Test doubles

While testing single units is simpler than testing entire systems, we still face challenges when unit testing classes that depend on other classes or on external infrastructure.

As an example, consider an application in which all SQL-related code has been encapsulated in an IssuedInvoices class. Other parts of the system that depend on this IssuedInvoices class, then also depend (directly or indirectly) on a database. This implicitly creates a dependency on the database when we write tests for those other parts, even though we're not immediately concerned with its behaviour. As we saw before, this can make testing more difficult. We'd really like to test this other code, assuming that the IssuedInvoices works.

In other words, we want to unit test a component A that depends on another component B, but setting up B for testing A is expensive.

This is where test doubles come in handy. We will create an object to mimic the behaviour of component B ("it looks like B, but it is not B"). Within the test, we have full control over what this "fake component B" does so we can make it behave as B would in the context of this test and cut the dependency on the real object. In our example above, suppose that A is a plain Java class that depends on an IssuedInvoices to retrieve values from a database, we can implement a fake IssuedInvoices that just returns a hard-coded list of values in its findAllInvoices() method rather than hitting an external database. This means that, in our test, we can control the environment around A so we can check how it behaves. We'll be showing some examples of how this works later in the chapter.

The use of objects that simulate the behaviour of other objects has advantages:

  • Firstly, we have more control. We can easily tell these objects what to do, without the need for complicated setups. If we want a method to throw an exception, we just tell the mock method to throw it; there is no need for complicated setups to "force" the dependency to throw the exception. Similarly, if we want a method that returns a Calendar object representing a date in 2009, we just tell the mock method to do it; there is no need to complicate your OS in order to make it go back in time. Note how hard it usually is to "force" a class to throw an exception, or to "force" a class to return a fake date. This effort is close to zero when we simulate the dependencies with mock objects.

  • Simulations are also faster. Imagine a dependency that communicates with a webservice or a database. A method in one of these classes might take a few seconds to process. On the other hand, if we simulate the dependency, it will no longer need to communicate with a database or with a webservice anymore and wait for a response; the simulation will simply return what it was configured to return, and it will cost nothing in terms of time.

Simulating dependencies is therefore a widely used technique in software testing, mainly to increase testability.

Such objects are also known as test doubles. In the rest of this chapter, we will:

  1. Explain the differences between dummies, fakes, stubs, spies, and mocks.
  2. Learn Mockito, a popular mocking framework in Java. Although Mockito has "mock" in the name, Mockito can be used for stubbing and spying as well. We also show, by means of a few examples, how simulations can help developers in writing unit tests more effectively.
  3. Discuss interaction testing and how mocks can be used as a design technique.
  4. Discuss what Google has learned about test doubles.

Dummies, fakes, stubs, spies, and mocks

Meszaros, in his book, defines five different types: dummy objects, fake objects, stubs, spies, and mocks:

  • Dummy objects: Objects that are passed to the class under test, but are never really used. This is common in business applications, where in many cases you need to fill a long list of parameters, but the test only exercises a few of them. Think of a unit test for a Customer class. Maybe this class depends on several other classes (Address, Last Order, etc). But a specific test case A wants to exercise the "last order business rule", and does not care much about which Address this Customer has. In this case, a tester would then set up a dummy Address object and pass it to the Customer class.

  • Fake objects: Fake objects have real working implementations of the class they simulate. However, they usually do the same task in a much simpler way. Imagine a fake database class that uses an array list instead of a real database.

  • Stubs: Stubs provide hard-coded answers to the calls that are performed during the test. Stubs do not actually have a working implementation, as fake objects do. In the examples above, where we used the word "simulation", we were actually talking about stub objects. Stubs do not know what to do if the test calls a method for which it was not programmed and/or set up.

  • Spies: As the name suggests, spies "spy" a dependency. It wraps itself around the object and observes its behaviour. Strictly speaking it does not actually simulate the object, but rather just delegates all interactions to the underlying object while recording information about these interactions. Imagine you just need to know how many times a method X is called in a dependency: that is when a spy would come in handy.

  • Mocks: Mock objects act like stubs that are pre-configured ahead of time to know what kind of interactions should occur with them. For example, imagine a mock object that is configured as follows: method A should be called twice and, for the first call it should return "1", and for the second call it should throw an exception, while method B should never be called.

As a tester, you should decide which test double to use. We will discuss some guidelines later.


