Question

For whatever reason, despite the fact that it’s entirely generic and works with integers whenever it’s tested with strings it seems to skip the last iteration of merging. I’ve scoured my code for a couple hours now and I can’t see why it isn’t working correctly so any insight anyone has would be well received!

import java.util.Iterator; import
java.util.LinkedList;

/** * * @author paul */ public
class MergeSort> {

LinkedList<T> theList;

MergeSort(LinkedList<T> toBeSorted) {
    theList = toBeSorted;
}

public LinkedList<T> sort() {
    return trueSort(theList);
}

private LinkedList<T> trueSort(LinkedList<T> sorting) {
    if (sorting.size() <= 1) {
        return sorting;
    }
    LinkedList<T> left, right, sorted;
    left = new LinkedList<T>();
    right = new LinkedList<T>();
    int middle = sorting.size() / 2;
    Iterator<T> sojourner = sorting.iterator();
    for (int i = 0; sojourner.hasNext(); i++) {
        if (i < middle) {
            left.add(sojourner.next());
        } else {
            right.add(sojourner.next());
        }
    }
    return trueMerge(trueSort(left),

trueSort(right));
}

private LinkedList<T> trueMerge(LinkedList<T> left,

LinkedList right) {
LinkedList result = new LinkedList();
while (left.size() > 0 || right.size() > 0) {
if (left.size() > 0 && right.size() > 0) {
if (left.getFirst().compareTo(right.getFirst())
< 0) {
result.add(left.pop());
} else {
result.add(right.pop());
}
} else if (left.size() > 0) {
result.add(left.pop());
} else {
result.add(right.pop());
}
}
return result;
} }

Here’s my main java file

import java.util.Iterator;
import java.util.LinkedList;
import java.util.Random;

/**
*
* @author paul
*/
public class Main {

    public static Random Rand;

    public static int randomNumber(int min, int max) {
  return min + (int) (Rand.nextDouble() * ((max - min) +

1));
}

    public static <T> String getString(LinkedList<T> linkInt) {
  String s = "";
  Iterator<T> interLink = linkInt.iterator();
  for (int i = 0; interLink.hasNext(); i++) {
    s = s + interLink.next().toString();
    if (interLink.hasNext()) {
    s = s + ", ";
    }
    if ((i + 1) % 10 == 0) {
    s = s + "\n";
    }
  }
  return s;
    }

    /**
  * @param args the command line arguments
  */
    public static void main(String[] args) {
  Rand = new Random();

  LinkedList<Integer> numbers = new LinkedList<Integer>();
  for(int i = 0; i< 100;i++){
    numbers.add(randomNumber(1,1000));
  }
  System.out.println(getString(numbers));
  MergeSort m = new MergeSort(numbers); //change this

later to be purely static
numbers = m.sort();
System.out.println(getString(numbers));

  LinkedList<String> words = new LinkedList<String>();
  words.add("Hello");
  words.add("MY");
  words.add("name");
  words.add("Is");
  words.add("Barthoal");
  words.add("I");
  words.add("Enjoy");
  words.add("long");
  words.add("beach");
  words.add("walks");
  words.add("would");
  words.add("you");
  words.add("like");
  words.add("to");
  words.add("come");
  words.add("Join");
  words.add("me");
  words.add("in");
  words.add("my");
  words.add("StarDestroyer-MobileHome?

(TM)”);
System.out.println(getString(words));
MergeSort mm = new MergeSort(words);
words = mm.sort();
System.out.println(getString(words));
}
}

This outputs:

304, 842, 342, 794, 574, 99, 250, 885,
408, 387, 899, 73, 391, 883, 771,
848, 968, 504, 129, 370, 994, 897,
649, 345, 983, 326, 688, 547, 541,
567, 777, 987, 201, 326, 298, 959,
166, 962, 864, 797, 512, 505, 609,
208, 21, 43, 458, 442, 138, 570, 455,
442, 516, 294, 406, 310, 215, 212,
397, 98, 938, 496, 263, 973, 571,
861, 687, 276, 927, 608, 421, 831,
820, 510, 68, 172, 504, 8, 976, 992,
68, 497, 33, 233, 607, 587, 611, 695,
834, 338, 448, 978, 359, 413, 1, 819,
18, 977, 693, 649

1, 8, 18, 21, 33, 43, 68, 68, 73, 98,
99, 129, 138, 166, 172, 201, 208, 212,
215, 233, 250, 263, 276, 294, 298,
304, 310, 326, 326, 338, 342, 345,
359, 370, 387, 391, 397, 406, 408,
413, 421, 442, 442, 448, 455, 458,
496, 497, 504, 504, 505, 510, 512,
516, 541, 547, 567, 570, 571, 574,
587, 607, 608, 609, 611, 649, 649,
687, 688, 693, 695, 771, 777, 794,
797, 819, 820, 831, 834, 842, 848,
861, 864, 883, 885, 897, 899, 927,
938, 959, 962, 968, 973, 976, 977,
978, 983, 987, 992, 994

Hello, MY, name, Is, Barthoal, I,
Enjoy, long, beach, walks, would,
you, like, to, come, Join, me, in, my,
StarDestroyer-MobileHome? (TM)

Barthoal, Enjoy, Hello, I, Is, Join,
MY, StarDestroyer-MobileHome? (TM),
beach, come, in, like, long, me, my,
name, to, walks, would, you

As you can see, the numbers are fully sorted. The strings seem to have missed the last iteration of merging. So what exactly went wrong?

Answer 1

String comparison is case sensitive. That is, all uppercase strings will be placed before the lower case ones. Your merge sort algorithm should allow to take an own comparator which could be String.CASE_INSENSITIVE_ORDER for your use case.