String x ="_hjk2io_jie@n3hf4_";  //一个字符串。

x="@hjkiio@jie_nhhhf4"

        String x = "_hjk2io_jie@n3hf4_";
        if (x.charAt(x.length() - 1) == '_')
            x = x.substring(0, x.length() - 1);
        char[] temp = x.toCharArray();
        StringBuffer y = new StringBuffer();

        for (int i = 0; i < x.length(); i++) {
            if (temp[i] == '_')
                y.append('@');
            else if (temp[i] == '@')
                y.append('_');
            else if (temp[i] >= '0' && temp[i] <= '9' && i < x.length() - 1
                    && ('0' < temp[i + 1] || temp[i + 1] < '9'))
                y.append(temp[i + 1]).append(temp[i + 1]);
            else
                y.append(temp[i]);
        }
        System.out.println(y);

------解决方案--------------------
Java code

public class Test
{
    public static String change(String str)
    {
        StringBuilder strBuilder = new StringBuilder(str);
        if (strBuilder.charAt(strBuilder.length() - 1) == '_')
        {
            strBuilder.setLength(strBuilder.length() - 1);
        }
        for (int i = 0; i < strBuilder.length(); i++)
        {
            if (strBuilder.charAt(i) == '_')
            {
                strBuilder.setCharAt(i, '@');
            }
            else if (strBuilder.charAt(i) == '@')
            {
                strBuilder.setCharAt(i, '_');
            }
            else if (Character.isDigit(strBuilder.charAt(i)) && i != strBuilder.length() - 1)
            {
                int t = Integer.parseInt(strBuilder.substring(i, i + 1));
                strBuilder.deleteCharAt(i);
                for (int j = 1; j < t; j++)
                {
                    strBuilder.insert(i, strBuilder.charAt(i));
                }
            }
        }
        return strBuilder.toString();
    }
    
    public static void main(String[] args)
    {
        String str = "_hjk2io_jie@n3hf4_";
        System.out.println(change(str));
    }
}

------解决方案--------------------
修正一下，当然用上正则能清晰很多，但是我看见正则就头晕!
Java code

        String x = "_hjk2io_jie@n3hf4_";
        if (x.charAt(x.length() - 1) == '_')
            x = x.substring(0, x.length() - 1);
        char[] temp = x.toCharArray();
        StringBuffer y = new StringBuffer();

        for (int i = 0; i < x.length(); i++) {
            if (temp[i] == '_')
                y.append('@');
            else if (temp[i] == '@')
                y.append('_');
            else if ((temp[i] >= '0' && temp[i] <= '9')
                    && i < x.length() - 1
                    && (('a' < temp[i + 1] || temp[i + 1] < 'z') && ('A' < temp[i + 1] || temp[i + 1] < 'Z')))
                y.append(temp[i + 1]).append(temp[i + 1]);
            else
                y.append(temp[i]);
        }
        System.out.println(y);