One of the most popular Java mocking/stubbing frameworks is Mockito (mockito.org). Mockito has a very simple API. Developers can set up stubs and/or define expectations in mock objects with just a few lines of code.

The most important methods one needs to know from Mockito are:

  • mock(<class>): creates a mock object/stub of a given class. The class can be retrieved from any class by <ClassName>.class.
  • when(<mock>.<method>).thenReturn(<value>): defines the behaviour when the given method is called on the mock. In this case <value> will be returned.
  • verify(<mock>).<method>: asserts that the mock object was exercised in the expected way for the given method.

Mockito is an extensive framework and we will only cover part of it in this chapter. To learn more, take a look at its documentation.


Let us learn how to use Mockito and setup stubs with a practical example.

Requirement: Low value invoices

The program must return all the issued invoices with values smaller than 100. The collection of invoices can be found in our database.

The following is a possible implementation of this requirement. Note that IssuedInvoices is a type responsible for retrieving all the invoices from the database. It connects to the database using some global properties.

import java.util.List;
import static java.util.stream.Collectors.toList;

public class InvoiceFilter {

  public List<Invoice> lowValueInvoices() {
    final var issuedInvoices = new IssuedInvoices();
    try {
      return issuedInvoices.all().stream()
              .filter(invoice -> invoice.value < 100)
    } finally {


Without stubbing the IssuedInvoices class, the InvoiceFilter test would need to also handle the database (making the unit testing more expensive). Note how the tests need to open and close the connection (see the @BeforeEach and @AfterEach methods) and save invoices to the database (see the many calls to invoices.save() in the filterInvoices test).

public class InvoiceFilterTest {
  private IssuedInvoices invoices;
  private DatabaseConnection dbConnection;

  @BeforeEach public void open() {
    dbConnection = new DatabaseConnection();
    invoices = new InvoiceDao(dbConnection.getConnection());

    // we need to clean up all the tables,
    // to make sure old data doesn't interfere with the test.

  @AfterEach public void close() {
    if (dbConnection != null) dbConnection.close();

  void filterInvoices() {
    final var mauricio = new Invoice("Mauricio", 20);
    final var steve = new Invoice("Steve", 99);
    final var arie = new Invoice("Arie", 300);


    final InvoiceFilter filter = new InvoiceFilter();

    assertThat(filter.lowValueInvoices()).containsExactlyInAnyOrder(mauricio, steve);


Note also the dbConnection.resetDatabase(); that resets the database at the beginning of every test. This is a common thing to do in tests that involve databases, in order to avoid "old data" (i.e., data that was inserted by previous test cases) interfering with the current test. So far, we have never had to "clean up our mess" in test code, as all the objects we created were in-memory only and recreated between each test.

Did you notice the assertThat...containsExactlyInAnyOrder assertion we used? This ensures that the list contains exactly the objects we pass, and in any order.

Such assertions do not come with JUnit 5. These assertions are part of the AssertJ project. AssertJ is a fluent assertions API for Java, giving us several interesting assertions that are especially useful when dealing with lists or complex objects. We recommend you get familiar with it!

Let us now re-write the test. This time we will stub the IssuedInvoices class which means that we need to find a way for the test to make the substitution. The current implementation of InvoiceFilters creates its instance of IssuedInvoices internally which means the test has no way of doing so. The most direct way to do this is to have the IssuedInvoices passed in as an explicit dependency, in this case through the constructor.

There are JVM frameworks that use runtime reflection to allow different implementations to be substituted when a class is instantiated, but that's missing the point here. That would continue to hide the dependency that already exists (on the underlying database) leaving the code diverging from the structure it represents. We can use the writing of the test to show us where we need to correct that.

import java.util.List;
import static java.util.stream.Collectors.toList;

public class InvoiceFilter {

  final IssuedInvoices issuedInvoices;

  public InvoiceFilter(IssuedInvoices issuedInvoices) {
    this.issuedInvoices = issuedInvoices;
  public List<Invoice> lowValueInvoices() {
      return issuedInvoices.all().stream()
              .filter(invoice -> invoice.value < 100)


