Errors & Exceptions

Array Index Out Of Bounds Exception in Java

Java supports the creation and manipulation of arrays as a data structure. The index of an array is an integer value that has value in the interval [0, n-1], where n is the size of the array. If a request for a negative or an index greater than or equal to the size of the array is made, then the JAVA throws an ArrayIndexOutOfBounds Exception. This is unlike C/C++, where no index of the bound check is done.

The ArrayIndexOutOfBoundsException is a Runtime Exception thrown only at runtime. The Java Compiler does not check for this error during the compilation of a program.

// A Common cause of index out of bound
public class NewClass2 {
public static void main(String[] args)
{
int ar[] = { 1, 2, 3, 4, 5 };
for (int i = 0; i <= ar.length; i++)
System.out.println(ar[i]);
}
}

Expected Output: 

1
2
3
4
5

Original Output:

Runtime error throws an Exception: 

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
    at NewClass2.main(NewClass2.java:5)

Here if you carefully see, the array is of size 5. Therefore while accessing its element using for loop, the maximum index value can be 4, but in our program, it is going till 5 and thus the exception.

Let’s see another example using ArrayList:

// One more example with index out of bound
import java.util.ArrayList;
public class NewClass2
{
public static void main(String[] args)
{
ArrayList<String> lis = new ArrayList<>();
lis.add("My");
lis.add("Name");
System.out.println(lis.get(2));
}
}

Runtime error here is a bit more informative than the previous time- 

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
    at java.util.ArrayList.rangeCheck(ArrayList.java:653)
    at java.util.ArrayList.get(ArrayList.java:429)
    at NewClass2.main(NewClass2.java:7)

Let us understand it in a bit of detail:

  • Index here defines the index we are trying to access.
  • The size gives us information on the size of the list.
  • Since the size is 2, the last index we can access is (2-1)=1, and thus the exception.

The correct way to access the array is : 
 

for (int i=0; i<ar.length; i++){

}