Financial functions, taken from Apache POI.

Financial.java

public class Financial {
    /**
     * Emulates Excel/Calc's PMT(interest_rate, number_payments, PV, FV, Type)
     * function, which calculates the payments for a loan or the future value of an
     * investment
     * 
     * @param r
     *            - periodic interest rate represented as a decimal.
     * @param nper
     *            - number of total payments / periods.
     * @param pv
     *            - present value -- borrowed or invested principal.
     * @param fv
     *            - future value of loan or annuity.
     * @param type
     *            - when payment is made: beginning of period is 1; end, 0.
     * @return <code>double</code> representing periodic payment amount.
     */
    static public double pmt(double r, int nper, double pv, double fv, int type) {
        return -r * (pv * Math.pow(1 + r, nper) + fv) / ((1 + r * type) * (Math.pow(1 + r, nper) - 1));
    }

    /**
     * Overloaded pmt() call omitting type, which defaults to 0.
     * 
     * @see #pmt(double, int, double, double, int)
     */
    static public double pmt(double r, int nper, double pv, double fv) {
        return pmt(r, nper, pv, fv, 0);
    }

    /**
     * Overloaded pmt() call omitting fv and type, which both default to 0.
     * 
     * @see #pmt(double, int, double, double, int)
     */
    static public double pmt(double r, int nper, double pv) {
        return pmt(r, nper, pv, 0);
    }

    /**
     * Emulates Excel/Calc's FV(interest_rate, number_payments, payment, PV, Type)
     * function, which calculates future value or principal at period N.
     * 
     * @param r
     *            - periodic interest rate represented as a decimal.
     * @param nper
     *            - number of total payments / periods.
     * @param pmt
     *            - periodic payment amount.
     * @param pv
     *            - present value -- borrowed or invested principal.
     * @param type
     *            - when payment is made: beginning of period is 1; end, 0.
     * @return <code>double</code> representing future principal value.
     */
    static public double fv(double r, int nper, double pmt, double pv, int type) {
        return -(pv * Math.pow(1 + r, nper) + pmt * (1 + r * type) * (Math.pow(1 + r, nper) - 1) / r);
    }

    /**
     * Overloaded fv() call omitting type, which defaults to 0.
     * 
     * @see #fv(double, int, double, double, int)
     */
    static public double fv(double r, int nper, double c, double pv) {
        return fv(r, nper, c, pv, 0);
    }

    /**
     * Present value of an amount given the number of future payments, rate, amount
     * of individual payment, future value and boolean value indicating whether
     * payments are due at the beginning of period (false =&gt; payments are due at
     * end of period)
     * 
     * @param r
     *            rate
     * @param n
     *            num of periods
     * @param y
     *            pmt per period
     * @param f
     *            future value
     * @param t
     *            type (true=pmt at beginning of period, false=pmt at end of period)
     */
    public static double pv(double r, double n, double y, double f, boolean t) {
        if (r == 0) {
            return -1 * ((n * y) + f);
        } else {
            double r1 = r + 1;
            return (((1 - Math.pow(r1, n)) / r) * (t ? r1 : 1) * y - f) / Math.pow(r1, n);
        }
    }

    /**
     * Present value of an amount given the number of future payments, rate, amount
     * of individual payment, defaults to 0 future value and payments at the end of
     * the period
     * 
     * @param r
     *            rate
     * @param n
     *            num of periods
     * @param y
     *            pmt per period
     */
    public static double pv(double r, double n, double y) {
        return pv(r, n, y, 0, false);
    }
    
    public static double mortgagePayment(double rate, int years, double mortgage) {
        return -pmt(rate/12.0, years*12, mortgage);
    }

    public static double mortgageForPayment(double rate, int years, double monthlyPayment) {
        return pv(rate/12.0, years*12, -monthlyPayment);
    }

    public static void main(String[] args) {
        System.out.println(mortgagePayment(.06, 30, 1_000_000));
        System.out.println(mortgageForPayment(.06, 30, 6000));
    }
}