Question

I wrote a linked list implementation for my java class earlier this year. This is a generic class called LList. We now have to write out the merge sort algorithm for a lab. Instead of creating a new List implementation that takes Ints, I decided to just reuse the generic list I had created before.

The problem is how do I compare two generic objects? java wont let me do something like

if(first.headNode.data > second.headNode.data) 

So, my question is, is their a way to implement some sort of comparison function that will work on any type of data? I tried the following:

        String one, two;         one = first.headNode.data.toString();         two = second.headNode.data.toString();         if(first.headNode.data.compareTo(second.headNode.data) < 0) {             result.add(first.headNode.data);             // remove head node. remove() takes care of list size.             first.remove(1);         } else {             // If compareTo returns 0 or higher, second.headNode is lower or             // equal to first.headNode. So it's safe to update the result             // list             result.add(second.headNode.data);             second.remove(1);         } 

Which wont even work properly. I tested with the numbers 6 and 12, the above adds 12 to the result list.

Relevant stuff:

 private LList<T> mergeSort(LList<T> list) {     LList<T> first = new LList();     LList<T> second = new LList();     if (list.length() == 1) {         return list;     }      int middle = list.length() / 2;     second.headNode = list.getNodeAt(middle + 1);     second.length = list.length() - (middle);     // Set first half to full list, then remove the 'second' half.     first.headNode = list.headNode;     first.length = middle;     first.getNodeAt(middle).next = null;      // Get the splitted halves.     first = mergeSort(first);     second = mergeSort(second);     return merge(first, second); }  private LList<T> merge(LList<T> first, LList<T> second) {     LList<T> result = new LList();      while((first.length > 0) && (second.length > 0)) {         // Ok, lets force toString to compare stuff since generics are a pain.         String one, two;         one = first.headNode.data.toString();         two = second.headNode.data.toString();         if(one.compareTo(two)) < 0) {             result.add(first.headNode.data);             // remove head node. remove() takes care of list size.             first.remove(1);         } else {             // If compareTo returns 0 or higher, second.headNode is lower or             // equal to first.headNode. So it's safe to update the result             // list             result.add(second.headNode.data);             second.remove(1);         }     }     return result; } 

NOTE: entire LList class can be found [here](http://rapidshare.com/files/219112739/LList.java.html MD5: BDA8217D0756CC171032FDBDE1539478)

Answer 1

Look into the Comparator and Comparable interfaces.

Your sort method should take Comparator or you should specify < T extends Comparable > so that the Comparable interface can be used.

public void sort(Comparable<T> comparator) {     sort(SortType.MERGE, comparator); } .... private LList<T> merge(LList<T> first, LList<T> second) {     ...         if(comparator.compare(first.headNode.data, second.headNode.data) < 0) {     ... }