简单的日历实现，只是显示了每一个月，没有显示当天和记事这些功能

主要是计算月初是周几，月末是周几，然后相应的显示上一月多少天和下一月多少天。

先看一下关于日期的用到的几个工具类

 /**
     * 获取该月天数
     */
    public static int getCurrentMonthDay(long millSec) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(millSec);
        calendar.set(Calendar.DATE, 1);
        calendar.roll(Calendar.DATE, -1);
        int dateCount = calendar.get(Calendar.DATE);
        return dateCount;
    }

    /**
     * 获取当月第一天
     */
    public static long getFirOfMonth(long millSec) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(millSec);
        calendar.set(Calendar.DATE, 1);
        return calendar.getTimeInMillis();
    }

    /**
     * 获取当前时间戳
     */
    public static long getCurrentTime() {
        Calendar calendar = Calendar.getInstance();
        return calendar.getTimeInMillis();
    }

    /**
     * 获取上一月/下一月
     */
    public static long getLastOrNextMonth(long millSec, int count) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(millSec);
        calendar.add(Calendar.MONTH, count);
        return calendar.getTimeInMillis();
    }

    /**
     * 格式化到月份
     */
    public static String long2str(long millSec) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM");
        return sdf.format(new Date(millSec));
    }

用一个List存放每一天，因为之前考虑到每一天有记事这些，所以封装了一个类，如果不需要的话可以直接一个时间戳就可以。
打开之后先获取当前的时间戳，然后初始化到当前月份的第一天，通过工具类中方法。
然后就是计算了，把上月结余的、本月所有、下月的一起添加到List中。

 private void setCalendarList(long millSecs) {
        beans.clear();
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(millSecs);
//        int nor = today.getDay();
        int max = DateUtil.getCurrentMonthDay(millSecs);
        /**
         * 当月日历
         */
        for (int i = 1; i <= max; i++) {
    
    
            DayBean bean = new DayBean();
            Calendar cc = Calendar.getInstance();
            cc.setTimeInMillis(millSecs);
            cc.add(Calendar.DAY_OF_MONTH, i - 1);
            setBean(bean, cc);
            bean.setIsCurrentMonth(true);
            beans.add(bean);
        }
        //上月结余
        int fir_day_of_week = beans.get(0).getCalendar().get(Calendar.DAY_OF_WEEK);
        Log.e("AAA", "week_last:" + fir_day_of_week);
        if (fir_day_of_week != 2) {
            if(fir_day_of_week == 1){
                for (int i = 0; i < 6; i++) {
    
    
                    DayBean bean = new DayBean();
                    Calendar cc = Calendar.getInstance();
                    cc.setTimeInMillis(millSecs);
                    cc.add(Calendar.DAY_OF_MONTH, -i -1);
                    setBean(bean, cc);
                    Log.e("AAA", "last:" + bean.getDay());
                    bean.setIsCurrentMonth(false);
                    beans.add(0, bean);
                }
            }else{
                for (int i = 0; i < fir_day_of_week-2; i++) {
    
    
                    DayBean bean = new DayBean();
                    Calendar cc = Calendar.getInstance();
                    cc.setTimeInMillis(millSecs);
                    cc.add(Calendar.DAY_OF_MONTH, -i-1);
                    setBean(bean, cc);
                    Log.e("AAA", "last:" + bean.getDay());
                    bean.setIsCurrentMonth(false);
                    beans.add(0, bean);
                }
            }

        }
        //下月
        int last_day_of_week = beans.get(beans.size() - 1).getCalendar().get(Calendar.DAY_OF_WEEK);
        Log.e("AAA", "week_next:" + last_day_of_week);
        if (last_day_of_week != 1) {
            for (int i = last_day_of_week; i < 8; i++) {
    
    
                DayBean bean = new DayBean();
                Calendar cc = Calendar.getInstance();
                cc.setTimeInMillis(millSecs);
                cc.add(Calendar.DAY_OF_MONTH, i - last_day_of_week);
                setBean(bean, cc);
                Log.e("AAA", "next:" + bean.getDay());
                bean.setIsCurrentMonth(false);
                beans.add(bean);
            }
        }
        GridViewAdapter adapter = new GridViewAdapter(this, beans);
        mGridView.setAdapter(adapter);
    }

    private void setBean(DayBean bean, Calendar cc) {
        bean.setCalendar(cc);
        bean.setDay(cc.get(Calendar.DAY_OF_MONTH));
    }

设置每一天时设置了一个字段为是否为本月，如果是，则设置为黑色，否则设为灰色显示，在Adapter中的getView方法中设置即可。

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if (convertView == null) {
            convertView = inflater.inflate(R.layout.item_layout, null);
            holder = new ViewHolder();
            holder.day_text = (TextView) convertView.findViewById(R.id.day_text);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.day_text.setText(beans.get(position).getDay() + "");
        if (beans.get(position).isToday()) {
            holder.day_text.setBackgroundColor(Color.RED);
        } else {
            holder.day_text.setBackgroundColor(Color.WHITE);
        }
        if (beans.get(position).isCurrentMonth()) {
            holder.day_text.setTextColor(Color.BLACK);
        } else {
            holder.day_text.setTextColor(Color.GRAY);
        }

        return convertView;
    }

    class ViewHolder {
        TextView day_text;
    }