獲取當月月初、下個月月初、上個月月初、去年當月月初、去年下個月月初

	public String getDayOfMonth(int year, int month, int type){
        // type爲1代表當月月初，2代表下個月月初，3代表上個月月初，4代表去年當月月初，5代表去年下個月月初
        if (type == 2 || type == 5)
            month = month + 1;
        if (type == 3)
            month = month - 1;
        if (type == 4 || type == 5)
            year = year - 1;
        Calendar a = Calendar.getInstance();
        a.set(Calendar.YEAR, year);
        a.set(Calendar.MONTH, month - 1);
        a.set(Calendar.DATE, 1);
        // 格式化日期
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        return sdf.format(a.getTime());
    }