Approx1.java

public class Main {

    public static void main(String[] args) throws Exception {
        boolean plus = true;
        double summe = 0;
        for (int nenner = 1; nenner < 1000; nenner += 2) {
            double bruch = 1.0 / nenner;
            if (plus) {
                summe += bruch;
            } else {
                summe -= bruch;
            }
            plus = !plus;
        }

        Out.println("pi ~ " + summe * 4);
    }
}

Approx2.java

public class Main {

    public static void main(String[] args) throws Exception {
        double zähler = 1;
        double nenner = 1;
        double summe = 1;
        for (int i = 1; i < 100; i++) {
            zähler *= i;
            nenner *= 2 * i + 1;
            summe += zähler / nenner;
        }

        Out.println("pi ~ " + summe * 2);
    }
}