This java.lang.ArrayIndexOutOfBoundsException is thrown when we are accessing an array with an index which is greater than the size of an array.

P.S Array index starts with 0

package com.mkyong;
public class ArrayExample {
    public static void main(String[] args) {
        // array of 3
        int[] num = new int[3];
        num[0] = 1;
        num[1] = 2;
        num[2] = 3;
        num[3] = 4; //ArrayIndexOutOfBoundsException: 3
        System.out.println("num[0] : " + num[0]);
        System.out.println("num[1] : " + num[1]);
        System.out.println("num[2] : " + num[2]);
        System.out.println("num[3] : " + num[3]); //ArrayIndexOutOfBoundsException: 3

Output

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
	at com.mkyong.calculator.ArrayExample.main(ArrayExample.java:13)