title: replacing all cases of ISO Control characters in a string with "CTRL"
body: <pre><code> static String clean(String identifier) {
      String firstString = &quot;&quot;;
      for (int i = 0; i &lt; identifier.length(); i++)
          if (Character.isISOControl(identifier.charAt(i))){
              firstString = identifier.replaceAll(identifier.charAt(i), 
                            &quot;CTRL&quot;);
           }
              
          return firstString;
  }
  </code></pre>
  <p>The logic behind the code above is to replace all instances of ISO Control characters in the string 'identifier' with &quot;CTRL&quot;. I'm however faced with this error: &quot;char cannot be converted to java.lang.String&quot;</p>
  <p>Can someone help me to solve and improve my code to produce the right output?</p>