使用Java插入双链双端列表时遇到问题

Question

public Link insert(String data, int where)
{
    Link temp = first;
    //If list is empty, insert as first
    if (first == null)
    {
        first = new Link(data, where);
        return first;
    }
    //List is not empty
    else
    {
        while (temp != null)
        {
            //If data is less than first, then insert as first
            if (data.compareTo(temp.getdData()) < 0 && temp == first)
            {
                Link newLink = new Link(data, where);
                newLink.setNext(first);
                first.setPrevious(newLink);
                first = newLink;
                return newLink;
            }
            //If data is less than current, then insert before current
            else if (data.compareTo(temp.getdData()) < 0)
            {
                Link newLink = new Link(data, where);
                newLink.setNext(temp);
                newLink.setPrevious(temp.getPrevious());
                temp.getPrevious().setNext(newLink);
                temp.setPrevious(newLink);
                return newLink;
            }
            //If data is more, but your at the end of the list.
            else if (data.compareTo(temp.getdData()) > 0 && temp.getNext() == null)
            {
                Link newLink = new Link(data, where);
                temp.setNext(newLink);
                newLink.setPrevious(temp);
                newLink.setNext(null);
                last = newLink;
                return newLink;
            }
            //Else keep going forward
            else
                temp = temp.next;
        }
    }
    return temp;
}

I can insert into the list, but for some reason it won't insert in order. For example, if a link's data is "Hello" and I want to insert a link with "aaa", it won't insert the new link before the "Hello" link.

Answer 1

Take a look at the way java compare string:

java> "aaa".compareTo("Hello")    // positive number
java> "Aaa".compareTo("Hello")    // negative number

The reason is that ASCII code of character 'a' is larger than 'H'.

BTW, you'd better use .equals() rather than == in your code to compare String in Java, take a look at this link: http://perso.ensta-paristech.fr/~diam/java/online/notes-java/data/expressions/22compareobjects.html

Answer 2

If you re-implement collections - it is the red flag you are doing something wrong.

Your task can be solved much simpler:

public class Link {
    private final String data;
    private final int where;
    ...
}

public class LinkComparator implements Comparator<Link> {
    @Override
    public int compare(Link o1, Link o2) {
        return o1.data.compareToIgnoreCase(o2.data);
    }
}

public static void main(String[] args) {
    NavigableSet<Link> links = new TreeSet<Link>(new LinkComparator());

    links.add(new Link(...));
}