Jul 29, 2017 1:00:15 PM | Java Exception Handling - java.lang.ArrayIndexOutOfBoundsException

A deeper look at the Java ArrayIndexOutOfBoundsException, including code samples showing both correct and incorrect array for-looping.

Today we'll take another journey through the "Land of Errors" of our ongoing Java Exception Handling series, with a deep dive into the java.lang.ArrayIndexOutOfBoundsException. As the name clearly indicates, the ArrayIndexOutOfBoundsException is thrown when an index is passed to an array which doesn't contain an element at that particular index location.

In this article we'll look a bit closer at the java.lang.ArrayIndexOutOfBoundsException by examining where it sits in the Java Exception Hierarchy. We'll also go over a few simple, functional code samples illustrating how ArrayIndexOutOfBoundsExceptions are commonly thrown, so let's get crackin'!

The Technical Rundown

When Should You Use It?

Since Java has internal classes and object structures to manage Arrays -- and because said objects will produce errors like the java.lang.ArrayIndexOutOfBoundsException on their own -- there will rarely be a situation where you'll need to explicitly throw your own ArrayIndexOutOfBoundsException. For example, if you were creating your own data structure object that contained a non-array collection of elements, you'd likely want to explicitly throw a java.lang.IndexOutOfBoundsException, as opposed to a java.lang.ArrayIndexOutOfBoundsException, since the JVM will handle that for you most of the time.

That said, to see how ArrayIndexOutOfBoundsExceptions are commonly thrown we'll start with the full working code sample, after which we'll explore it in more detail:

package io.airbrake;

import io.airbrake.utility.*;


public class Main {


 public static void main(String[] args) {
 // Create list of Books.
 Book[] library = {
 new Book("The Pillars of the Earth", "Ken Follett", 973),
 new Book("Gone Girl", "Gillian Flynn", 555),
 new Book("His Dark Materials", "Philip Pullman", 399),
 new Book("Life of Pi", "Yann Martel", 460)
 };
 // Iterate over array.
 iterateArray(library);
 Logging.lineSeparator();
 iterateArrayInvalid(library);
 }


 /**
 * Iterate over passed Array, logging each element.
 *
 * @param list
 */
 private static void iterateArray(Book[] list) {
 try {
 // Loop through each element by index.
 for (int index = 0; index < list.length; index++) {
 // Output element.
 Logging.log(list[index]);
 }
 } catch (ArrayIndexOutOfBoundsException exception) {
 // Output expected ArrayIndexOutOfBoundsException.
 Logging.log(exception);
 } catch (Exception exception) {
 // Output unexpected Exceptions.
 Logging.log(exception, false);
 }
 }


 /**
 * Iterate over passed Array, logging each element.
 *
 * For-loop limit includes list.length, which is invalid.
 *
 * @param list
 */
 private static void iterateArrayInvalid(Book[] list) {
 try {
 // Loop through each element by index.
 // Less-than or equal to (<=) limit results in an exception.
 for (int index = 0; index <= list.length; index++) {
 // Output element.
 Logging.log(list[index]);
 }
 } catch (ArrayIndexOutOfBoundsException exception) {
 // Output expected ArrayIndexOutOfBoundsException.
 Logging.log(exception);
 } catch (Exception exception) {
 // Output unexpected Exceptions.
 Logging.log(exception, false);
 }
 }
 }


// Book.java
 package io.airbrake;


/**
 * Simple example class to store book instances.
 */
 public class Book
 {
 private String author;
 private String title;
 private Integer pageCount;


 /**
 * Constructs an empty book.
 */
 public Book() { }


 /**
 * Constructs a basic book.
 *
 * @param title Book title.
 * @param author Book author.
 */
 public Book(String title, String author) {
 setAuthor(author);
 setTitle(title);
 }


 /**
 * Constructs a basic book, with page count.
 *
 * @param title Book title.
 * @param author Book author.
 * @param pageCount Book page count.
 */
 public Book(String title, String author, Integer pageCount) {
 setAuthor(author);
 setPageCount(pageCount);
 setTitle(title);
 }


 /**
 * Get author of book.
 *
 * @return Author name.
 */
 public String getAuthor() {
 return author;
 }


 /**
 * Get page count of book.
 *
 * @return Page count.
 */
 public Integer getPageCount() {
 return pageCount;
 }


 /**
 * Get a formatted tagline with author, title, and page count.
 *
 * @return Formatted tagline.
 */
 public String getTagline() {
 return String.format("'%s' by %s is %d pages.", this.title, this.author, this.pageCount);
 }


 /**
 * Get title of book.
 *
 * @return Title.
 */
 public String getTitle() {
 return title;
 }


 /**
 * Set author of book.
 *
 * @param author Author name.
 */
 public void setAuthor(String author) {
 this.author = author;
 }


 /**
 * Set page count of book.
 *
 * @param pageCount Page count.
 */
 public void setPageCount(Integer pageCount) {
 this.pageCount = pageCount;
 }


 /**
 * Set title of book.
 *
 * @param title Title.
 */
 public void setTitle(String title) {
 this.title = title;
 }


 /**
 * Throw an Exception.
 */
 public void throwException(String message) throws Exception {
 throw new Exception(message);
 }
 }


// Logging.java
 package io.airbrake.utility;


import java.util.Arrays;
 import org.apache.commons.lang3.builder.*;


/**
 * Houses all logging methods for various debug outputs.
 */
 public class Logging {


