code: public void checkin(AdminClient client) {
          if (isClosed.get()) {
              throw new IllegalStateException("Pool is closing");
          }
  
          if (client == null) {
              throw new IllegalArgumentException("client is null");
          }
  
          boolean isCheckedIn = clientCache.offer(client);
  
          if (!isCheckedIn) {
              // Cache is already full, close this AdminClient
              client.close();
          }
      }
docs: submit the adminClient after usage is completed.
  Behavior is undefined, if checkin is called with objects not retrieved
  from checkout.
  
  @param client AdminClient retrieved from checkout