利用char转ascill码，得到字符的下一位，达到A1到z9的规则序列

/**
	 * 得到当前最大类别编码
	 * @param maxItemNo 当前编码，输入A1自动获取A2，输入B9自动获取C1
	 * @modifier: Memory
	 * @date 2012-04-17
	 * @return
	 */
	private String getNextItemNo(String maxItemNo){
		/*
		 *notice:编码规则:A1，A2，A3......A9,B1,B2,B3......Z9,a1......z9
		 *规则用完，编码则已用尽 
		 */
		
		//下一个编码
		String nextItemNo = null;
		if(maxItemNo == null){
			nextItemNo = "A1";
		}else{
			char[] itemNo = maxItemNo.toCharArray();
			char vcode = itemNo[0];
			int vid = Integer.parseInt(String.valueOf((itemNo[1])));
			if(vid < 9){
				int nextVid = vid + 1;
				nextItemNo = String.valueOf(vcode) + nextVid;
			}else{
				String nextcode = null;
				if(vcode == 'Z'){
					nextcode = "a";
				}else{
					nextcode = String.valueOf((char)(((int)vcode)+1));
				}
				nextItemNo = nextcode + "1";
			}
		}
		return nextItemNo;
	}