 /**
 * Outputs any kind of Object.
 * Uses ReflectionToStringBuilder from Apache commons-lang library.
 *
 * @param value Object to be output.
 */
 public static void log(Object value)
 {
 if (value instanceof String)
 {
 System.out.println(value);
 }
 else
 {
 System.out.println(new ReflectionToStringBuilder(value, ToStringStyle.MULTI_LINE_STYLE).toString());
 }
 }


 /**
 * Outputs passed in Throwable exception or error instance.
 * Can be overloaded if expected parameter should be specified.
 *
 * @param throwable Throwable instance to output.
 */
 public static void log(Throwable throwable)
 {
 // Invoke call with default expected value.
 log(throwable, true);
 }


 /**
 * Outputs passed in Throwable exception or error instance.
 * Includes Throwable class type, message, stack trace, and expectation status.
 *
 * @param throwable Throwable instance to output.
 * @param expected Determines if this Throwable was expected or not.
 */
 public static void log(Throwable throwable, boolean expected)
 {
 System.out.println(String.format("[%s] %s", expected ? "EXPECTED" : "UNEXPECTED", throwable.toString()));
 throwable.printStackTrace();
 }


 /**
 * Output a dashed line separator of default (40) length.
 */
 public static void lineSeparator()
 {
 // Invoke default length method.
 lineSeparator(40);
 }


 /**
 * Output a dashed lin separator of desired length.
 *
 * @param length Length of line to be output.
 */
 public static void lineSeparator(int length)
 {
 // Create new character array of proper length.
 char[] characters = new char[length];
 // Fill each array element with character.
 Arrays.fill(characters, '-');
 // Output line of characters.
 System.out.println(new String(characters));
 }
 }

To illustrate a common problem when using arrays we have two similar methods, iterateArray(Book[] list) and iterateArrayInvalid(Book[] list):

/**

* Iterate over passed Array, logging each element.

*

* @param list

*/

private static void iterateArray(Book[] list) {

    try {

        // Loop through each element by index.

        for (int index = 0; index < list.length; index++) {

            // Output element.

            Logging.log(list[index]);

        }

    } catch (ArrayIndexOutOfBoundsException exception) {

        // Output expected ArrayIndexOutOfBoundsException.

        Logging.log(exception);

    } catch (Exception exception) {

        // Output unexpected Exceptions.

        Logging.log(exception, false);

    }

}

/**
 * Iterate over passed Array, logging each element.
 *
 * For-loop limit includes list.length, which is invalid.
 *
 * @param list
 */
 private static void iterateArrayInvalid(Book[] list) {
 try {
 // Loop through each element by index.
 // Less-than or equal to (<=) limit results in an exception.
 for (int index = 0; index <= list.length; index++) {
 // Output element.
 Logging.log(list[index]);
 }
 } catch (ArrayIndexOutOfBoundsException exception) {
 // Output expected ArrayIndexOutOfBoundsException.
 Logging.log(exception);
 } catch (Exception exception) {
 // Output unexpected Exceptions.
 Logging.log(exception, false);
 }
 }

These methods don't do anything fancy and, in fact, basically only serve as wrappers to stick our try-catch blocks in, and to differentiate the slight differences in loop logic between the two. Specifically, as indicated by the code comments, the iterateArrayInvalid(Book[] list) method contains a termination expression that allows index to be less than or equal to the length of list. Since, like most languages, Java uses zero-based numbering to index Arrays and other collections, an index equal to the length of an array will be one greater than the largest index. To better illustrate, consider this table of Arrays showing each array's length and its maximum index:

Length/Count Maximum Index
1 0
2 1
3 2
4 3
5 4
etc. etc.

With that in mind, we'll get started testing both of the iteration methods. We'll use our Book class just to keep things a little more interesting by creating a few elements for our array, then pass it to both methods so we can review the output of each:

public static void main(String[] args) {

    // Create list of Books.

    Book[] library = {

        new Book("The Pillars of the Earth", "Ken Follett", 973),

        new Book("Gone Girl", "Gillian Flynn", 555),

        new Book("His Dark Materials", "Philip Pullman", 399),

        new Book("Life of Pi", "Yann Martel", 460)

    };

    // Iterate over array.

    iterateArray(library);

    Logging.lineSeparator();

    iterateArrayInvalid(library);

}

The output from iterateArray(Book[] list) is just as expected, outputting all four elements before execution is completed:

io.airbrake.Book@532760d8[

  author=Ken Follett

  title=The Pillars of the Earth

  pageCount=973

]

io.airbrake.Book@5679c6c6[

  author=Gillian Flynn

  title=Gone Girl

  pageCount=555

]

io.airbrake.Book@27ddd392[

  author=Philip Pullman

  title=His Dark Materials

  pageCount=399

]

io.airbrake.Book@19e1023e[

  author=Yann Martel

  title=Life of Pi

  pageCount=460

]

On the other hand, invoking iterateArrayInvalid(Book[] list) runs into a problem. While we successfully output all four elements, as mentioned above, the for loop iterates one too many times, resulting in a call to list[4], which is an unknown index. This throws a java.lang.ArrayIndexOutOfBoundsException, indicating the index value that was out of bounds:

io.airbrake.Book@532760d8[

  author=Ken Follett

  title=The Pillars of the Earth

  pageCount=973

]

io.airbrake.Book@5679c6c6[

  author=Gillian Flynn

  title=Gone Girl

  pageCount=555

]

io.airbrake.Book@27ddd392[

  author=Philip Pullman

  title=His Dark Materials

  pageCount=399

]

io.airbrake.Book@19e1023e[

  author=Yann Martel

  title=Life of Pi

  pageCount=460

]

[EXPECTED] java.lang.ArrayIndexOutOfBoundsException: 4

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Written By: Frances Banks