reverse for loop for array countdown

You declared array on integers of 10 elements. And you are iterating from i=0 to i=10 and i=10 to i=0 that's 11 elements. Obviously it's an index out of bounds error.

Change your code to this

public class Reverse {  public static void main(String [] args){    int i, j;    System.out.print("Countdown\n");    int[] numIndex = new int[10]; // array with 10 elements.    for (i = 0; i<10 ; i++) {  // from 0 to 9      numIndex[i] = i;// element i = number of iterations (index 0=0, 1=1, ect.)    }    for (j=9; j>=0; j--){ // from 9 to 0      System.out.println(numIndex[j]);//indexes should print in reverse order from here but it throws an exception?       }   }}

Remember indices starts from 0.

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Java uses 0-based array indexes. When you create an Array of size 10 new int[10] it creates 10 integer 'cells' in the array. The indexes are: 0, 1, 2, ...., 8, 9.

Your loop counts to the index which is 1 less than 11, or 10, and that index does not exist.

The array is of size 10, which means it is indexable from 0 to 9. numIndex[10] is indeed out of bounds. This is a basic off-by-one error.