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bubblesort an array of objects using a comapreTo() and a bubbleSort()

Posted on 2007-08-11
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Last Modified: 2013-12-29
doing a bubblesort... stupid i know. anyway i got a compareTo(Object o) method going on, getting the -1, 1, 0 values and passing them back to a bubbleSort() for all the swapping. I am fairly familiar with java, but not with this kinda of sorting. normally I would just use a java library sorting technique.

just want to know what wrong with this. i'm getting a nullpointerexception, but not sure why since i know for a fact that the object is not null. all I have to do is sort the "mark" highest to lowest. I am not at all concerned with O notation.

public int compareTo(Object o) {
         
          if (this.competitorMark < ((Competitor)o).competitorMark)
                return -1;
          else if (this.competitorMark == ((Competitor)o).competitorMark)
                return 0;
          else
                return +1;
}

public static Competitor[] bubbleSort(Competitor[] a) {

            boolean swappedOnPrevRun = true;
            while (swappedOnPrevRun) {

                  swappedOnPrevRun = false;
                  for (int i = 0; i < a.length - 1; i++) {
                        int current = i;
                        
                        if (a[current].compareTo(a[current + 1]) == 1)
                              swappedOnPrevRun = true;
                              Competitor temp = a[i];
                              a[current] = a[current + 1];
                              a[current + 1] = temp;
                        
                  }
            }
            return a;
      }

this is what is being passed to it (id, name, mark)
 6241 Cathy 8.53

  3792 Patrick 8.25

  2910 John 7.32

  8371 Mary 9.72

  9372 Tony 9.12

  3532 Jane 6.37
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Question by:deomega22
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3 Comments
 
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Expert Comment

by:marklorenz
ID: 19677897
"current+1"  goes beyond the array size.
In your example data, the array index goes from 0 to 5. When your for loop gets to length-1 (5), current+1 = 6, which is beyond your array size.  Remember, arrays don't automatically grow like Collection subclasses.

and you are correct, there are better ways than writing your own, see here:

Sorting Arrays
http://www.leepoint.net/notes-java/data/arrays/70sorting.html

So, i am assuming you are doing this as a learning exercise.
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Author Comment

by:deomega22
ID: 19677923
yea.. learning. if i could use array.sort i would in a heart beat.

i see what you are saying about current + 1 is out of bounds,  so i tried i put the for loop to be i < a.length -1.

is my approach all wrong?
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Accepted Solution

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marklorenz earned 1500 total points
ID: 19677933
Take a look at the algorithm here:

Bubble Sorts
http://www.leepoint.net/notes-java/data/arrays/32arraybubblesort.html

You'll see that they don't index the way you are doing.  I'd suggest you follow their lead.

Mark
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