code: public void assertHasSize(Description description, Set<?> actual, int expectedSize) {
      assertNotNull(description, actual);
      if (expectedSize < 0) {
        throw failures.failure(description, shouldNotBeLessThan(expectedSize, 0));
      }
      int sizeOfActual = actual.size();
      if (sizeOfActual != expectedSize) {
        throw failures.failure(description, shouldHaveSize(actual, sizeOfActual, expectedSize));
      }
    }
docs: Asserts that the number of values in the given {@code Set} is equal to the expected one.
  
  @param description contains information about the assertion.
  @param actual the given {@code Set}.
  @param expectedSize the expected size of {@code actual}.
  @throws AssertionError if the given {@code Set} is {@code null}.
  @throws AssertionError if the number of values in the given {@code Set} is different than the expected one.