How to test exceptions with assertThrows - unit-testing

Question

I have a function called "foo" which produces an ArrayList of strings. foo always returns a non-null array of size 2. When I try to reach an element which is out of the scope, I should get "IndexOutOfBounds" exception. There are two variations of the test method. The second one fails while the first one works just fine. What do you think is causing the second one to fail?
I've tried checking the exception using assertThrows but it didn't work as desired.
import java.util.ArrayList;
public class fooClass {
static ArrayList<String> foo (){
ArrayList<String> arrayOfSize2 = new ArrayList<String>();
arrayOfSize2.add("x");
arrayOfSize2.add("y");
return arrayOfSize2;
}
}
The following test chunk is working
#Test
void testFoo() {
ArrayList<String> dummyArrayList = FooClass.foo();
assertEquals("x",dummyArrayList.get(0));
assertEquals("y",dummyArrayList.get(1));
assertThrows(IndexOutOfBoundsException.class, () -> dummyArrayList.get(2));
}
But this one is not working
#Test
void testFoo() {
ArrayList<String> dummyArrayList = new ArrayList<String>();
dummyArrayList=fooClass.foo();
assertEquals("x", dummyArrayList.get(0));
assertEquals("y", dummyArrayList.get(1));
assertThrows(IndexOutOfBoundsException.class, () -> dummyArrayList.get(2));
}
I expect the output of assertThrows to be True for the second code as well but the actual output is Local variable dummyArrayList defined in an enclosing scope must be final or effectively final

Answer 1

I think your approach is wrong, as it will also terminate exceptionally in the off-chance the foo() method returns a list of less than 2 elements, or null.
I'd test it in this order:
#Test
void testFoo() {
ArrayList<String> dummyArrayList = FooClass.foo();
assertNotNull(dummyArrayList);
assertEquals(2,dummyArrayList.size());
assertEquals("x", dummyArrayList.get(0));
assertEquals("y", dummyArrayList.get(1));
}
[edit] or even simpler:
#Test
void testFoo() {
ArrayList<String> dummyArrayList = FooClass.foo();
assertIterableEquals(Arrays.asList("x", "y"), dummyArrayList);
}

Answer 2

Variables used in lambda expression should be effectively final. the assertion is fine but the lambda is complaining