I've read various articles about mocking vs stubbing in testing, including Martin Fowler's Mocks Aren't Stubs, but still don't understand the difference.
Foreword
There are several definitions of objects, that are not real. The general term is test double. This term encompasses: dummy, fake, stub, mock.
Reference
According to Martin Fowler's article:
Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example).
Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
Style
Mocks vs Stubs = Behavioral testing vs State testing
Principle
According to the principle of Test only one thing per test, there may be several stubs in one test, but generally there is only one mock.
Lifecycle
Test lifecycle with stubs:
Setup - Prepare object that is being tested and its stubs collaborators.
Exercise - Test the functionality.
Verify state - Use asserts to check object's state.
Teardown - Clean up resources.
Test lifecycle with mocks:
Setup data - Prepare object that is being tested.
Setup expectations - Prepare expectations in mock that is being used by primary object.
Exercise - Test the functionality.
Verify expectations - Verify that correct methods has been invoked in mock.
Verify state - Use asserts to check object's state.
Teardown - Clean up resources.
Summary
Both mocks and stubs testing give an answer for the question: What is the result?
Testing with mocks are also interested in: How the result has been achieved?
Stub
I believe the biggest distinction is that a stub you have already written with predetermined behavior. So you would have a class that implements the dependency (abstract class or interface most likely) you are faking for testing purposes and the methods would just be stubbed out with set responses. They would not do anything fancy and you would have already written the stubbed code for it outside of your test.
Mock
A mock is something that as part of your test you have to setup with your expectations. A mock is not setup in a predetermined way so you have code that does it in your test. Mocks in a way are determined at runtime since the code that sets the expectations has to run before they do anything.
Difference between Mocks and Stubs
Tests written with mocks usually follow an initialize -> set expectations -> exercise -> verify pattern to testing. While the pre-written stub would follow an initialize -> exercise -> verify.
Similarity between Mocks and Stubs
The purpose of both is to eliminate testing all the dependencies of a class or function so your tests are more focused and simpler in what they are trying to prove.
A stub is a simple fake object. It just makes sure test runs smoothly.
A mock is a smarter stub. You verify your test passes through it.
Here's a description of each one followed by with real world sample.
Dummy - just bogus values to satisfy the API.
Example: If you're testing a method of a class which requires many mandatory parameters in a constructor which have no effect on your test, then you may create dummy objects for the purpose of creating new instances of a class.
Fake - create a test implementation of a class which may have a dependency on some external infrastructure. (It's good practice that your unit test does NOT actually interact with external infrastructure.)
Example: Create fake implementation for accessing a database, replace it with in-memory collection.
Stub - override methods to return hard-coded values, also referred to as state-based.
Example: Your test class depends on a method Calculate() taking 5 minutes to complete. Rather than wait for 5 minutes you can replace its real implementation with stub that returns hard-coded values; taking only a small fraction of the time.
Mock - very similar to Stub but interaction-based rather than state-based. This means you don't expect from Mock to return some value, but to assume that specific order of method calls are made.
Example: You're testing a user registration class. After calling Save, it should call SendConfirmationEmail.
Stubs and Mocks are actually sub types of Mock, both swap real implementation with test implementation, but for different, specific reasons.
In the codeschool.com course, Rails Testing for Zombies, they give this definition of the terms:
Stub
For replacing a method with code that returns a specified result.
Mock
A stub with an assertion that the method gets called.
So as Sean Copenhaver described in his answer, the difference is that mocks set expectations (i.e. make assertions, about whether or how they get called).
Stubs don't fail your tests, mock can.
Reading all the explanations above, let me try to condense:
Stub: a dummy piece of code that lets the test run, but you don't care what happens to it. Substitutes for real working code.
Mock: a dummy piece of code that you verify is called correctly as part of the test. Substitutes for real working code.
Spy: a dummy piece of code that intercepts and verifies some calls to real working code, avoiding the need to substitute all the real code.
