Quicksort - Java Implementation

Sorting Algorithm Tutorials - Herong's Tutorial Examples - Version 6.01, by Dr. Herong Yang

Sorting Algorithm Tutorials - Herong's Tutorial Examples

∟Quicksort - Java Implementation

This section provides a tutorial on how to implement the Quicksort algorithm in Java. An implementation diagram is also provided.

Here is my Java implementation of Quicksort algorithm:

/**
 * HyArrays.java
 * This class contains sorting methods similar to java.util.Arrays.
 * All sorting methods should have a signature of 
 * %Sort(Object[] a, int fromIndex, int toIndex)
 * where "fromIndex" is inclusive, and "toIndex" is exclusive.
 * Copyright (c) 2011 by Dr. Herong Yang, herongyang.com
 */
public class HyArrays {
   public static void quickSort(Object[] a, int fromIndex, 
      int toIndex) {
      Object d;
      if (toIndex-1>fromIndex) {
         int j = fromIndex;
         for (int i=j+1; i<toIndex; i++) {
            if (((Comparable)a[fromIndex]).compareTo(a[i])>0) {
               j++;
               d = a[i];
               a[i] = a[j];
               a[j] = d;
            }
         }
         d = a[j];
         a[j] = a[fromIndex];
         a[fromIndex] = d;
         quickSort(a, fromIndex, j+1);
         quickSort(a, j+1, toIndex);
      }
   }
}

The following diagram illustrates how this implementation works:

                               53---------------
                                   39  ... 92   |  67  ...         42
                                    |       |   |   |               |
        +---+-----------+---+---+---+-------+---+---+---------------+
        |   |           |   |   |                
       36  29    ...   17  24   ---------------11                       
        |                   |                   |                   |
fromIndex                   j                   i           toIndex-1

Elements to be sorted are stored from "fromIndex" to "toIndex-1" inclusive.
At the beginning of each iteration of loop, elements from "fromIndex+1" to "j" are lower in order than element at "fromIndex".
At the beginning of each iteration of loop, elements from "j+1" to "i-1" are higher than or equal to element at "fromIndex" in order.
If the element at "i" is lower than element at "fromIndex", we need to place it in the "lower" section by swapping it with element at "j+1".

Last update: 2011.

Table of Contents

About This Book

Introduction of Sorting Algorithms

Java API for Sorting Algorithms

Insertion Sort Algorithm and Implementation

Selection Sort Algorithm and Implementation

Bubble Sort Algorithm and Implementation

►Quicksort Algorithm and Implementation

Quicksort - Algorithm Introduction

►Quicksort - Java Implementation

Quicksort - Performance