有一隻兔子，從出生後第3個月起每個月都生只兔子，小兔子長到第三個月後每個月又生一隻兔子，假如兔子都不死，問每個月的兔子總數為多少？

public class CalculateRabbit{
    public static void calculate(int month, List<Rabbit> rabbits, List<Rabbit> extRabbits) {
        if (month > 12) {
            return;
        }
        for (Rabbit rabbit : rabbits) {
            if (rabbit.getMonth() >= 3) {
                Rabbit extRabbit = new Rabbit();
                extRabbit.setMonth(extRabbit.getMonth() + 1);
                extRabbits.add(extRabbit);
            }
            rabbit.setMonth(rabbit.getMonth() + 1);
        }
        rabbits.addAll(extRabbits);
        extRabbits.clear();
        System.out.println("第" + month + "月有" + rabbits.size() + "只兔子");
        month++;
        calculate(month, rabbits, extRabbits);
    }

    public static void main(String[] args) {
        List<Rabbit> rabbits = new ArrayList<Rabbit>();
        rabbits.add(new Rabbit());
        List<Rabbit> extRabbits = new ArrayList<Rabbit>();
        calculate(1, rabbits, extRabbits);
    }
}

class Rabbit {
    private int month = 1;

    public int getMonth() {
        return this.month;
    }

    public void setMonth(int month) {
        this.month = month;
    }
}