What happens if an array index is out of bounds?

When an array index is out of bounds in Java, it results in a runtime error known as an ArrayIndexOutOfBoundsException. This happens when code attempts to access an index that is either negative or greater than or equal to the length of the array. Arrays in Java have fixed sizes defined at the time of creation, so any attempt to access an index outside these bounds violates the rules of the array, triggering the exception.

Handling such an exception is crucial for creating robust applications. When the exception occurs, the program will terminate the current execution context; thus, it is important for developers to implement error handling mechanisms like try-catch blocks if there's any possibility of such errors arising. This explains why the stated answer is accurate, as encountering an out-of-bounds index is a direct cause of a runtime error in Java.