List with primitive array versus List with Object array

Question

I have the below piece of code for swapping.

public static <E> void swap(List<E> list, int i, int j){
        E temp = list.get(i);
        list.set(i, list.get(j));
        list.set(j, temp);
}

Now when I use List backed by Integer array like below

Integer[] ar = new Integer[]{1,2};      
swap(Arrays.asList(ar),1,0);

It works fine and gives output as [2,1] But I use List backed by int array like below

int[] ar = new int[]{1,2};
swap(Arrays.asList(ar),1,0);

It hrows ArrayIndexOutOfBounds exception. I don't understand why this is happening. List should treat int element as object only. Little help please.

Adding some garbage text to meet the minimum requirements for posting a question is ridiculous. — Aniket Sahrawat
– Aniket Sahrawat, Commented Apr 11, 2019 at 13:10
@AniketSahrawat I'm actually surprised the garbage text was even needed. Certainly, I have seen shorter questions today, so it can't be a length requirement alone. — Tim Biegeleisen
– Tim Biegeleisen, Commented Apr 11, 2019 at 13:18
@TimBiegeleisen I think it's code vs text ratio which is considered. Never really read about it though. — Aniket Sahrawat
– Aniket Sahrawat, Commented Apr 11, 2019 at 13:19
If it's the ratio of text to code, @UtpalKumar should have been fine if he hadn't included most of his questions inside of the code block. — Jordan
– Jordan, Commented Apr 11, 2019 at 13:22

Tim Biegeleisen · Accepted Answer · 2019-04-11 13:16:18Z

2

Here is what is happening. Your current code is actually creating a List<Object>, which happens to contain just a single int[], not a list of actual integers:

int[] ar = new int[]{1,2};
List<Object> list = Arrays.asList(ar);

The reason for the ArrayIndexOutOfBounds exception is that the list you pass in to the swap method has only one entry, at index zero.

In any case, it is not possible to use Arrays.asList to directly convert an array of primitives to a list of some boxed type. The first version of your code is correct, and is what you should be using:

Integer[] ar = new Integer[] {1, 2};
swap(Arrays.asList(ar), 1, 0);

answered Apr 11, 2019 at 13:16

Tim Biegeleisen

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3 Comments

Utpal Kumar Over a year ago

In both cases i.e primitive and Wrapper object array, i am passing only Array object reference to swap method, where it is viewed as List and being referred by a list object. I still don't get how i am passing a single int[]. Is it something to do with internal structure of both the arrays.

Tim Biegeleisen Over a year ago

That's the thing, viewed as List doesn't mean what you think it does. The way you are calling Arrays.asList without specifying a type means that it defaults to List<Object>, with one entry for the primitive array. It doesn't "know" that you want a list of integers.

Utpal Kumar Over a year ago

I guess you are right, when i do this "int[] ar1 = new int[]{1,2}; System.out.println(Arrays.asList(ar1).size());" it returns 1.

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