Linked List sorting issue

Question

YES, this is a homework project. That being said, I'm looking to learn from my mistakes rather than just have someone do it for me.

My project is a word frequency list - I accept a text file (or website URL) and count the:
- Number of unique words, and
- How many times they appear.

All methods are provided for me except for one: the insert(E word) method, where the argument is a generic type word. The word is stored in a Node (Linked List project) that also has a 'count' value, which is the value representing the number of times the word appears in the text being read.

What this method has to do is the following:

1. If the argument is already in the list, increment the count of that element. I have done this part
2. If the argument is not found in the list, append it to the list. I also have done this part.
3. sort the list by descending count value. i.e. highest -> lowest count 3.5. If two elements have the same count value, they are sorted by the dictionary order of their word.

I am VERY unfamiliar with Linked Lists, so as such I am running into a lot of NullPointerExceptions. This is my current insert method:

public void insert(E word){
    if(word.equals("")){
        return;
    }

    if(first == null){//if list is null (no elements)
        /*Node item = new Node(word);
        first = item;*/
        first = new Node(word);
    }

    else{//first != null

        Node itemToAdd = new Node(word);

        boolean inList = false;

        for(Node x = first; x != null; x=x.next){
            if (x.key.equals(word)){// if word is found in list
                x.count++;//incr
                inList = true;//found in list

                break;//get out of for
            }//end IF
            if(x.next == null && inList == false){//if end of list && not found
                x.next = itemToAdd;//add to end of list
                break;
            }//end IF
        }//end FOR

        //EVERYTHING ABOVE THIS LINE WORKS. 
        if (!isSorted()){
            countSort();
        }

    }//end ELSE
}//end method

My isSorted() method:

public boolean isSorted(){
    for(Node copy = first; copy.next != null; copy = copy.next){
        if (copy.count < copy.next.count){
            return false;
        }
    }
    return true;
}

and last but not least, the part where I'm struggling, the sort method:

public void countSort(){

        for (Node x = first, p = x.next; p != null; x=x.next, p=p.next){
            // x will start at the first Node, P will always be 1 node ahead of X.

            if(x == first && (x.count < p.count)){
                Node oldfirst = first; 
                x.next = p.next;
                first = p;
                first.next = oldfirst;
                break;
            }

            if (x.count < p.count){
                //copy.next == x.
                Node oldfirst = first;
                oldfirst.next = first.next; 
                x.next = p.next;
                first = p;
                first.next = oldfirst;
                break;
            }

            if (x.count == p.count){
                if(x.toString().charAt(0) < p.toString().charAt(0)){
                    //[x]->[p]->[q]

                    Node oldfirst = first;
                    x.next = p.next;
                    first = p;
                    first.next = oldfirst;
                    break;
                }
            }
        }
    }

Here is the output of my insert method when called by the classes/methods given to me:

Elapsed time:0.084
(the,60)
(of,49)
(a,39)
(is,46)
(to,36)
(and,31)
(can,9)
(in,19)
(more,7)
(thing,7)
(violent,3)
(things,3)
(from,9)
(collected,1)
(quotes,1)
(albert,1)
(einstein,2)
(any,2)
(intelligent,1)
(fool,1)
(make,1)
(bigger,1)
(complex,1)
(it,11)
(takes,1)
(touch,1)
(genius,1)
(lot,1)
(courage,1)
(move,1)
(opposite,1)
(direction,1)
(imagination,1)
(important,5)
(than,3)
(knowledge,3)
(gravitation,1)
(not,17)
(responsible,1)
(for,14)
(people,2)
(falling,1)
(love,2)
(i,13)
(want,1)
(know,3)
(god,4)
(s,8)
(thoughts,2)
(rest,2)
(are,11)
(details,2)
(hardest,1)
(world,7)
(understand,3)
(income,1)
(tax,1)
(reality,3)
(merely,1)
(an,7)
(illusion,2)
(albeit,1)
(very,3)
(persistent,2)
(one,12)
(only,7)
(real,1)
(valuable,1)
(intuition,1)
(person,1)
(starts,1)
(live,2)
(when,3)
(he,11)
(outside,1)
(himself,4)
(am,1)
(convinced,1)
(that,14)
(does,5)
(play,2)
(dice,1)
(subtle,1)
(but,8)
(malicious,1)
(weakness,2)
(attitude,1)
(becomes,1)
(character,1)
(never,3)
(think,1)
(future,2)
(comes,1)
(soon,1)
(enough,1)
(eternal,1)
(mystery,1)
(its,4)
(comprehensibility,1)
(sometimes,1)

My initial idea has been to try and loop the if(!isSorted()){ countSort();} part to just repeatedly run until it's sorted, but I seem to run into an infinite loop when doing that. I've tried following my professor's lecture notes, but unfortunately he posted the previous lecture's notes twice so I'm at a loss.

I'm not sure if it's worth mentioning, but they provided me an iterator with methods hasNext() and next() - how can I use this as well? I can't imagine they'd provide it if it were useless.

Where am I going wrong?

Answer 1

You are close. First the function to compare the items is not complete, so isSorted() could yield wrong results (if the count is the same but the words are in wrong order). This is also used to sort, so it's best to extract a method for the comparison:

// returns a value < 0 if a < b, a value > 0 if a > b and 0 if a == b
public int compare(Node a, Node b) {
    if (a.count == b.count)
        return a.word.compareTo(b.word);
        // case-insensitive: a.word.toLoweCase().compareTo(b.word.toLowerCase())
    } else {
        return a.count - b.count;
    }
}

Or simplified which is enough in your case:

public boolean correctOrder(Node a, Node b) {
    if (a.count > b.count)
       return true;
    else if (a.count < b.count)
       return false;
    else
       return a.word.compareTo(b.word) <= 0;
}

For the sort you seem to have chosen bubble sort, but you are missing the outer part:

boolean change;
do {
   change = false;
   Node oldX = null;
   // your for:
   for (Node x = first; x.next != null; x = x.next) {
       if (!correctOrder(x, x.next)) {
            // swap x and x.next, if oldX == null then x == first
            change = true;
       }
       oldX = x;
   }
} while (change);

We could use the help of Java native library implementation or more efficient sort algorithms, but judging from the exercise the performance of the sort algorithm is of no concern yet, first need to grasp basic concepts.

Answer 2

With looking your codes, it sounds like to me that two things can be done:

Firstly, you can make use of Comparable class method. So, I assume you wrote the class Node, thus you may want to inherit from Comparable class. When you inherited from that class, java will automatically provide you the compareTo method, and all you need to do is to specify in that method that "I want to compare according to your counts and I want it to be in ascending order." **Edit(1):By the way, I forgot the mention before but after you impelement your compareTo method, you can use Collections.sort(LinkedList list), and it will be done.

The second solution came to mind is that you can sort your list during the countSort() operation with the technique of adding all to an another list with sorting and after add all them back to the real list. The sorting technique I'm trying to say is, keep going towards to the end of the list until you find a Node in the list that has a count smaller than currently adding Node's counts. Hope that doesn't confuse your head, but by this way you can achieve more clear method and less complicated view. To be clear I want to repeat the procedure:

Look the next
If (next is null), add it //You are at the end.
else{
if (count is smaller than current count), add it there
else, keep moving to the next Node. //while can be used for that.
}