Java Stack Overflow Error

I'm trying to program a GCD algorithm and everything seems to be correct except the StackOverflowError. Here is the code:

public class Gcd {
    public static BigInteger gcdNaive(BigInteger a, BigInteger b) {
        int res = a.compareTo(b);

        if ( res == 0 ) {
            BigInteger g = a;
            return g;
        }
        else if ( res == 1 ) {
            BigInteger h = a.subtract(b);
            a = h;
            return gcdNaive(a, b);
        }
        else if ( res == -1 ) {
            BigInteger g = b.subtract(a);
            b = g;
            return gcdNaive(a, b);
        }

        return BigInteger.ZERO;
    }

    public static BigInteger gcdEuclid(BigInteger a, BigInteger b) {
        if( b == BigInteger.ZERO ) {
            BigInteger g = a;
            return g;
        }
        else if ( b != BigInteger.ZERO ) {
            BigInteger g = b;
            BigInteger h = a.mod(b);
            a = g;
            b = h;
            return gcdEuclid(a, b);
        }

        return BigInteger.ZERO;
    }
}

Exception:

Exception in thread "main" java.lang.StackOverflowError 


    at Gcd.gcdNaive(Gcd.java:7) 
    at Gcd.gcdNaive(Gcd.java:19)