请使用JAVA代码进行 IO练习：从键盘上输入任意年份和月份，输出该月份的日历，并输出到文件中。

好的，这是一个比较实用的IO练习，以下是Java代码实现：

import java.io.*;

public class CalendarIO {
    public static void main(String[] args) {
        // 从键盘上输入年份和月份
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int year = 0, month = 0;
        try {
            System.out.print("请输入年份：");
            year = Integer.parseInt(br.readLine());
            System.out.print("请输入月份：");
            month = Integer.parseInt(br.readLine());
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 输出该月份的日历
        String[] weekArr = {"日", "一", "二", "三", "四", "五", "六"};
        int[][] daysArr = new int[6][7];
        int daysInMonth = getDaysInMonth(year, month);
        int firstDayOfWeek = getFirstDayOfWeek(year, month);
        int row = 0, col = firstDayOfWeek;
        for (int i = 1; i <= daysInMonth; i++) {
            daysArr[row][col] = i;
            col++;
            if (col == 7) {
                col = 0;
                row++;
            }
        }

        StringBuilder sb = new StringBuilder();
        sb.append("日\t一\t二\t三\t四\t五\t六\n");
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < 7; j++) {
                sb.append(daysArr[i][j]).append("\t");
            }
            sb.append("\n");
        }

        // 输出到文件中
        try (PrintWriter writer = new PrintWriter(new FileWriter("calendar.txt"))) {
            writer.println(year + "年" + month + "月");
            writer.println(sb.toString());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // 获取指定年月的天数
    private static int getDaysInMonth(int year, int month) {
        int days = 0;
        switch (month) {
            case 2:
                if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) {
                    days = 29;
                } else {
                    days = 28;
                }
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                days = 30;
                break;
            default:
                days = 31;
                break;
        }
        return days;
    }

    // 获取指定年月的第一天是星期几（0代表星期日）
    private static int getFirstDayOfWeek(int year, int month) {
        int dayOfWeek = 0;
        try {
            String dateString = year + "-" + month + "-01";
            java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat("yyyy-MM-dd");
            java.util.Date date = sdf.parse(dateString);
            java.util.Calendar calendar = java.util.Calendar.getInstance();
            calendar.setTime(date);
            dayOfWeek = calendar.get(java.util.Calendar.DAY_OF_WEEK) - 1;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return dayOfWeek;
    }
}

这里使用了`BufferedReader`、`StringBuilder`、`PrintWriter`和`FileWriter`等类来实现从键盘输入和输出到文件，需要注意的是，在使用这些类时，我们需要在代码块的最后使用`try-with-resources`语句，以确保资源能够被正确释放。另外，为了实现获取指定年月的天数和第一天是星期几的功能，我们还实现了两个辅助方法：`getDaysInMonth`和`getFirstDayOfWeek`。