Let us now stub IssuedInvoices. Note that now our test does not need to do anything that is related to databases. The full control of the stub enables us to try different cases (even exceptional ones) very quickly:

import static java.util.Arrays.asList;
import static org.assertj.core.api.Assertions.assertThat;
import static org.mockito.Mockito.when;

public class InvoiceFilterTest {
    private final IssuedInvoices issuedInvoices = Mockito.mock(IssuedInvoices.class);
    private final InvoiceFilter filter = new InvoiceFilter(issuedInvoices);

    void filterInvoices() {
      final var mauricio = new Invoice("Mauricio", 20);
      final var steve = new Invoice("Steve", 99);
      final var arie = new Invoice("Arie", 300);

      when(issuedInvoices.all()).thenReturn(asList(mauricio, arie, steve));

      assertThat(filter.lowValueInvoices()).containsExactlyInAnyOrder(mauricio, steve);


Note how we setup the stub, using Mockito's when method. In this example, we tell the stub to return a list containing mauricio, arie, and steve (the three invoices we instantiate as part of the test case). The test then invokes the method under test, filter.lowValueInvoices(). As a consequence, the method under test invokes issuedInvoices.all(). However, at this point, issuedInvoices is actually a stub that returns the list with the three invoices. The method under test continues its execution, returns a new list with only the two invoices that are below 100, causing the assertion to pass.

Note that, besides making the test easier to write, the use of stubs also made the test class more cohesive. The InvoiceFilterTest only tests the InvoiceFilter class. It does not test the usage of the IssuedInvoices class. Clearly, IssuedInvoices deserves to be tested, but in another place, and by means of an integration test.

Note that a cohesive test has less chances of failing because of something else. In the old version, the filterInvoices test could fail because of a bug in the InvoiceFilter class or because of a bug in the IssuedInvoices class. The new tests can now only fail because of a bug in the InvoiceFilter class, and never because of IssuedInvoices. This is handy, as a developer will spend less time debugging in case this test starts to fail.

Our new approach for testing InvoiceFilter is faster, easier to write, and more cohesive.

In a real software system, the business rule implemented by the InvoiceFilter would probably be best done directly in the database. After all, a simple SQL query would do the job in a much more performatic manner.

Nevertheless, abstract away the example. Imagine that for a class to do its job, it needs some data from (or to interact with) another dependency.

Mocks and expectations

Suppose our system has a new requirement:

Requirement: Send low valued invoices to SAP

All low valued invoices should be sent to our SAP system. SAP offers a /sendInvoice webservice that receives invoices.

Let us follow the idea of using test doubles to facilitate the development of our production and test code. To that aim, let us create an SAP interface that represents the communication with SAP:

public interface SAP {
  void send(Invoice invoice);

We now need a class that will coordinate the process: retrieve the low valued invoices from the InvoiceFilterand pass them to the SAP service. Let us create a new SAPInvoiceSender class:

public class SAPInvoiceSender {

  private final InvoiceFilter filter;
  private final SAP sap;

  public SAPInvoiceSender(InvoiceFilter filter, SAP sap) {
    this.filter = filter;
    this.sap = sap;

  public void sendLowValuedInvoices() {
      .forEach(invoice -> sap.send(invoice));

Let us now test the SAPInvoiceSender class.

Note that, for this test, we now mock the InvoiceFilter class. After all, for the SAPInvoiceSender, InvoiceFilter class is "just a class that returns a list of invoices". As it is not the goal of the current test to test the filter itself, we should mock this class, in order to facilitate the testing of the method under test.

After the execution of the method under test (sendLowValuedInvoices()), we should expect that the SAP mock received mauricio's and steve's invoices. For that, we use Mockito's verify() method:

public class SAPInvoiceSenderTest {

  private static final InvoiceFilter filter = mock(InvoiceFilter.class);
  private static final SAP sap = mock(SAP.class);
  private static final SAPInvoiceSender sender = new SAPInvoiceSender(filter, sap);

  void sendToSap() {
    final var mauricio = new Invoice("Mauricio", 20);
    final var steve = new Invoice("Steve", 99);

    when(filter.lowValueInvoices()).thenReturn(asList(mauricio, steve));



Note how we defined the expectations of the mock object. We "knew" exactly how the InvoiceFilter class had to interact with the mock. When the test is executed, Mockito will check whether these expectations were met, and fail the test if they were not. (If you want to test it, simply comment out the forEach... line in the sendLowValuedInvoices and see the test failing).

This example illustrates the main difference between stubbing and mocking. Stubbing means simply returning hard-coded values for a given method call. Mocking means not only defining what methods do, but also explicitly defining how the interactions with the mock should be.

