Reversing list throws IndexOutOfBoundsException

Question

The method is supposed to revers the order of items in the List

public class ZrcalnaSlikaSeznama {
    public static <T> List<T> zrcalnaSlika(List<T> seznam) {
        int length = seznam.size();
        List<T> seznam2 = new ArrayList<>();
        for (T x : seznam){
            seznam2.add(length-1, x);
            length--;
        }
        return seznam2;
    }
}

But it throws IndexOutOfBoundsException . Why? And how to fix the method?

Answer 1

Explanation

The main issue of your code is that you assumed you could directly add something to a later position in an ArrayList , using add(index, element) .

But it is not an array, it is a list. This is not possible. The add method does not allow adding beyond its current size , which is still 0 . As explained in the documentation of this method:

Throws: IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())

---

Fix

To fix your code, and still going this approach, you would first have to ensure that the list already has the desired size by prefilling it with garbage data, like lots of null s. And then doing what you tried to do, but by using set , not add (otherwise you would not change existing entries but shift them around). So something like this:

List<T> reversed = new ArrayList<>();

// Fill with garbage
for (int i = 0; i < seznam.size(); i++) {
    reversed.add(null);
}

// Exchange against elements
int i = seznam.size() - 1;
for (T element : seznam) {
    reversed.set(i, element);
    i--;
}

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Proper solution

Obviously, filling the list first with garbage data is not an ideal approach. You can do much better. How about adding to your new list forwards, but iterating the original items backwards. So if you have [1, 2, 3] you add, 3 , 2 and finally 1 . This can be accomplished easily:

List<T> reversed = new ArrayList<>();

for (int i = seznam.size() - 1; i >= 0; i--) {
    reversed.add(seznam.get(i));
}

---

Note

There are also other approaches to this and Java also has already built-in methods for this, if that is an option for you, ie Collections.reverse(list) .

For example, see How to get a reversed list view on a list in Java?

Answer 2

Another alternative to reverse the order of your items in the List using the for-loop() could be :

public class ZrcalnaSlikaSeznama {

    public static <T> List<T> zrcalnaSlika(List<T> seznam) {
        int length = seznam.size();
        List<T> seznam2 = new ArrayList<>();

        for (int i = length-1; i>=0; i--){
            seznam2.add(seznam.get(i));
        }

        return seznam2;
    }

}