Why do I get an ArrayIndexOutOfBoundsException For My ArrayList

Question

I cannot see anything wrong with my code:-

import java.util.ArrayList;

public class Main {

    public static void main(String[] args) {

       int[] A = new int[] {2, 1, 1, 2, 3, 1}; 
       ArrayList<Integer> foundNumbers = new ArrayList<>();
        int distinct = 0;

        for(int i = 0; i < A.length-1; i++) {
            if(foundNumbers.get(i-1) == null) {

                foundNumbers.set((i-1), A[i]);
                distinct++;
            }


    }
        System.out.println(distinct);
}    
}

I want to check if the value of Array element i has already been assigned to ArrayList element i-1, then increment the distinct variable and print how many distinct values are in the array.

Here's the exception when I change the value of i to 1:-

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
    at java.util.ArrayList.rangeCheck(ArrayList.java:604)
    at java.util.ArrayList.get(ArrayList.java:382)
    at tdd.Main.main(Main.java:19)

`Index: 0, Size: 0` -- What does that tell you? – Hot Licks Jan 03 '15 at 13:29 — Hot Licks, Jan 03 '15 at 13:29
@HotLicks ArrayLists expand automatically don't they? – Angela Marie-Daley Jan 03 '15 at 13:43 — Angela Marie-Daley, Jan 03 '15 at 13:43
Not if you don't put stuff in them. – Hot Licks Jan 03 '15 at 14:04 — Hot Licks, Jan 03 '15 at 14:04

Boann · Accepted Answer · 2015-01-03T13:49:21.717

The list is completely empty. You haven't put anything in it, yet you're trying to read elements from it with the foundNumbers.get call, so any index will be out of bounds.

To add unique elements from A to the list, get and set are simply the wrong list methods to call, use contains and add if that's what you want to do:

for (int x : A) {
    if (!foundNumbers.contains(x))
        foundNumbers.add(x);
}

Here is the same logic as above written in a more verbose fashion that might make it easier to understand what is involved:

for (int i = 0; i < A.length; i++) {
    boolean found = false;
    for (int j = 0; j < foundNumbers.size(); j++) {
        if (A[i] == foundNumbers.get(j)) {
            found = true;
            break;
        }
    }
    if (!found) {
        foundNumbers.add(A[i]);
    }
}

You don't need the separate distinct variable, since it is simply foundNumbers.size().

Although this works, a List is not very efficient for eliminating duplicates if the count of elements is large, since every contains call requires another loop through the contents of the list. A Set automatically prevent duplicates and internally structures its contents in a way that makes it efficient to do so:

Set<Integer> distinct = new TreeSet<>();
for (int x : A) distinct.add(x);
System.out.println(distinct); // [1, 2, 3]

I don't want to fill it with the contents of the array, only with distinct values that appear in the array, for instance, in this example, the array list should have 1, 2 and 3 inside it — Angela Marie-Daley, Jan 03 '15 at 13:28
When I change the code to your edit, I get IndexOutOfBoundsException: Index: 1, Size: 0 — Angela Marie-Daley, Jan 03 '15 at 13:38
@AngelaMarie-Daley Don't call `get` or `set`. There is no purpose in doing so here. — Boann, Jan 03 '15 at 13:43

score 1 · Answer 2 · answered Jan 03 '15 at 13:23

1

for(int i = 0; i < A.length-1; i++) {
    if(foundNumbers.get(i-1) == null) {

In the first iteration of that loop, i will be set to zero, so the second line is doing .get(-1).

answered Jan 03 '15 at 13:23

paxdiablo

854,327
234
1,573
1,953

Even when I set it to 1, I still get an exception – Angela Marie-Daley Jan 03 '15 at 13:25

score 1 · Answer 3 · answered Jan 03 '15 at 13:29

There are multiple issues:

when i is 0, you try to get i-1=-1th element which is not valid
Even when you fix this, since you dont have elements in list, you will still get IndexOutOfBoundsException, since you havent stored any elements yet and your list is empty.

Your loop should be:

for (int i = 0; i < A.length - 1; i++) {
        if (foundNumbers.size() > i && foundNumbers.get(i) == null) {//or better you use contains method of list like foundNumbers.contains(someNumber);
            foundNumbers.add(A[i]);
            distinct++;
        }
}

Why do I get an ArrayIndexOutOfBoundsException For My ArrayList

3 Answers3