unit_test.md

Unit Test

Test Double

            Test Double
            
    Dummy   Test    Test    Mock    Fack
    Object  Stub    Spy     Object  Object

Reference: Test Double

Dummy Object

An object that has no implementation which is used purely to populate arguments of method calls which are irrelevant to the test.

Example

Test addProduct method of a Category which add a Product. The content of product is irrelevant to this test, so Mockito helps us to create a dummy object.

@Test
public void testAddProductWithDummyTest() {
    Product dummy = mock(Product.class);
    Category category = new Category();
    category.addProduct(dummy);
    Assert.assertEquals(1, category.getNumberOfProducts());
}

Test stub

Test stub is to return controlled values to the object being tested. These are described as indirect inputs to the test. Test stub have two types: Responder’s and Saboteur's.

Example

Test getHighestPrice method of the Category which return the highest price by providing a test stub of category.

Responder's

@Test
public void testGetHighestPrice() throws Exception {
  Price price1 = new Price(10); 
  Price price2 = new Price(15);
  Price price3 = new Price(25);
 
  Category category = mock(Category.class);
  when(category.getPrice(any(Product.class)))
    .thenReturn(price1, price2, price3);
   
  Category category = new Category();
  Price highestPrice = category.getHighestPrice());
  
  assertEquals(price3.getPrice(), highestPrice.getPrice());
}

Saboteur's.

@Test(expected=ProductNotFoundException.class)
public void testGetProductById() throws Exception {
    ...
    when(category.getProductById(anyLong()))
        .thenThrow(new ProductNotFoundException());
    ...
}

Mock Object

Mock objects are used to verify object behaviour during a test.

Example

In our implementation, we have a function a in where Object b.doSomething(c). Then we can use Mock Object to verfiy where doSomething has been invoked.

// declare Mock Object
@Mock
B b
// invoke once
a();
// verify
verify(b).doSomething(c)

Test Spy

Fake Object

Fake objects are usually hand crafted or light weight objects only used for testing and not suitable for production. A good example would be an in-memory database or fake service layer.