StringBuilder getting error using .append (Java)

I get the following errors:

nameCombine.java:18: error: cannot find symbol
sb.append(s);"
  ^

symbol:   method append(String)
location: variable sb of type StringBuilder
nameCombine.java:19: error: cannot find symbol
        sb.append("\t");
          ^
symbol:   method append(String)
location: variable sb of type StringBuilder
StringBuilder.java:16: error: cannot find symbol
        sb.append(s);
          ^
symbol:   method append(String)
location: variable sb of type StringBuilder
StringBuilder.java:17: error: cannot find symbol
        sb.append("\t");
          ^
symbol:   method append(String)
location: variable sb of type StringBuilder

After trying to compile the following code:

import java.util.ArrayList;

public class nameCombine {
   public static void main(String[] args) {
      ArrayList<String> list = new ArrayList<String>();
      list.add("firstName");
      list.add("middleName");
      list.add("lastName");

      StringBuilder sb = new StringBuilder();

      for (String s : list) {
         sb.append(s);
         sb.append("\t");
      }

      System.out.println(sb.toString());
   }
}

I'm sorry if this is a repeat question (I'm pretty sure it is) but a quick search didn't yield anything useful, for me anyways.

What im trying to do (if you can't already tell):
Combine an arrayList (firstName, middleName, lastName) into one new string. Any suggestions as to how I can do that in Java is appreciated.

Copyright License：
Author:「user1413110」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/10724451/stringbuilder-getting-error-using-append-java