Java Collections Sort

How can I sort an ArrayList of objects:

ArrayList<CommandModel> codes;
          
Collections.sort(codes);  ===> Bound Mismatch on compile

CommandModel contains:

                 private String code;

      public String toString() {
            return code;
      }
      
      public int compareTo(Object o1) {                      
            return (this.code.compareTo(((CommandModel) o1).toString()) );    
      }