Mockito actually enables us to define even more specific expectations. For example, see the expectations below:

verify(sap, times(2)).send(any(Invoice.class));
verify(sap, times(1)).send(mauricio);
verify(sap, times(1)).send(steve);

These expectations are more restrictive than the ones we had before. We now expect the SAP mock to have its send method invoked precisely two times (for any given Invoice). We then expect the send method to called once for the mauricio invoice and once for the steve invoice. We point the reader to Mockito's manual for more details on how to configure expectations.

You might be asking yourself now: Why did you not put this new SAP sending functionality inside of the existing InvoiceFilter class?

If we were to do it, the lowValueInvoices method would then be both a "command" and a "query". By "query", we mean that the method returns data to the caller while with "command", we mean that the method also performs an action in the system. Mixing both concepts in a single method is not a good idea, as it may confuse developers who will eventually call this method. How would they know this method had some extra side-effect, besides just returning the list of invoices?

If you want to read more about this, search for Command-Query Separation, or CQS, a concept devised by Bertrand Meyer.

How to stub static methods (or with APIs you do not control)

Imagine the following requirement:

Requirement: Christmas Discount

The program gives a 15% discount on the total amount of an order if the current date is Christmas day. Otherwise there is no discount.

A possible implementation for this requirement is as follows:

public class ChristmasDiscount {

public double applyDiscount(double amount) {
    Calendar today = Calendar.getInstance();

    double discountPercentage = 0;
    boolean isChristmas = today.get(Calendar.DAY_OF_MONTH) == 25 &&
          today.get(Calendar.MONTH) == Calendar.DECEMBER;

      discountPercentage = 0.15;

    return amount - (amount * discountPercentage);


The implementation is quite straightforward. And given the characteristics of the class, unit testing seems to be a perfect fit for testing it. The question is: how can we write unit tests for it? To test both cases (i.e., Christmas/not Christmas), we need to be able to control/stub the Calendar class, so that it returns the dates we want.

We can then ask a more specific question: how can we stub the Calendar API?

You might have noted that the call to Calendar.getInstance() is a static call. Mockito does not allow us to stub static methods (although some other more magical mock frameworks do). Static calls are indeed enemies of testability, as they do not allow for easy stubbing.

In such cases, a pragmatic solution is to create an abstraction on top of the static call. The abstraction encapsulates the "not-so-easy-to-be-stubbed" method, and offers an "easy-to-be-stubbed" method to the rest of the program. For this particular case, a Clock abstraction can do the job:

import java.util.Calendar;

public interface Clock {
    Calendar now();

With this interface, we can follow the same approach as before. Let us inject Clock to the ChristmasDiscount class:

import java.util.Calendar;

public class ChristmasDiscount {

  private final Clock clock;

  public ChristmasDiscount(Clock clock) {
    this.clock = clock;

  public double applyDiscount(double rawAmount) {
    Calendar today = clock.now();

    double discountPercentage = 0;
    boolean isChristmas = today.get(Calendar.DAY_OF_MONTH) == 25 &&
          today.get(Calendar.MONTH) == Calendar.DECEMBER;

      discountPercentage = 0.15;

    return rawAmount - (rawAmount * discountPercentage);

With Clock being an interface, we can stub it the way we want and/or need:

import org.junit.jupiter.api.Test;
import org.mockito.Mockito;
import java.util.Calendar;
import java.util.GregorianCalendar;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.when;

public class ChristmasDiscountTest {

    private final Clock clock = Mockito.mock(Clock.class);
    private final ChristmasDiscount cd = new ChristmasDiscount(clock);

    public void christmas() {
      Calendar christmas = new GregorianCalendar(2015, Calendar.DECEMBER, 25);

      double finalValue = cd.applyDiscount(100.0);
      assertEquals(85.0, finalValue, 0.0001);

    public void notChristmas() {
      Calendar christmas = new GregorianCalendar(2015, Calendar.JANUARY, 25);

      double finalValue = cd.applyDiscount(100.0);
      assertEquals(100.0, finalValue, 0.0001);

Note again that we were able to develop the main logic of the requirement without depending on the concrete implementation of Clock. For completeness, let us implement Clock:

public class DefaultClock implements Clock {
    public Calendar now() {
      return Calendar.getInstance();

Creating abstractions on top of dependencies that you do not own, as a way to gain more control, is a common technique among developers.

