问题是您的代码有很多错误。我已经修好了一些，但剩下的我相信你能修好

public class Rainfalls {
    public static void main(String[] args) {

        String[] monthName = { "January", "February", "March", "April", "May", "June",

                "July", "August", "September", "October", "November", "December" }; // declare the month

        double[] rainfallSize = { 11.5, 11.2, 11.4, 0.7, 0.5, 0.1, 0.1, 0.0, 1.2, 4.3, 7.9, 8.1 }; // declare the
                                                                                                    // rainfall size

        double total_rain = 0;
        for (double i : rainfallSize) {
            total_rain += i;
        }

        for (int i = 0; i <= 11; i++) {

            System.out.printf("Month: %s, Rainfall amount: %.1f mm \n", monthName[i], rainfallSize[i]);
        }
        // calculate average monthly rainfall
        System.out.printf("Average montly Rainfall: \n");
        System.out.printf("Total Rainfall Yearly: \n", total_rain);
        // calculate max monthly rainfall
        System.out.printf("Max Value: \n");
        // calculate min monthly rainfall
        System.out.printf("Min value: \n");

    }

}

如果你觉得这个回答有帮助，你会投票给我的答案，这样我就可以帮助其他人（在这个论坛上得到其他人的帮助）。
提前谢谢
哦，输出看起来是这样的：

Month: January, Rainfall amount: 11.5 mm 
Month: February, Rainfall amount: 11.2 mm 
Month: March, Rainfall amount: 11.4 mm 
Month: April, Rainfall amount: 0.7 mm 
Month: May, Rainfall amount: 0.5 mm 
Month: June, Rainfall amount: 0.1 mm 
Month: July, Rainfall amount: 0.1 mm 
Month: August, Rainfall amount: 0.0 mm 
Month: September, Rainfall amount: 1.2 mm 
Month: October, Rainfall amount: 4.3 mm 
Month: November, Rainfall amount: 7.9 mm 
Month: December, Rainfall amount: 8.1 mm 
Average montly Rainfall: 
Total Rainfall Yearly: 
Max Value: 
Min value:

在java中，数组是否有一个函数来查找sum和mean以及stdev？
据我所知，不是所有的。但是有一个类可以让你计算 count, sum, average, min, and max 指一系列的值。它被称为 DoubleSummaryStatistics 首先，声明该类的一个示例（您可以阅读javadoc以查看默认情况下这些初始值是什么）。

DoubleSummaryStatistics dss = new DoubleSummaryStatistics();

然后，只需迭代您的值，如下所示。

for (double rainFall : rainfallSize) {
          dss.accept(rainFall);
}

完成后，可以使用类的getter打印适当的值。

System.out.printf("Total Rainfall Yearly: %.2f mm%n",
        dss.getSum());
System.out.printf("Average monthly Rainfall: %.2f mm%n",
        dss.getAverage());
System.out.printf("Max Value: %.2f mm%n", dss.getMax());
System.out.printf("Min value: %.2f mm%n", dss.getMin());

印刷品

Total Rainfall Yearly: 57.00 mm
Average monthly Rainfall: 4.75 mm
Max Value: 11.50 mm
Min value: 0.00 mm

当您计算其他统计数据（如标准偏差）时（查看wiki中的各种算法），您还需要遍历数据。因此，在填充summary stats类时，使用与上面相同的循环并收集数据。