Commons Lang、Fraction#invert()

import org.apache.commons.lang.math.Fraction;

public class C2009022700 {
    public static void main(String[] args) {
        System.out.println(Fraction.ONE.invert());
        System.out.println(Fraction.ONE_HALF.invert());
        System.out.println(Fraction.ONE_THIRD.invert());
        System.out.println(Fraction.TWO_THIRDS.invert());
        System.out.println(Fraction.getFraction(1, 2, 3).invert());
    }
}

で、

1/1
2/1
3/1
3/2
3/5