为Java TreeSet创建比较器类

Question

I have created a comparator class for Java's TreeSet function that I wish to use to order messages. This class looks as follows

public class MessageSentTimestampComparer
{
/// <summary>
/// IComparer implementation that compares the epoch SentTimestamp and MessageId
/// </summary>
/// <param name="x"></param>
/// <param name="y"></param>
/// <returns></returns>

public int compare(Message x, Message y)
{
    String sentTimestampx = x.getAttributes().get("SentTimestamp");
    String sentTimestampy = y.getAttributes().get("SentTimestamp");

    if((sentTimestampx == null) | (sentTimestampy == null))
    {
        throw new NullPointerException("Unable to compare Messages " +
                "because one of the messages did not have a SentTimestamp" +
                " Attribute");
    }

    Long epochx = Long.valueOf(sentTimestampx);
    Long epochy = Long.valueOf(sentTimestampy);

    int result = epochx.compareTo(epochy);

    if (result != 0)
    {
        return result;
    }
    else
    {
        // same SentTimestamp so use the messageId for comparison
        return x.getMessageId().compareTo(y.getMessageId());
    }
}
}

But when I attempt to use this class as the comparator Eclipse gives and error and tells me to remove the call. I have been attempting to use the class like this

private SortedSet<Message> _set = new TreeSet<Message>(new MessageSentTimestampComparer());

I have also attempted to extend the MessageSentTimestampComparer as a comparator with no success. Can someone please explain what I am doing wrong.

Answer 1

Your MessageSentTimestampComparer doesn't implement Comparator . Try this:

public class MessageSentTimestampComparer implements Comparator<Message> {
  @Override
  public int compare(Message x, Message y) {
    return 0;  // do your comparison
  }
}

Answer 2

If you check the constructor signatue - public TreeSet(Comparator<? super E> comparator) , the parameter type is java.util.Comparator .

So your comparator must implement the Comparator interface (for the compiler to not complain) as follows -

public class MessageSentTimestampComparer implements Comparator<Message> {