One might ask: Won't that increase the overall complexity of my system? After all, it requires maintaining another abstraction. Yes. There is more complexity when we add new abstractions in our software, and that is what we are doing here. However, the point is: does the ease in testing the system that we get from adding this abstraction compensate for the cost of the increased complexity? Often, the answer is yes, it does pay off.

When to mock/stub?

Mocks and stubs are a useful tool when it comes to simplifying the process of writing unit tests. However, as expected mocking too much might also be problem. We do not want to mock a dependency that should not be mocked. Imagine you are testing class A, which depends on a class B. Should we mock/stub B?

Pragmatically, developers often mock/stub the following types of dependencies:

  • Dependencies that are too slow: If the dependency is too slow, for any reason, it might be a good idea to simulate that dependency.

  • Dependencies that communicate with external infrastructure: If the dependency talks to (external) infrastructure, it might be too complex to be set up. Consider stubbing it.

  • Hard to simulate cases: If we want to force the dependency to behave in a hard-to-simulate way, mocks/stubs can help. A common example is when we would like the dependency to throw an exception. Forcing an exception in the real dependency might be tricky, but easy to do in a stub/mock.

On the other hand, developers tend not to mock/stub:

  • Entities. An entity is a simple class that mirrors a collection in a database, while instances of this class mirror the entries of that collection. In Java we tend to call these POJOs (Plain Old Java Objects), as they mostly consist of fields only. In business systems, it is quite common that entities depend on other entities, e.g., a Order depends on OrderItem. When testing Order, developers tend not to stub OrderItem. The reason is that OrderItem is probably also a well-contained class that is easy to set up. Mocking it would take more time than the actual implementation would take. Exceptions can be made for heavy entities.

  • Native libraries and utility methods. It is not common to mock/stub libraries that come with our programming language and utility methods. For example, why would one mock ArrayList or a call to String.format? As shown with the Calendar example above, any library or utility methods that harm testability can be abstracted away.

Ultimately, remember that whenever you mock, you reduce the reality of the test. It is up to you to understand this trade-off.

Mocks as a design technique

From a developer's perspective, the use of mocks enables them to develop their software, without caring too much about external details. Imagine a developer working on the "low value invoices" requirement. The developer knows that the invoices will come from the database. However, while developing the main logic of the requirement (i.e., the filtering logic), the developer "does not care about the database"; they only care about the list of invoices that will come from it.

In other words, the developer only cares about the existence of a method that returns all the existing invoices. In object-oriented languages, that can be represented by means of an interface:

public interface IssuedInvoices {
 List<Invoice> all();
 void save(Invoice inv);

Having such an interface, the developer can then proceed to the InvoiceFilter and develop it completely. After all, its implementation never depended on a database, but solely on the issued invoices. Look at it again:

public class InvoiceFilter {

  final IssuedInvoices issuedInvoices;

  public InvoiceFilter(IssuedInvoices issuedInvoices) {
    this.issuedInvoices = issuedInvoices;
  public List<Invoice> lowValueInvoices() {
      return issuedInvoices.all().stream()
              .filter(invoice -> invoice.value < 100)

Once the InvoiceFilter and all its tests are done, the developer can then focus on finally implementing the IssuedInvoices class and its integration tests.

Hence, once you get used to this way of developing, you will notice how your code will become easier to test. We will talk more about design for testability in future chapters.

Mock Roles, Not Objects!

In their seminal work Mock Roles, Not Objects, Freeman et al. show how Mock Objects can be used for thinking about design as much as for testing. By treating an object (or a cluster of objects) as a closed box, the programmer can focus on how it interacts with its environment: if the object receives a triggering event (one or more method calls), what does it need to find out (stubs) and how does it change the world around it (mocks)? This, in turn, means that the programmer needs to make explicit what else the target object depends on. Because these tests focus on how an object interacts with its environment, this style of testing is sometimes called "Interaction Testing."

This thinking of objects in terms of input and outputs was inspired by the "Roles, Responsibilities, and Collaborators" technique, best described by Wirfs-Brock and McKean. It has a history going back to the programming language Smalltalk, which used a biological metaphor of communicating cells (Ingalls).

Interaction testing is most effective when combined with TDD. While writing a test, the programmer has to define what the object needs from its environment. For a new feature, this might require a new collaborating object, which introduces a new dependency. The type of this new dependency is defined in terms of what the target object needs - its caller, not its implementation; in Java, this is usually an interface. Following this process, with regular refactoring, a programmer can grow a codebase written in the terminology of the domain. The result is a set of pluggable objects, with clear dependencies, that are combined to build a system.