I think the simplest and clearer answer about this question is given from Roy Osherove in his book The art of Unit Testing (page 85)
The easiest way to tell we’re dealing with a stub is to notice that the stub can never fail the test. The asserts the test uses are always against
the class under test.
On the other hand, the test will use a mock object to verify whether the
test failed or not. [...]
Again, the mock object is the object we use to see if the test failed or not.
Stub and mock are both fakes.
If you are making assertions against the fake it means you are using the fake as a mock, if you are using the fake only to run the test without assertion over it you are using the fake as a stub.
A Mock is just testing behaviour, making sure certain methods are called.
A Stub is a testable version (per se) of a particular object.
What do you mean an Apple way?
If you compare it to debugging:
Stub is like making sure a method returns the correct value
Mock is like actually stepping into the method and making sure everything inside is correct before returning the correct value.
This slide explain the main differences very good.
*From CSE 403 Lecture 16 , University of Washington (slide created by "Marty Stepp")
To be very clear and practical:
Stub: A class or object that implements the methods of the class/object to be faked and returns always what you want.
Example in JavaScript:
var Stub = {
method_a: function(param_a, param_b){
return 'This is an static result';
}
}
Mock: The same of stub, but it adds some logic that "verifies" when a method is called so you can be sure some implementation is calling that method.
As #mLevan says imagine as an example that you're testing a user registration class. After calling Save, it should call SendConfirmationEmail.
A very stupid code Example:
var Mock = {
calls: {
method_a: 0
}
method_a: function(param_a, param_b){
this.method_a++;
console.log('Mock.method_a its been called!');
}
}
let see Test Doubles:
Fake: Fakes are objects that have working implementations, but not the same as production one. Such as: in-memory implementation of Data Access Object or Repository.
Stub: Stub is an object that holds predefined data and uses it to answer calls during tests. Such as: an object that needs to grab some data from the database to respond to a method call.
Mocks: Mocks are objects that register calls they receive.
In test assertion, we can verify on Mocks that all expected actions were performed. Such as: a functionality that calls e-mail sending service.
for more just check this.
Using a mental model really helped me understand this, rather than all of the explanations and articles, that didn't quite "sink in".
Imagine your kid has a glass plate on the table and he starts playing with it. Now, you're afraid it will break. So, you give him a plastic plate instead. That would be a Mock (same behavior, same interface, "softer" implementation).
Now, say you don't have the plastic replacement, so you explain "If you continue playing with it, it will break!". That's a Stub, you provided a predefined state in advance.
A Dummy would be the fork he didn't even use... and a Spy could be something like providing the same explanation you already used that worked.
I think the most important difference between them is their intentions.
Let me try to explain it in WHY stub vs. WHY mock
Suppose I'm writing test code for my mac twitter client's public timeline controller
Here is test sample code
twitter_api.stub(:public_timeline).and_return(public_timeline_array)
client_ui.should_receive(:insert_timeline_above).with(public_timeline_array)
controller.refresh_public_timeline
STUB: The network connection to twitter API is very slow, which make my test slow. I know it will return timelines, so I made a stub simulating HTTP twitter API, so that my test will run it very fast, and I can running the test even I'm offline.
MOCK: I haven't written any of my UI methods yet, and I'm not sure what methods I need to write for my ui object. I hope to know how my controller will collaborate with my ui object by writing the test code.
By writing mock, you discover the objects collaboration relationship by verifying the expectation are met, while stub only simulate the object's behavior.
I suggest to read this article if you're trying to know more about mocks: http://jmock.org/oopsla2004.pdf
I like the explanantion put out by Roy Osherove [video link].
Every class or object created is a Fake. It is a Mock if you verify
calls against it. Otherwise its a stub.
Stub
A stub is an object used to fake a method that has pre-programmed behavior. You may want to use this instead of an existing method in order to avoid unwanted side-effects (e.g. a stub could make a fake fetch call that returns a pre-programmed response without actually making a request to a server).
