Why are empty collections of different type equal?

Question

What is mechanism below that makes equal different types?

import static org.testng.Assert.assertEquals;

@Test
public void whyThisIsEq         


        

Answer 1

Testng calls through to a method implemented this way.

  public static void assertEquals(Collection actual, Collection expected, String message) {
    if (actual == expected) {
      return;
    }

    if (actual == null || expected == null) {
      if (message != null) {
        fail(message);
      } else {
        fail("Collections not equal: expected: " + expected + " and actual: " + actual);
      }
    }

    assertEquals(
        actual.size(),
        expected.size(),
        (message == null ? "" : message + ": ") + "lists don't have the same size");

    Iterator actIt = actual.iterator();
    Iterator expIt = expected.iterator();
    int i = -1;
    while (actIt.hasNext() && expIt.hasNext()) {
      i++;
      Object e = expIt.next();
      Object a = actIt.next();
      String explanation = "Lists differ at element [" + i + "]: " + e + " != " + a;
      String errorMessage = message == null ? explanation : message + ": " + explanation;
      assertEqualsImpl(a, e, errorMessage);
    }
  }

This is trying to be helpful but is poor for a number of reasons.

Two equals collections can appear to be different.

Set a = new HashSet<>();
a.add(82);
a.add(100);
System.err.println(a);
Set b = new HashSet<>();
for (int i = 82; i <= 100; i++)
    b.add(i);
for (int i = 83; i <= 99; i++)
    b.remove(i);
System.err.println(b);
System.err.println("a.equals(b) && b.equals(a) is " + (a.equals(b) && b.equals(a)));
assertEquals(a, b, "a <=> b");

and

Set a = new HashSet<>();
a.add(100);
a.add(82);
System.err.println(a);
Set b = new HashSet<>(32);
b.add(100);
b.add(82);
System.err.println(b);
System.err.println("a.equals(b) && b.equals(a) is " + (a.equals(b) && b.equals(a)));
assertEquals(a, b, "a <=> b");

prints

[82, 100]
[100, 82]
a.equals(b) && b.equals(a) is true
Exception in thread "main" java.lang.AssertionError: a <=> b: Lists differ at element [0]: 100 != 82
    at ....

Two collections can be the same or different depending on how they are compared.

assertEquals(a, (Iterable) b); // passes

assertEquals(a, (Object) b); // passes

assertEquals(Arrays.asList(a), Arrays.asList(b)); // passes