[Tiny] How the conversion of int[] to List<Integer> can be buggy?

Imagine we have an array of ints and we want to convert it to a List. Here is the sample:

int[] array = new int[]{42, 5, 1, 3, 4};
List<Integer> list = new ArrayList(Arrays.asList(array));
System.out.println(list.get(0));

What do you think will be printed? That's right, something like

[I@2ff4acd0

It does not look like 42 at all. So, what's the problem?

First of all, let's look at the warnings IDE shows:

Raw use of parameterized class 'ArrayList'
Call to 'asList()' with only one argument

Okay, so let's fix the first one by adding diamond operator <> to new ArrayList:

int[] array = new int[]{42, 5, 1, 3, 4};
List<Integer> list = new ArrayList<>(Arrays.asList(array));
System.out.println(list.get(0));

And now we have a compilation error on the ArrayList creation:

Required type: List<Integer>
Provided: ArrayList<int[]>

It looks like it matches the second warning we saw earlier. The reason is that Arrays.asList(array) does not box int to Integer and creates a List of <int[]>, not a List of Integer as we wanted it to.

There are several ways to fix it:

• The type of array can be changed from int[] to Integer[]:

Integer[] array = new Integer[]{42, 5, 1, 3, 4};
List<Integer> list = new ArrayList<>(Arrays.asList(array));
System.out.println(list.get(0));

• array can be converted to List<Integer> using IntStream with boxing:

int[] array = new int[]{42, 5, 1, 3, 4};
List<Integer> list = new ArrayList<>(Arrays.stream(array).boxed().toList());
System.out.println(list.get(0));

• Another way to create IntStream from int[] is to use IntStream.of():

int[] array = new int[]{42, 5, 1, 3, 4};
List<Integer> list = new ArrayList<>(IntStream.of(array).boxed().toList());
System.out.println(list.get(0));

All three fixes will work as expected, and the code will print 42.

As a conclusion, don't ignore IDE warnings, as they can indicate real bugs, not just code smells.

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