尝试反转数组时出现ArrayIndexOutOfBoundsException

Question

This seems simple enough but I get the error "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3 at reverse.main(reverse.java:28)"

I initially take inputs from the user to write an array, and then I want to print the array backwards. I understand there are other ways of doing this, but I mainly want to know why this is not working. Going through it line by line makes sense?

PS. If it's not a problem, is there any better way of doing this?

import java.util.Scanner;


public class reverse {

    /**
     * @param args
     */
    public static void main(String[] args) {
        System.out.printf("Enter the number of values in array: ");
        Scanner scanner = new Scanner(System.in);
        int n;
        n = scanner.nextInt();

        double[] a1 = new double[n];
        int i;

        System.out.printf("Enter the value in the array: ");
        for (i = 0; i < n; i++){
            Scanner scanner2 = new Scanner(System.in);
            a1[i] = scanner2.nextInt();
            }
         double j;
         double k;

            for (i = 0; i < n/2; i++){
                j = a1[i];
                k = a1[n-i]; //error line;
                a1[i]=k;
                a1[n-i]=j;
            }
         for(i = 0; i < n; i++){
        System.out.println(" "+a1[i]);
    }}


}

Answer 1

When i = 0, ni will result in n, which is one larger than the available indexes( 0 -> n-1 ).

for (i = 0; i < n/2; i++){
  j = a1[i];
  k = a1[n-i]; //error line;
  a1[i]=k;
  a1[n-i]=j;
}

Answer 2

Collections.reverse(Arrays.asList(array))

Will reverse an array for you, then just print its values out. It's great to do these kinds of problems as exercises but if you're ever woring in the industry it's usually better to rely on the Java API for trivial things like this. Probably going to be faster and a lot more simpler than anything you can come up with.

Answer 3

As said by Samhain, when i = 0 , then ni == n , which is greater than the last index of the array (since arrays start with index 0).

The simplest solution is to just subtract an additional 1 from ni .

j = a1[i];
k = a1[n-i-1];
a1[i]=k;
a1[n-i-1]=j;

Also, creating a new Scanner is totally unnecessary. Just continue to use the first one you created.

for (i = 0; i < n; i++){
    a1[i] = scanner.nextInt();
}

Finally, for what it's worth, if you're using nextInt you don't need to declare your array as a double[] (nor do j and k need to be doubles). You can just use ints.

Here's it running on ideone .