The authors summarized their best practices for interaction testing as:

  • Only mock types you own. Developers should not mock objects that they do not "own", e.g., a class from an external library. Rather, they should build abstractions on top of those.

  • Be explicit about things that should not happen. Writing tests for interactions that should not happen makes intentions clearer.

  • Specify as little as possible in the test. Overspecified tests tend to be more brittle. Focus on the interactions that really matter.

  • Don't use mocks to test boundary objects. Objects that have no relationships to other objects do not need to be mocked. In practice, these objects tend to "only" store data or represent atomic values.

  • Don't add behaviour. Mocks are still stubs, and you should avoid adding any extra behavior on it.

  • Only mock your immediate neighbours. Mocking an entire network of objects adds extra complexity to the test. This might be a symptom that the component needs a better design.

  • Avoid too many mocks. After all, mock objects add complexity to the overall test.

The authors also discuss some common misconception when using mocks:

  • Mocks are just stubs. Mock objects indeed act as stubs. However, their main goal (or, what makes it different from "just" stubs) is to assert the interaction of the target object with its neighbours.

  • Mocks should not be used only to the boundaries of the system. Some developers might argue that only classes that are at the boundaries of the system should be mocked (e.g., classes that talk to external systems). Mocks might be even more helpful when used within the system, as they can also drive developers to better class design.

  • Gather state during the test and assert against it afterwards. Making assertions only at the end of the test makes failing tests less easy to be understood. Favour immediate failures.

  • Testing using Mock Objects duplicates the code. This might mean that the code under test isn't doing very much. Perhaps it should be treated as a policy object and tested in a larger cluster of objects.

To Mock or Not To Mock?

A very common (and heated) discussion among practitioners is about whether to use mocks or not. Up to this point, we have discussed many advantages of test doubles. Let us now discuss a possible disadvantage.

Some developers strongly affirm that the use of mocks might lead to test suites that "test the mock, not the code".

That, in fact, can happen. Suppose some class A that depends on class B. Suppose class B offers a method sum() that always returns positive numbers (i.e., the post-condition of sum()). When testing class A, the developer decides to mock B. Everything seems to work. Months later, a developer decides to change the post-conditions of B's sum(): now, it also returns negative numbers. In a common development workflow, a developer would apply these changes in B and update B's tests to reflect such change. It is easy for the developer to forget to check whether A handles this new post-condition well. Even worse: A's test suite will still pass! After all, A mocks B. And the mock does not really know that B changed. Now, imagine this in a large-scale software. It can be really easy to lose control of your mocks in the sense that mocks might not really represent the real contract of the class.

For mock objects to work well, developers have to design careful (and hopefully stable) contracts. Note that, from a design perspective, there is always a lesson to be learned from the situation. Mocks should have some locality. If a contract change affects an unknown range of uses, there is probably a better way to design the system such that this would not happen. Remember that mocks are also a design technique and are there to also give you feedback.

Moreover, note that, although we use the example of a contract break as a disadvantage of mocks, remember that whatever change in contract there is, it has to be part of the job of the coder to find its dependencies, and check that the new contract is covered, irrespective of mocks or not.

Test doubles at Google

The "Software Engineering at Google" book has an entire chapter dedicated to test doubles. In the following, we provide you with a summary:

  • Using test doubles requires the system to be designed for testability. Dependency injection is the common technique to enable test doubles.
  • Building test doubles that are faithful to the real implementation is challenging. Test doubles have to be as faithful as possible.
  • Prefer realism over isolation. When possible, opt for the real implementation, instead of fakes, stubs, or mocks.
  • Some trade-offs to consider when deciding whether to use a test double: the execution time of the real implementation, how much non-determinism we would get from using the real implementation.
  • When using the real implementation is not possible or too costly, prefer fakes over mocks. An in-memory database, for example, might be better (or more real) than a mock.
  • Excessive mocking can be dangerous, as tests become unclear (i.e., hard to comprehend), brittle (i.e., might break too often), and less effective (i.e., reduced fault capability detection).
  • When mocking, prefer state testing rather than interaction testing. In other words, make sure you are asserting a change of state and/or the consequence of the action under test, rather than the precise interaction that the action has with the mocked object. After all, interaction testing tends to be too coupled with the implementation of the system under test.
  • Use interaction testing when state testing is not possible, or when a bad interaction might have an impact in the system (e.g., calling the same method twice would make the system twice as slow).
  • Avoid overspecified interaction tests. Focus on the relevant arguments and functions.
  • Good interaction testing requires strict guidelines when designing the system under test. Google engineers tend not to do it.


