Sorting array list in Java

Question

I have following class. In this, Iris is another class with some attributes.

public class Helper {

    Iris iris;
    double distance;

    public Helper(Iris iris, double distance) {
        this.iris = iris;
        this.distance = distance;
    }
}

I want to sort an array list of this (i.e List < Helper > helperList), descending based on distance parameter. I have written the following method but it is not working.

public void sort(){
for(int k=0; k < helperList.size(); k++)
        {
            double distance = helperList.get(k).distance; 

             for(int l=0; l < helperList.size(); l++)
             {
                 Helper temp = helperList.get(l);
                 if( distance < temp.distance )
                 {
                     helperList.set(l, helperList.get(k));
                     helperList.set(k, temp);
                 }
            }
        }
}

Can anybody suggest a solution?

Consider what happens with two elements, say [1,2]. for k = 0, l = 1, because list.get(1) > 1, you swap, giving [2,1]. Then for k = 1, l = 0 you swap again. You should compare each element only to those on one side of it. — Daniel Fischer
– Daniel Fischer, Commented Jan 1, 2012 at 8:00
@Daniel Fischer ! In my case the list consists of 120 elements. More than half of the starting elements are sorted correctly but some at the ending aren't. I couldn't understand this behavior. — Ammar
– Ammar, Commented Jan 1, 2012 at 8:08
It's because you're swapping back and forth. I haven't analyzed, perhaps if you start the inner loop at k+1 instead of 0 it works. However, the proper way would be to create a Comparator meeting your needs and use a library sort. Or implement one of the classical sorting algorithms. — Daniel Fischer
– Daniel Fischer, Commented Jan 1, 2012 at 8:15

COD3BOY · Accepted Answer · 2012-01-18 07:55:34Z

17

Why don't you get your Helper class to implement Comparable interface and then use the in-built sort method offered by the Collections class.

Collections.sort(helperList)

I think that would solve the problem. Plus, this sort method is stable.

http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort%28java.util.List%29

http://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html

Implementing Comparable interface:

public class Helper implements Comparable{

    Iris iris;
    double distance;

    public Helper(Iris iris, double distance) {
        this.iris = iris;
        this.distance = distance;
    }

    public int compareTo(Helper other) {
        return new Double(this.distance).compareTo(new Double(other.distance));

    }
}

edited Jan 18, 2012 at 7:55

COD3BOY

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answered Jan 1, 2012 at 8:00

Divya

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4 Comments

jackrabbit Over a year ago

If the two distances are less than 1 apart, this compareTo implementation will return 0 (if you would cast correctly). return this.distance < other.distance ? -1 : this.distance > other.distance ? 1 : 0;

Ammar Over a year ago

It works. Thanx to all and specially to @Divya for her suggestion.

jackrabbit Over a year ago

Why would you create 2 Double objects every time you do a comparison? And even if you want to, use Double.valueOf().

COD3BOY Over a year ago

When you add link to javadoc, try to include the latest one :)

Community · Accepted Answer · 2017-05-23 10:08:31Z

4

Divya's answer is good, but if you don't want to implement Comparable interface, following might be help:

Collections.sort(helperList, new Comparator<Helper>() {
    public int compare(Helper helper1, Helper helper2) {
        return Double.compare(helper1.distance, helper2.distance);
    }
})

edited May 23, 2017 at 10:08

CommunityBot

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answered Jan 1, 2012 at 8:16

Chen-Hai Teng

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1 Comment

Divya Over a year ago

helper1.distance - helper2.distance gives a double and not an int. I made the same mistake.

Shawn Janas · Accepted Answer · 2012-01-01 08:15:47Z

1

The problem is that the loop looses track of where the distance index is located after it has been swapped. This algorithm should work fine.

 for(int k = 1; k < helperList.size(); k++) {
    double distance = helperList.get(k).distance;
    int j = k - 1;
    boolean done = false;
    while(!done) {
       Helper temp = helperList.get(j);
       if(temp.distance < distance) {
           helperList.set(j+1, temp);
           j = j - 1;
           if(j < 0) {
              done = true;
           }
       } else {
          done = true;
       }
       helperList.set(j+1, value);
   }
}

answered Jan 1, 2012 at 8:15

Shawn Janas

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1 Comment

Ammar Over a year ago

Thanx Shawn. I used Collections.sort(object) as suggested by Divya and it works fine.

jackrabbit · Accepted Answer · 2012-01-01 08:11:08Z

0

The article on bubble sort on Wikipedia contains pseudo code and also some optimized versions. Compare with that one to see where you are going wrong.

Bubble sort is one of the most obvious sorting algorithms, but not exactly the most efficient. Why don't you let the platform do the sort? java.util.Collections contains a sort method that lets you supply your own Comparator. All that comparator has to do is decide which of two Helper instances should come first.

edited Jan 1, 2012 at 8:11

answered Jan 1, 2012 at 8:04

jackrabbit

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