Approximating PI (Madhava-Leibniz series) (Java)

public class Main {
    private String operator = "-";
    private double denom = 3.0;
    private double num = 1.0;
    private int power = 1;

    public double calc() {
        for (int x = 0; x < 21; x++) {
            if (operator.equals("-")) {
                num -= (1.0/(denom*(Math.pow(3, power))));
                operator = "+";
            }
            else {
                num += (1.0/(denom*(Math.pow(3, power))));
                operator = "-";
            }
            denom += 2;
            power += 1;
        }
        return Math.sqrt(12) * num;
    }


    public static void main(String[] args) {
        System.out.println(new Main().calc());
    }

}