatFactorization(), and I got the output that I expected.

What am I overlooking? Could the compiler alter a and b in fermatFactorization() since they are only being assigned to?(doubtful)

public static void fermatFactorization(Long n, Long a, Long b)   
//PRE:  n is the integer to be factored
//POST: a and b will be the factors of n
{
    Long v = 1L;
    Long x = ((Double)Math.ceil(Math.sqrt(n))).longValue();
    //System.out.println("x: " + x);
    Long u = 2*x + 1;
    Long r = x*x - n;

    while(r != 0)                 //we are looking for the condition x^2 - y^2 - n to be zero
    {
        while(r>0)
        {
            r = r - v;            //update our condition
            v = v + 2;            //v keeps track of (y+1)^2 - y^2 = 2y+1, increase the "y"
        }
        while(r<0)
        {
            r = r + u;
            u = u + 2;            //keeps track of (x+1)^2 - x^2 = 2x+1, increases the "x"
        }
    }

    a = (u + v - 2)/2;            //remember what u and v equal; --> (2x+1 + 2y+1 - 2)/2 = x+y
    b = (u - v)/2;                //                             --> (2x+1 -(2y+1))/2 = x-y
}

public static void testFermatFactorization(Long number)
{
    Long a = 0L;
    Long b = 0L;
    fermatFactorization(number, a, b);
    System.out.printf("Fermat Factorization(%d) = (%d)(%d)\n", number, a, b);
}

Answer 1

Java is pass by value. If you assign a new value to the argument, it won’t affect the value in the caller method.

You have two options:

• make your method return a and b – either in a int[] or using a separate FactorizationRezult class that has two fields. That way you will declare a and b as local variables in your called method, rather than taking them as parameters. This is the most advisable approach.

• An alternative approach is to use a MutableLong and use a setValue(..) method – that way the changes will affect the object in the caller method. This is less advisable