Problem with this solution is that it will return incorrect results if the collection contains numerical strings. For example, ["13", "42"] should result in [ ] because the array contains no ints, but your solution will return [13, 42].

I can see one problem with failure messages tho. String elements are not quoted, what makes it impossible to tell 7 from "7" on failure. (EDIT: fixed)

It is not a proper way to raise an issue. I cannot fix anything if I do not know the languag you attempt, and your code.

For input [908, 134, 416, , o, 5, R] expected:<[908, 134, 416]> but was:<[908, 134, 416, 5]>

For input [1, 2, 'aasf', '1', '123', 123] expected:<[1, 2, 123]> but was:<[1, 2, 1, 123, 123]>

Groovy Translation

not a question

OP solved the kata

you altered the return type of the method. it should return a List<Object>, not a List<Integer>. the input list can contain potentially any type, therefore so can the output, expect Strings

Where is my error ? :src/test/java/SolutionTests.java:43: error: incompatible types: List cannot be converted to List
List result = Kata.filterList(List.copyOf(input));
^
1 error

Agreed.

You have a typo in the example namely in the output of the last number:
ListFilterer.GetIntegersFromList(new List<object>(){1, 2, "a", "b", "aasf", "1", "123", 123}) => {1, 2, 231}

There should be:
ListFilterer.GetIntegersFromList(new List<object>(){1, 2, "a", "b", "aasf", "1", "123", 123}) => {1, 2, 123}

This kata is a subject to deduplication process here: https://github.com/codewars/content-issues/issues/227.
Please join the discussion to help us identify duplicate kata and retire them.

Posting a spoiler solution is not a kata issue, it's a user issue.