Reduce Rational

public class Rational {
    public static int GCD( int numerator, int denominator ) {
        if( denominator == 0 ) {
            return( numerator );
        }
        else {
            return( GCD( denominator, numerator % denominator ) );
        }
    }

    public static String ratString( int numerator, int denominator ) {
        int gcd = GCD( numerator, denominator );

        numerator /= gcd;
        denominator /= gcd;

        boolean numerChanged = false;

        if( numerator < 0 ) {
            if( denominator < 0 ) {
                numerator = -numerator;
                denominator = -denominator;

                numerChanged = true;
            }
        }

        if( denominator < 0 ) {
            if( !numerChanged ) {
                denominator = -denominator;
                numerator = -numerator;
            }
        }

        if( denominator == 1 ) {
            return( String.format( "%1$d", numerator ) );
        }
        else {
            return( String.format( "%1$d/%2$d", numerator, denominator ) );
        }
    }

    public static void main( String[] args ) {
        System.out.println( ratString( -6, 3 ) );
    }
}