Sorting Lists in Java

October 16, 2002 Timothy Prickett Morgan

Hey, David:

I have a servlet program that builds a list of products. I have several ways of sorting the list of products, and in most cases, I can just change the order by clause in my select statement to return the correct order. In one view, I need to sort the list by the length of a product component name. I built my own sort routine, but I wonder whether SQL or Java provides a way to sort a list of items by their length.

— Michael

You could use a universal disk format (UDF) to return a value to use in your sort, but since you are using Java it makes sense to use the sort facilities provided with the Collections class. The Collections class is part of the collections framework that became available with the 1.2 JDK. In addition to sorting, the Collections class provides static methods that allow you do a multitude of actions, such as search, shuffle, swap, reverse, rotate, retain sets, and remove sets.

The sort routine provided by the Collections class is also very fast. The Collections sort routine is a merge sort that is far more efficient than a quick sort or bubble sort. Here is an example that sorts a list of string values first by length and then by the string value:

package demo;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;


/**
 * Class LengthSort shows how to sort a list by the length of its values.
 * @author David Morris
 */
public class LengthSort {
    public static void main(String[] args) {
        ArrayList values = new ArrayList();

        values.add("one");
        values.add("two");
        values.add("three");
        values.add("four");
        values.add("five");
        values.add("six");

        Iterator i = values.iterator();
        System.out.println("Before:");

        while (i.hasNext())
            System.out.println(i.next());

        Collections.sort(values, new Comparator() {
            public int compare(Object o1, Object o2) {
                String s1 = (String) o1;
                String s2 = (String) o2;

                if (s1.length() < s2.length()) {
                    return -1;
                }
                else if (s1.length() > s2.length()) {
                    return 1;
                }
                else {
                    return s1.compareTo(s2);
                }
            }
        });

        i = values.iterator();
        System.out.println("nAfter:");

        while (i.hasNext())
            System.out.println(i.next());
    }
}

Running this program generates the following list:

Before:
one
two
three
four
five
six

After:
one
six
two
five
four
three

Passing a comparator to Collections.sort allows you to sort a list in any order. The comparator returns a negative 1 (-1) to indicate that the first value is less than the second, a zero (0) when both values are equal, and a positive 1 (+1) when the first value is greater than the second.

— David

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