code: public Smartsheet build() {
          if(baseURI == null){
              baseURI = DEFAULT_BASE_URI;
          }
  
          if(accessToken == null){
              accessToken = System.getenv("SMARTSHEET_ACCESS_TOKEN");
          }
  
          SmartsheetImpl smartsheet = new SmartsheetImpl(baseURI, accessToken, httpClient, jsonSerializer);
  
          if (changeAgent != null) { smartsheet.setChangeAgent(changeAgent); }
          if (assumedUser != null) { smartsheet.setAssumedUser(assumedUser); }
          if (maxRetryTimeMillis != null) { smartsheet.setMaxRetryTimeMillis(maxRetryTimeMillis); }
  
          return smartsheet;
      }
docs: <p>Build the Smartsheet instance.</p>
  
  @return the Smartsheet instance
  @throws IllegalStateException if accessToken isn't set yet.