public class Rational {

    private int mNumber;
    private int mDenom;

    public Rational(int numerator, int denominator) {
    //assert denominator != 0 : "denom = 0"; <-- Não use assertivas
        mNumber = numerator;
        mDenom = denominator;
    }

    public int getNumber() {
        return mNumber;
    }

    public int getDenom() {
        return mDenom;
    }

    public Rational multiply(Rational ration) {
        return new Rational(getNumber() * ration.getNumber(), getDenom() * ration.getDenom());
    }

    @override
    public String toString() {
        return getNumber() + " / " + getDenom(); //Não faça esse tipo de concatenação
    }

    public static void main(String[] args) {
        System.out.println(new Rational(2,3).multiply(new Rational(5,4)));
    }

}