code: private void initializeUsernameEditText() {
          usernameEditText = (EditText) findViewById(R.id.username_edit_text);
          usernameEditText.addValidator(
                  Validators.notEmpty(this, R.string.not_empty_validator_error_message));
          usernameEditText.addValidator(Validators
                  .maxLength(this, R.string.max_length_validator_error_messsage, MAX_CHARACTERS));
          usernameEditText.addValidator(Validators
                  .letterOrNumber(this, R.string.letter_or_number_validator_error_message,
                          Case.CASE_INSENSITIVE, false, new char[]{'-', '_'}));
      }
docs: Initializes the edit text, which allows to enter an username.