Java Notes

Sorting Arrays

Why you shouldn't write your own sort

A favorite computer science topic it sorting, putting a collection of data in some order. Typically this is done for arrays. Textbooks cover both the slow, O(n²), sorts (eg, selection, insertion, and bubble sorts) and fast, O(n log n), sorts (quick sort, heap sort, etc). These are interesting problems, give students a lot of practice with arrays, bring up important tradeoffs, and are generally quite educational.

However, the Java library already has sort methods that are more efficient, more general, and more correct than anything you are likely to write yourself. And they are already written. Professional programmers use one of the java.util.Arrays.sort() methods; they do not write their own, except perhaps in unusual cases.

Other data structures. There are sort methods in the java.util.Collections class which can be used to sort other kinds of data structures (eg, ArrayList), and there are data structures such as TreeMap that keep elements sorted based on a key.

Sorting Arrays with Arrays.sort(...)

The java.util.Arrays class contains a number of static methods for sorting arrays, both arrays of primitive types and Object types. The sort method can be applied to entire arrays, or only a particular range.

Example - Sorting arrays using Arrays.sort()

This example sorts an array of Strings and an array of doubles. All object types that implement Comparable (ie, defines compareTo() method), can be sorted with using a comparator. The Arrays.sort() method is also defined for primitive arrays.

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// File   : arrays/-objectsort/Dblsrt.java
// Purpose: To show how Arrays.sort() works with objects
//          that implement Comparable (ie, define compareTo().
//          It will also sort primitive types.
// Author : Fred Swartz 2006-03-10.  Public domain.

import java.util.Arrays;

public class Dblsrt {
    //========================================================= main
    public static void main(String[] args) {
        //... Sort strings - or any other Comparable objects.
        String[] names = {"Zoe", "Alison", "David"};
        print(names);       // Print unsorted array.
        Arrays.sort(names); // Sort array
        print(names);       // Print sorted array

        //... Sort doubles or other primitives.
        double[] lengths = {120.0, 0.5, 0.0, 999.0, 77.3};
        Arrays.sort(lengths);
        for (double d : lengths) {
            System.out.print(d + " ");
        }
        System.out.println();
    }

    //======================================================== print
    // Print array of any objects using their toString() method.
    private static void print(Object[] oa) {
        for (Object obj : oa) {
            System.out.print(obj + " ");
        }
        System.out.println();
    }
}

Comparators

All sorts that Java uses are comparison sorts, which means that they make all ordering decisions by comparing two values. If there is no natural ordering, or you don't want to use it, you can specify a Comparator to use in comparing two values. See Comparators.

Sorting ArrayLists

In a similar way, you can use the methods below to sort ArrayLists.

   Collections.sort(alist);
   Collections.sort(alist, comparator);