簡單的星期幾獲取

• 獲取本週的周幾

        Calendar cal = Calendar.getInstance();
        //n爲推遲的週數，1本週，-1向前推遲一週，2下週，依次類推
        int n = 1;
        cal.add(Calendar.DATE, n*7);
        //想周幾，這裏就傳幾Calendar.MONDAY（TUESDAY...）
        cal.set(Calendar.DAY_OF_WEEK,Calendar.SUNDAY);
        String date = new SimpleDateFormat("yyyy-MM-dd").format(cal.getTime());
        System.out.println(date);

• 獲取本月第一個周幾

        Calendar cal = Calendar.getInstance();
        // n=1 爲本月, 0位爲上個月,1爲下個月...一次類推
        int n = 1;
        cal.set(Calendar.DAY_OF_MONTH, n);
        int i = 1;
        while(cal.get(Calendar.DAY_OF_WEEK) != Calendar.MONDAY){
            cal.set(Calendar.DAY_OF_MONTH, i++);
        }
        Date firstMonday = cal.getTime();
        String dtStr = new SimpleDateFormat("yyyy-MM-dd").format(firstMonday);
        System.out.println(dtStr);