assertEquals("Cause should be available.", exception, handler.exception);
    assertEquals("EventBus should be available.", eventBus, handler.context.getEventBus());
    assertEquals("Event should be available.", EVENT, handler.context.getEvent());
    assertEquals("Subscriber should be available.", subscriber, handler.context.getSubscriber());
    assertEquals(
        "Method should be available.",

    assertEquals("Should be able to peek() at first element", "A", peekingIterator.peek());
    assertEquals(
        "Should be able to peek() first element multiple times", "A", peekingIterator.peek());
    assertEquals(
        "next() should still return first element after peeking", "A", peekingIterator.next());

    assertEquals("Should be able to peek() at middle element", "B", peekingIterator.peek());

	}

	// RelationModel3 should be existed
	_, err = findColumnType(&RelationModel3{}, "id")
	AssertEqual(t, nil, err)

	// RelationModel1 should not be existed
	_, err = findColumnType(&RelationModel1{}, "id")
	if err == nil {
		t.Errorf("RelationModel1 should not be migrated")
	}

	// RelationModel2 should not be existed
	_, err = findColumnType(&RelationModel2{}, "id")

In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

  public void testPut_supportedPresent() {
    assertEquals("put(present, value) should return the old value", v0(), getMap().put(k0(), v3()));
    expectReplacement(entry(k0(), v3()));
  }

  @MapFeature.Require(SUPPORTS_PUT)
  public void testPut_supportedNotPresent() {
    assertNull("put(notPresent, value) should return null", put(e3()));
    expectAdded(e3());
  }

		t.Errorf(fmt.Sprintf("Count should work, but got err %v", err))
	}
	if count1 != 1 {
		t.Errorf("Count with group should be 1, but got count: %v", count1)
	}

	var count2 int64
	if err := DB.Model(&Company{}).Where("name in ?", []string{"company_count_group_b", "company_count_group_c"}).Group("name").Count(&count2).Error; err != nil {
		t.Errorf(fmt.Sprintf("Count should work, but got err %v", err))
	}
	if count2 != 2 {

    assertFalse(
        "removeAll(emptyCollection) should return false",
        collection.removeAll(MinimalCollection.of()));
    expectUnchanged();
  }

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  public void testRemoveAll_nonePresent() {
    assertFalse(
        "removeAll(disjointCollection) should return false",
        collection.removeAll(MinimalCollection.of(e3())));
    expectUnchanged();

  //   where the endpoint is located. This should match the corresponding
  //   node label.
  // * topology.kubernetes.io/zone: the value indicates the zone where the
  //   endpoint is located. This should match the corresponding node label.
  // * topology.kubernetes.io/region: the value indicates the region where the
  //   endpoint is located. This should match the corresponding node label.

		t.Fatalf("No error should happened when create scanner valuer struct, but got %v", err)
	}

	var result ScannerValuerStruct

	if err := DB.Find(&result, "id = ?", data.ID).Error; err != nil {
		t.Fatalf("no error should happen when query scanner, valuer struct, but got %v", err)
	}

	if result.ExampleStructPtr.Val != "value2" {

    assertTrue("containsValue(present) should return true", getMap().containsValue(v0()));
  }

  public void testContains_no() {
    assertFalse("containsValue(notPresent) should return false", getMap().containsValue(v3()));
  }

  @MapFeature.Require(ALLOWS_NULL_VALUE_QUERIES)
  public void testContains_nullNotContainedButAllowed() {
    assertFalse("containsValue(null) should return false", getMap().containsValue(null));
  }