String s = "user_nick_name";
    // build result in a StringBuilder
    StringBuilder result= new StringBuilder();
    for (int pos=0; pos<s.length(); pos++)
    { // iterate through input
        char c= s.charAt(pos);
        if (c!='_') // if not _ just append char unaltered
            result.append(c);
        else
        {
            // is there a next char?
            if (pos==s.length()-1) break; // no, so _ was a final char
            // get next char
            char cNext=s.charAt(pos+1);
            // convert to upper case and append
            result.append(Character.toUpperCase(cNext));
            pos++;
        }
    }
    s=new String(result); // back to a String
    System.out.println(s);