Mock
A mock is an object used to fake a method that has pre-programmed behavior as well as pre-programmed expectations. If these expectations are not met then the mock will cause the test to fail (e.g. a mock could make a fake fetch call that returns a pre-programmed response without actually making a request to a server which would expect e.g. the first argument to be "http://localhost:3008/" otherwise the test would fail.)
Difference
Unlike mocks, stubs do not have pre-programmed expectations that could fail your test.
Stubs vs. Mocks
Stubs
provide specific answers to methods calls
ex: myStubbedService.getValues() just return a String needed by the code under test
used by code under test to isolate it
cannot fail test
ex: myStubbedService.getValues() just returns the stubbed value
often implement abstract methods
Mocks
"superset" of stubs; can assert that certain methods are called
ex: verify that myMockedService.getValues() is called only once
used to test behaviour of code under test
can fail test
ex: verify that myMockedService.getValues() was called once; verification fails, because myMockedService.getValues() was not called by my tested code
often mocks interfaces
I was reading The Art of Unit Testing, and stumbled upon the following definition:
A fake is a generic term that can be used to describe either a stub or a mock object (handwritten or otherwise), because they both look like the real object. Whether a fake is a stub or a mock depends on how it's used in the current test. if it's used to check an interaction (asserted against), it's a mock object. Otherwise, it's a stub.
The generic term he uses is a Test Double (think stunt double). Test Double is a generic term for any case where you replace a production object for testing purposes. There are various kinds of double that Gerard lists:
Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an InMemoryTestDatabase is a good example).
Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test.
Spies are stubs that also record some information based on how they were called. One form of this might be an email service that records how many messages it was sent(also called Partial Mock).
Mocks are pre-programmed with expectations which form a specification of the calls they are expected to receive. They can throw an exception if they receive a call they don't expect and are checked during verification to ensure they got all the calls they were expecting.
Source
A fake is a generic term that can be used to describe either a stub
or a mock object (handwritten or otherwise), because they both look like the
real object. Whether a fake is a stub or a mock depends on how it’s used in
the current test. If it’s used to check an interaction (asserted against), it’s a
mock object. Otherwise, it’s a stub.
Fakes makes sure test runs smoothly. It means that reader of your future test will understand what will be the behavior of the fake object, without needing to read its source code (without needing to depend on external resource).
What does test run smoothly mean?
Forexample in below code:
public void Analyze(string filename)
{
if(filename.Length<8)
{
try
{
errorService.LogError("long file entered named:" + filename);
}
catch (Exception e)
{
mailService.SendEMail("admin#hotmail.com", "ErrorOnWebService", "someerror");
}
}
}
You want to test mailService.SendEMail() method, to do that you need to simulate an Exception in you test method, so you just need to create a Fake Stub errorService class to simulate that result, then your test code will be able to test mailService.SendEMail() method. As you see you need to simulate a result which is from an another External Dependency ErrorService class.
Mocks: help to emulate and examine outcoming interactions. These interactions
are calls the SUT makes to its dependencies to change their state.
Stubs: help to emulate incoming interactions. These interactions are calls the
SUT makes to its dependencies to get input data.
source : Unit Testing Principles, Practices, and Patterns - Manning
Right from the paper Mock Roles, not Objects, by the developers of jMock :
Stubs are dummy implementations of production code that return canned
results. Mock Objects act as stubs, but also include assertions to
instrument the interactions of the target object with its neighbours.
So, the main differences are:
expectations set on stubs are usually generic, while expectations set on mocks can be more "clever" (e.g. return this on the first call, this on the second etc.).
stubs are mainly used to setup indirect inputs of the SUT, while mocks can be used to test both indirect inputs and indirect outputs of the SUT.
To sum up, while also trying to disperse the confusion from Fowler's article title: mocks are stubs, but they are not only stubs.
I came across this interesting article by UncleBob The Little Mocker. It explains all the terminology in a very easy to understand manner, so its useful for beginners. Martin Fowlers article is a hard read especially for beginners like me.
a lot of valid answers up there but I think worth to mention this form uncle bob:
https://8thlight.com/blog/uncle-bob/2014/05/14/TheLittleMocker.html
the best explanation ever with examples!
