code: private boolean isInterestRegistered(SelectableChannel channel, int operation) {
      SelectionKey key = channel.keyFor(selector);
  
      return key != null && (key.interestOps() & operation) != 0;
    }
docs: Check whether an operation interest was registered on a SelectableChannel
  There are two cases that interest is not registered
  1. The whole key does not exist; no interests ever registered for this channel
  2. The key exists due to other interests registered but not the one we are adding
  If the key exists, the key for SelectableChannel has to be valid.
  Otherwise, InvalidValid Exception would throw.
  
  @param channel The Selectable to check
  @param operation The operation interest to check