Exercise 1. See the following class:

public class OrderDeliveryBatch {

  public void runBatch() {
    OrderBook orderBook = new OrderBook();
    DeliveryStartProcess delivery = new DeliveryStartProcess();


class OrderBook {
  // accesses a database

class DeliveryStartProcess {
  // communicates with a third-party webservice

Which of the following Mockito lines would never appear in a test for the OrderDeliveryBatch class?

  1. OrderBook orderBook = Mockito.mock(OrderBook.class);
  2. Mockito.verify(delivery).start(order); (assume order is an instance of Order)
  3. Mockito.when(orderBook.paidButNotDelivered()).thenReturn(orders); (assume orderBook is an instance of OrderBook and orders is an instance of List<Order>)
  4. OrderDeliveryBatch batch = Mockito.mock(OrderDeliveryBatch.class);

Exercise 2. You are testing a system that triggers advanced events based on complex combinations of Boolean external conditions relating to the weather (outside temperature, amount of rain, wind, ...). The system has been designed cleanly and consists of a set of co-operating classes that each have a single responsibility. You create a decision table for this logic, and decide to test it using mocks. Which is a valid test strategy?

  1. You use mocks to support observing the external conditions.
  2. You create mock objects to represent each variant you need to test.
  3. You use mocks to control the external conditions and to observe the event being triggered.
  4. You use mocks to control the triggered events.

Exercise 3. Below, we show the InvoiceFilter class. This class is responsible for returning the invoices for an amount that is smaller than 100.0. It makes use of the IssuedInvoices type, which is responsible for communication with the database.

public class InvoiceFilter {

    private IssuedInvoices invoices;

    public InvoiceFilter(IssuedInvoices invoices) {
        this.invoices = invoices;

    public List<Invoice> filter() {
      return invoices.all().stream()
              .filter(invoice -> invoice.getValue() < 100.0)

Which of the following statements about this class are false?

  1. Integration testing is the only way to achieve 100% branch coverage.
  2. Its implementation allows for dependency injection, which enables mocking.
  3. It is possible to write completely isolated unit tests for it by, e.g., using mocks.
  4. The IssuedInvoices type (a direct dependency of the InvoiceFilter) itself should be tested by means of integration tests.

Exercise 4. Class A depends on a static method in another class B. If you want to test class A, which of the following two action(s) should you apply to do this properly?

  • Approach 1: Mock class B to control the behavior of the methods in class B.
  • Approach 2: Refactor class A, so the outcome of the method of class B is now used as a parameter.
  1. Only approach 1.
  2. Neither.
  3. Only approach 2.
  4. Both.

Exercise 5. Referring to the previous chapter, apply boundary testing techniques to the InvoiceFilter example discussed in this chapter.


  • Fowler, Martin. Mocks aren't stubs. https://martinfowler.com/articles/mocksArentStubs.html

  • Meszaros, G. (2007). xUnit test patterns: Refactoring test code. Pearson Education.

  • Mockito's website: https://site.mockito.org

  • Lee Houghton: TestDouble: Don't mock types you don't own: https://github.com/testdouble/contributing-tests/wiki/Don%27t-mock-what-you-don%27t-own. Last access on March, 2020.

  • Spadini, Davide, Maurício Aniche, Magiel Bruntink, and Alberto Bacchelli. "Mock objects for testing java systems." Empirical Software Engineering 24, no. 3 (2019): 1461-1498.

  • Freeman, S., & Pryce, N. (2009). Growing object-oriented software, guided by tests. Pearson Education.

  • Winters, T., Manshreck, T., Wright, H. Software Engineering at Google: Lessons Learned from Programming Over Time. O'Reilly, 2020. Chapter 13, "Test doubles".

  • Wirfs-Brock, R. & McKean, A., "Object Design", Addison-Wesley

  • Ingalls, D. "The Design Principles behind Smalltalk", http://www.cs.virginia.edu/~evans/cs655/readings/smalltalk.html

  • Freeman, S., Mackinnon, T., Pryce, N., & Walnes, J. (2004, October). Mock roles, not objects. In Companion to the 19th annual ACM SIGPLAN conference on Object-oriented programming systems, languages, and applications (pp. 236-246).

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