A mock is both a technical and a functional object.
The mock is technical. It is indeed created by a mocking library (EasyMock, JMockit and more recently Mockito are known for these) thanks to byte code generation.
The mock implementation is generated in a way where we could instrument it to return a specific value when a method is invoked but also some other things such as verifying that a mock method was invoked with some specific parameters (strict check) or whatever the parameters (no strict check).
Instantiating a mock :
#Mock Foo fooMock
Recording a behavior :
when(fooMock.hello()).thenReturn("hello you!");
Verifying an invocation :
verify(fooMock).hello()
These are clearly not the natural way to instantiate/override the Foo class/behavior. That's why I refer to a technical aspect.
But the mock is also functional because it is an instance of the class we need to isolate from the SUT. And with recorded behaviors on it, we could use it in the SUT in the same way than we would do with a stub.
The stub is just a functional object : that is an instance of the class we need to isolate from the SUT and that's all.
That means that both the stub class and all behaviors fixtures needed during our unit tests have to be defined explicitly.
For example to stub hello() would need to subclass the Foo class (or implements its interface it has it) and to override hello() :
public class HelloStub extends Hello{
public String hello {
return "hello you!";
}
}
If another test scenario requires another value return, we would probably need to define a generic way to set the return :
public class HelloStub extends Hello{
public HelloStub(String helloReturn){
this.helloReturn = helloReturn;
}
public String hello {
return helloReturn;
}
}
Other scenario : if I had a side effect method (no return) and I would check that that method was invoked, I should probably have added a boolean or a counter in the stub class to count how many times the method was invoked.
Conclusion
The stub requires often much overhead/code to write for your unit test. What mock prevents thanks to providing recording/verifying features out of the box.
That's why nowadays, the stub approach is rarely used in practice with the advent of excellent mock libraries.
About the Martin Fowler Article : I don't think to be a "mockist" programmer while I use mocks and I avoid stubs.
But I use mock when it is really required (annoying dependencies) and I favor test slicing and mini-integration tests when I test a class with dependencies which mocking would be an overhead.
Plus useful answers, One of the most powerful point of using Mocks than Subs
If the collaborator [which the main code depend on it] is not under our control (e.g. from a third-party library),
In this case, stub is more difficult to write rather than mock.
Stub
A stub is an object that holds predefined data and uses it to answer calls during tests. It is used when you can’t or don’t want to involve objects that would answer with real data or have undesirable side effects.
An example can be an object that needs to grab some data from the database to respond to a method call. Instead of the real object, we introduced a stub and defined what data should be returned.
example of Stub:
public class GradesService {
private final Gradebook gradebook;
public GradesService(Gradebook gradebook) {
this.gradebook = gradebook;
}
Double averageGrades(Student student) {
return average(gradebook.gradesFor(student));
}
}
Instead of calling database from Gradebook store to get real students grades, you preconfigure stub with grades that will be returned. You define just enough data to test average calculation algorithm.
public class GradesServiceTest {
private Student student;
private Gradebook gradebook;
#Before
public void setUp() throws Exception {
gradebook = mock(Gradebook.class);
student = new Student();
}
#Test
public void calculates_grades_average_for_student() {
//stubbing gradebook
when(gradebook.gradesFor(student)).thenReturn(grades(8, 6, 10));
double averageGrades = new GradesService(gradebook).averageGrades(student);
assertThat(averageGrades).isEqualTo(8.0);
}
}
Mock
Mocks are objects that register calls they receive. In test assertion you can verify on Mocks that all expected actions were performed. You use mocks when you don’t want to invoke production code or when there is no easy way to verify, that intended code was executed. There is no return value and no easy way to check system state change. An example can be a functionality that calls e-mail sending service.
