Junit5 testing flattened ArrayList results in false

Why does Junit prints false for my test?

I am flattening this structure and running Junit5 test against it.

Arrays.asList("a", 
    Arrays.asList("b",
        Arrays.asList("c", "d")), "e")

Junit test:

@Test
public void shouldFlattenAListOfList() throws Exception {
    List<String> flatten = Problem07.flatten(Arrays.asList("a", Arrays.asList("b",
            Arrays.asList("c", "d")), "e"), String.class);
    assertEquals(flatten.size(), 5);
    System.out.println(flatten == Arrays.asList("a", "b", "c", "d", "e")); // prints: false
    assertEquals(flatten, Arrays.asList("a", "b", "c", "d", "e"));
}

Results in error, AssertionFailedError. I see that difference is in whitespaces, and cannot solve this issue.

org.opentest4j.AssertionFailedError: 
Expected :[a,  b,  c,  d,  e]
Actual   :[a, b, c, d, e]

Plain class with static method:

public class Problem07 {
    static List<String> flatten(Collection<?> objects, Object aClass) {
        if (objects == null) {
            throw new NoSuchElementException();
        }

        if (objects.isEmpty()) {
            return Collections.emptyList();
        }

        List<String> strings = new ArrayList<>();

        /*TODO generify for other classes, not only hardcoded String*/
        objects.forEach(o -> {
            if (o instanceof ArrayList) {
                ArrayList<String> o1 = (ArrayList<String>) o;
                strings.addAll(o1);
            } else {
                strings.add(o.toString());
            }
        });

        String formattedString = strings.toString()
                .replace("[", "")  //remove the right bracket
                .replace("]", "");  //remove the left bracket

        List<String> list = new ArrayList<>(Arrays.asList(formattedString.split(",")));

        System.out.println(list);//prints: [a,  b,  c,  d,  e]

        return list;
    }
}