import java.util.function.UnaryOperator;
import java.math.BigInteger;

// Methods compatible with a functional interface
class FunctionalFactorial {
    public static BigInteger factorial(BigInteger i) {
        if (i.equals(BigInteger.ZERO))
            return BigInteger.ONE;
        else
            return i.multiply(factorial(i.subtract(BigInteger.ONE)));
    }

    public BigInteger instanceFactorial(BigInteger n) {
        return factorial(n);
    }

    // Prints 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000
    public static void main(String args[]) {
        UnaryOperator<BigInteger> f;

        f = FunctionalFactorial::factorial;
        System.out.println(f.apply(new BigInteger("100")));

        f = new FunctionalFactorial()::instanceFactorial;
        System.out.println(f.apply(new BigInteger("100")));
    }
}