Computing Pi to the nth decimal in Java

Pi.java

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Pi {

  private static final BigDecimal TWO = new BigDecimal("2");
  private static final BigDecimal FOUR = new BigDecimal("4");
  private static final BigDecimal FIVE = new BigDecimal("5");
  private static final BigDecimal TWO_THIRTY_NINE = new BigDecimal("239");

  private Pi() {
  }

  public static BigDecimal pi(int numDigits) {

    int calcDigits = numDigits + 10;

    return FOUR.multiply((FOUR.multiply(arccot(FIVE, calcDigits)))
        .subtract(arccot(TWO_THIRTY_NINE, calcDigits)))
        .setScale(numDigits, RoundingMode.DOWN);
  }

  private static BigDecimal arccot(BigDecimal x, int numDigits) {

    BigDecimal unity = BigDecimal.ONE.setScale(numDigits, RoundingMode.DOWN);
    BigDecimal sum = unity.divide(x, RoundingMode.DOWN);
    BigDecimal xpower = new BigDecimal(sum.toString());
    BigDecimal term = null;

    boolean add = false;

    for (BigDecimal n = new BigDecimal("3"); term == null || term.compareTo(BigDecimal.ZERO) != 0; n = n.add(TWO)) {
      xpower = xpower.divide(x.pow(2), RoundingMode.DOWN);
      term = xpower.divide(n, RoundingMode.DOWN);
      sum = add ? sum.add(term) : sum.subtract(term);
      add = !add;
    }
    return sum;
  }

  public static void main(String[] args) {
    System.out.println(pi(1_000));
  }
}