code: public Credential setExpirationTimeMilliseconds(Long expirationTimeMilliseconds) {
      lock.lock();
      try {
        this.expirationTimeMilliseconds = expirationTimeMilliseconds;
      } finally {
        lock.unlock();
      }
      return this;
    }
docs: Sets the expected expiration time in milliseconds relative to the
  {@link System#currentTimeMillis() Java epoch}, or {@code null} for none.
  
  <p>
  Overriding is only supported for the purpose of calling the super implementation and changing
  the return type, but nothing else.
  </p>