public class Test {

    public static void main(String[] args) {    
        String asciiStr = stringToAscii("0123456789abcdefghijklmnopqrstuvwxyz");
        System.out.println(asciiStr);//48,49,50,51,52,53,54,55,56,57,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,
 

        System.out.println(asciiToString(asciiStr));//0123456789abcdefghijklmnopqrstuvwxyz
    }

    /**
     * 第33～126號(共94個)是字元，其中第48～57號為0～9十個阿拉伯數字 
     * @param val
     * @return
     * 
     * @author HuangSL
     * @since 2017/12/01
     */
    public static String stringToAscii(String val) {

        StringBuilder builder = new
 
 StringBuilder();
        for (int i = 0; i < val.length(); i++) {
            char c = val.charAt(i);
            builder.append((byte) c).append(",");
        }

        return builder.toString();
    }

    public static String asciiToString(String val) {

        String[] asciiArr = val.split(","
 
);
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < asciiArr.length; i++) {
            builder.append((char)Integer.parseInt(asciiArr[i]));
        }

        return builder.toString();
    }

}

關於 ASCII的相關資訊，請關注以下文章：

需要注意的是:
ASCII 編碼與 Unicode 編碼不一樣,0到9的unicode值如下:
\u0030 數字 0
\u0031 數字 1
\u0032 數字 2
\u0033 數字 3
\u0034 數字 4
\u0035 數字 5
\u0036 數字 6
\u0037 數字 7
\u0038 數字 8
\u0039 數字 9