double pi = 0.0;

//        次数越多越精确

        int count =0x7fffffff;

        double denominator = 1.0;

        for (double i = 1.0; i <= count; i++) {

        pi +=denominator/(2*i-1); 

        denominator = -denominator;

}

        pi=pi*4;

        System.out.println(pi);