java 获取当月第一天和最后一天

获取当月第一天的Java代码：

Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.DAY_OF_MONTH, 1); // 设置为当月第一天
Date firstDayOfMonth = calendar.getTime();

获取当月最后一天的Java代码：

Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH)); // 设置为当月最后一天
Date lastDayOfMonth = calendar.getTime();

或者使用Java 8的新日期时间API：

LocalDate now = LocalDate.now();
LocalDate firstDayOfMonth = now.withDayOfMonth(1);
LocalDate lastDayOfMonth = now.withDayOfMonth(now.lengthOfMonth());