In this post, I will be sharing how to fix java.lang.IndexOutOfBoundsException in Java. According to Oracle docs, IndexOutOfBoundsException is a subclass of RuntimeException. It is an unchecked exception that is thrown to indicate that an index of some sort (such as to a string, to a vector, or to an array) is out of range.

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What is Index?

The index is the representation of the position of an element inside an array, a list, or other data structure that has order. Indexes in Java are 0 based. It means the 0th index represents the position of the 1st element, the 1st index will represent the 2nd element, the 2nd index will represent the 3rd element, and so on.

[Fixed] java.lang.IndexOutOfBoundsException

First, we will produce the java.lang.IndexOutOfBoundsException before moving on to the solution.

 import java.util.List;
import java.util.ArrayList;
public class IndexOutOfBoundsExceptionExample {
    public static void main(String args[]) {
        List<String> list = new ArrayList<>();
        list.add("Alive");
        list.add("is");
        list.add("Awesome");
        list.get(5);
    }
}

Output:
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 5 out of bounds for length 3

Explanation


We have added three elements to the list. "Alive" is the 1st element at index 0. "is" is the second element at index 1. "Awesome" is the third element at index 2.
When we are executing
 list.get(5)

We are trying to fetch the element at index 5. But we have elements stored till index 2. Hence, we get the IndexOutOfBoundsException.

Solution


We can easily solve this exception by adding an if block condition that the index should always be smaller than the list size as shown below in the example:
 if (index >= 0 && index < list.size())
     list.get(index);

Subclasses of IndexOutOfBoundsException

There are two subclasses of IndexOutOfBoundsException:

1. ArrayIndexOutOfBoundsException

2. StringIndexOutOfBoundsException

1. ArrayIndexOutOfBoundsException


According to Oracle docs, ArrayIndexOutOfBoundsException is a subclass of IndexOutOfBoundsException. This exception is thrown to indicate that an array has been accessed with an illegal index. The illegal index means
a. If the index is negative or
b. If the index is greater than or equal to the size of the array

Producing the ArrayIndexOutOfBoundsException

Example1: When the index is negative

 public class ArrayIndexOutOfBoundsExceptionExample {
    public static void main(String args[]) {
        int[] arr = {2, 5, 29};
        // Passing negative index
        int exception1 = arr[-1];
    }
}


Output:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 3

Example2: When the index is greater than or equal to the size of the array

 public class ArrayIndexOutOfBoundsExceptionExample2 {
    public static void main(String args[]) {
        int[] arr = {2, 5, 29};
        // Passing index greater than the size of the array
        int exception2 = arr[8];
    }
}


Output:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 8 out of bounds for length 3

Solution


We can easily solve the ArrayIndexOutOfBoundsException by adding an if block condition that the index should always be smaller than the array size as shown below in the example:
 public class ArrayIndexOutOfBoundsExceptionSolution {
    public static void main(String args[]) {
        int[] arr = {2, 5, 29};
        int num = 0;
        int index = 5;
        // if condition check to solve ArrayIndexOutOfBoundsException 
        if (index >= 0  && index < arr.length)
            num = arr[index];
        else
            throw new ArrayIndexOutOfBoundsException("Invalid Index"); 
        System.out.println(num);
    }
}


Output:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Invalid Index

2. StringIndexOutOfBoundsException


According to Oracle docs, StringIndexOutOfBoundsException is a subclass of  IndexOutOfBoundsException. This exception is thrown by String methods to indicate that an index is either negative or greater than the size of the String. For some methods like the charAt() method, this exception is also thrown when the index is equal to the size of the string.

Producing the StringIndexOutOfBoundsException

Example1: When the index is negative

 public class StringIndexOutOfBoundsExceptionExample {
    public static void main(String args[]) {
        String str = "Love Yourself"; // length = 13
        // Passing index having negative value
        char exception1 = str.charAt(-4);
    }
}


Output:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -4

Example2: When the index is greater than or equal to the size of the String

 public class StringIndexOutOfBoundsExceptionExample2 {
    public static void main(String args[]) {
        String str = "Alive is Awesome"; // length = 16
        // Passing index greater than the size of the string
        char exception1 = str.charAt(23);
    }
}


Output:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 23

Solution


We can easily solve the StringIndexOutOfBoundsException by adding an if block condition that the index should always be smaller than the String size as shown below in the example:
 public class StringIndexOutOfBoundsExceptionSolution {
    public static void main(String args[]) {
        char ch = Character.MIN_VALUE;
        int index = 22;
        String str = "Love Yourself"; // length = 13
        // if condition check to solve StringIndexOutOfBoundsException 
        if (index >= 0 && index < str.length())
            ch = str.charAt(index);
        else
            throw new StringIndexOutOfBoundsException("Invalid Index"); 
        System.out.println(ch);
    }
}


Output:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: Invalid Index

That's all for today, please mention in the comments in case you have any questions related to java.lang.IndexOutOfBoundsException in Java.