Calendar getActualMaximum

有问题的代码：

    /**
     * 获取每月的最后那天的数值
     *
     * @param year
     * @param month 1月传0，2月传1，依次类推
     * @return
     */
    public static int getActualMaximum(int year, int month) {
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month);
        return cal.getActualMaximum(Calendar.DAY_OF_MONTH);
    }

今天（2018/3/30）却发现一个小问题，当我要拿2018年2月的最后一天是，返回31。

原因解析：

原来是因为这样子的写法，改calendar为2018/2/30号，又没有这么一天所以Calender跳到了3月。
自然就返回了31。

解决方法1：

    /**
     * 获取每月的最后那天的数值
     *
     * @param year
     * @param month 1月传0，2月传1，依次类推
     * @return
     */
    public static int getActualMaximum(int year, int month) {
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month);
        // add code here
        cal.set(Calendar.DATE, 1);
        return cal.getActualMaximum(Calendar.DAY_OF_MONTH);
    }

原因解析：

Calendar类在set的时候，并不会立即生效，只有在get的时候才会生效

解决方法2：

在实例化Calendar类之后，要先调用clear()方法清空缓存。

    /**
     * 获取每月的最后那天的数值
     *
     * @param year
     * @param month 1月传0，2月传1，依次类推
     * @return
     */
    public static int getActualMaximum(int year, int month) {
        Calendar cal = Calendar.getInstance();
        cal.clear();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month);
        return cal.getActualMaximum(Calendar.DAY_OF_MONTH);
    }

    /**
     * Field number for <code>get</code> and <code>set</code> indicating the
     * month. This is a calendar-specific value. The first month of
     * the year in the Gregorian and Julian calendars is
     * <code>JANUARY</code> which is 0; the last depends on the number
     * of months in a year.
     *
     * @see #JANUARY
     * @see #FEBRUARY
     * @see #MARCH
     * @see #APRIL
     * @see #MAY
     * @see #JUNE
     * @see #JULY
     * @see #AUGUST
     * @see #SEPTEMBER
     * @see #OCTOBER
     * @see #NOVEMBER
     * @see #DECEMBER
     * @see #UNDECIMBER
     */
    public final static int MONTH = 2;

java.util.Calendar#DAY_OF_MONTH

    /**
     * Field number for <code>get</code> and <code>set</code> indicating the
     * day of the month. This is a synonym for <code>DATE</code>.
     * The first day of the month has value 1.
     *
     * @see #DATE
     */
    public final static int DAY_OF_MONTH = 5;

java.util.Calendar#getActualMaximum 获取指定 calendar field的最大值。

    /**
     * Returns the maximum value that the specified calendar field
     * could have, given the time value of this
     * <code>Calendar</code>. For example, the actual maximum value of
     * the <code>MONTH</code> field is 12 in some years, and 13 in
     * other years in the Hebrew calendar system.
     *
     * <p>The default implementation of this method uses an iterative
     * algorithm to determine the actual maximum value for the
     * calendar field. Subclasses should, if possible, override this
     * with a more efficient implementation.
     *
     * @param field the calendar field
     * @return the maximum of the given calendar field for the time
     * value of this <code>Calendar</code>
     * @see #getMinimum(int)
     * @see #getMaximum(int)
     * @see #getGreatestMinimum(int)
     * @see #getLeastMaximum(int)
     * @see #getActualMinimum(int)
     * @since 1.2
     */
    public int getActualMaximum(int field) {
        int fieldValue = getLeastMaximum(field);
        int endValue = getMaximum(field);

        // if we know that the maximum value is always the same, just return it.
        if (fieldValue == endValue) {
            return fieldValue;
        }

        // clone the calendar so we don't mess with the real one, and set it to
        // accept anything for the field values.
        Calendar work = (Calendar)this.clone();
        work.setLenient(true);

        // if we're counting weeks, set the day of the week to Sunday.  We know the
        // last week of a month or year will contain the first day of the week.
        if (field == WEEK_OF_YEAR || field == WEEK_OF_MONTH) {
            work.set(DAY_OF_WEEK, firstDayOfWeek);
        }

        // now try each value from getLeastMaximum() to getMaximum() one by one until
        // we get a value that normalizes to another value.  The last value that
        // normalizes to itself is the actual maximum for the current date
        int result = fieldValue;

        do {
            work.set(field, fieldValue);
            if (work.get(field) != fieldValue) {
                break;
            } else {
                result = fieldValue;
                fieldValue++;
            }
        } while (fieldValue <= endValue);

        return result;
    }