BigDecimal is driving me crazy

[Randall Twede]

i wrote a class to compute e

class E implements Algorithm
{
	public String calculate(int howMany)
	{
		double e = 2;
		for(int i = 2; i < howMany; i++)
		{
			long nFactorial = 1;
			for(int j = 1; j < (i + 1); j++)
			{
				nFactorial = nFactorial * j;
			}
			e = e + (1.0 / nFactorial);
		}
		return Double.toString(e);
	}	
}

this works fine but only to a certain precision. so i am trying to use BigDecimal. at first i was getting an exception when i divided but i learned i had to specify the rounding. i did that but now i get 2 as an answer

import java.math.BigDecimal;

class New implements Algorithm
{
	public String calculate(int howMany)
	{
		BigDecimal e = new BigDecimal(2);
		for(int i = 2; i < howMany; i++)
		{
			//BigDecimal one = new BigDecimal(1);
			BigDecimal nFactorial = new BigDecimal(1);
			for(int j = 1; j < (i + 1); j++)
			{
				BigDecimal iteration = new BigDecimal(j);
				nFactorial = nFactorial.multiply(iteration);
			}
			e = e.add(BigDecimal.ONE.divide(nFactorial, BigDecimal.ROUND_HALF_EVEN));
			System.out.println(e);
		}
		return e.toString();
	}	
}

[Paul Clapham]

The first thing that I thought when I read that code was "How does BigDecimal.divide know how many decimal places to use?"

[Randall Twede]

yeah i think that might be the problem. i guess i was assuming a large default value for that. this is the first time i have tried using this class.

[Rob Spoor]

Off topic, but I have two small improvements for you:

			BigDecimal nFactorial = new BigDecimal(1);
...
				BigDecimal iteration = new BigDecimal(j);

Use BigDecimal.ONE and BigDecimal.valueOf(j). The former will definitely reuse an existing object, the latter will probably do so as well. Currently, all values from 0 to 10 are cached; that already saves you up to 10 extra objects per loop.