public class UpperCase {

    public static void main(String[] args) {

        System.out.println(getTheUpperCase("good", 0));

    }
    static String getTheUpperCase(String src, int pos)
    {
        if(pos>=src.length())
            throw new RuntimeException("指定的位置不合法!");
        
        StringBuilder sb = new StringBuilder(src);
        sb.setCharAt(pos, Character.toUpperCase(src.charAt(pos)));
        return sb.toString();
    }
}

------解决方案--------------------
字符位置从1开始表示，返回null表示传入的参数有误
Java code

    public static String upperCaseString(String str,int pos)
    {
        if(pos < 1 || pos > str.length() || null == str | "".equals(str)) return null;
        char[] strChars = str.toCharArray();
        strChars[pos-1] = Character.toUpperCase(strChars[pos-1]);
        return String.valueOf(strChars);
    }

------解决方案--------------------
public static String upperCaseString(String str,int pos)
   {
       if(pos < 1 || pos > str.length() || null == str | "".equals(str)) return null;
       char[] strChars = str.toCharArray();
       strChars[pos-1] = Character.toUpperCase(strChars[pos-1]);
       return String.valueOf(strChars);
   }