You don’t want to send e-mails each time you run a test. Moreover, it is not easy to verify in tests that a right email was send. Only thing you can do is to verify the outputs of the functionality that is exercised in our test. In other worlds, verify that the e-mail sending service was called.
Example of Mock:
public class SecurityCentral {
private final Window window;
private final Door door;
public SecurityCentral(Window window, Door door) {
this.window = window;
this.door = door;
}
void securityOn() {
window.close();
door.close();
}
}
You don’t want to close real doors to test that security method is working, right? Instead, you place door and window mocks objects in the test code.
public class SecurityCentralTest {
Window windowMock = mock(Window.class);
Door doorMock = mock(Door.class);
#Test
public void enabling_security_locks_windows_and_doors() {
SecurityCentral securityCentral = new SecurityCentral(windowMock, doorMock);
securityCentral.securityOn();
verify(doorMock).close();
verify(windowMock).close();
}
}
Thanks a lot to Michał Lipski for his good article. For further reading:
Test Double – Martin Fowler https://martinfowler.com/bliki/TestDouble.html
Test Double – xUnit Patterns http://xunitpatterns.com/Test%20Double.html
Mocks Aren’t Stubs – Martin Fowler https://martinfowler.com/articles/mocksArentStubs.html
Command Query Separation – Martin Fowler https://martinfowler.com/bliki/CommandQuerySeparation.html
Stub helps us to run test. How? It gives values which helps to run test. These values are itself not real and we created these values just to run the test. For example we create a HashMap to give us values which are similar to values in database table. So instead of directly interacting with database we interact with Hashmap.
Mock is an fake object which runs the test. where we put assert.
See below example of mocks vs stubs using C# and Moq framework. Moq doesn't have a special keyword for Stub but you can use Mock object to create stubs too.
namespace UnitTestProject2
{
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;
[TestClass]
public class UnitTest1
{
/// <summary>
/// Test using Mock to Verify that GetNameWithPrefix method calls Repository GetName method "once" when Id is greater than Zero
/// </summary>
[TestMethod]
public void GetNameWithPrefix_IdIsTwelve_GetNameCalledOnce()
{
// Arrange
var mockEntityRepository = new Mock<IEntityRepository>();
mockEntityRepository.Setup(m => m.GetName(It.IsAny<int>()));
var entity = new EntityClass(mockEntityRepository.Object);
// Act
var name = entity.GetNameWithPrefix(12);
// Assert
mockEntityRepository.Verify(m => m.GetName(It.IsAny<int>()), Times.Once);
}
/// <summary>
/// Test using Mock to Verify that GetNameWithPrefix method doesn't call Repository GetName method when Id is Zero
/// </summary>
[TestMethod]
public void GetNameWithPrefix_IdIsZero_GetNameNeverCalled()
{
// Arrange
var mockEntityRepository = new Mock<IEntityRepository>();
mockEntityRepository.Setup(m => m.GetName(It.IsAny<int>()));
var entity = new EntityClass(mockEntityRepository.Object);
// Act
var name = entity.GetNameWithPrefix(0);
// Assert
mockEntityRepository.Verify(m => m.GetName(It.IsAny<int>()), Times.Never);
}
/// <summary>
/// Test using Stub to Verify that GetNameWithPrefix method returns Name with a Prefix
/// </summary>
[TestMethod]
public void GetNameWithPrefix_IdIsTwelve_ReturnsNameWithPrefix()
{
// Arrange
var stubEntityRepository = new Mock<IEntityRepository>();
stubEntityRepository.Setup(m => m.GetName(It.IsAny<int>()))
.Returns("Stub");
const string EXPECTED_NAME_WITH_PREFIX = "Mr. Stub";
var entity = new EntityClass(stubEntityRepository.Object);
// Act
var name = entity.GetNameWithPrefix(12);
// Assert
Assert.AreEqual(EXPECTED_NAME_WITH_PREFIX, name);
}
}
public class EntityClass
{
private IEntityRepository _entityRepository;
public EntityClass(IEntityRepository entityRepository)
{
this._entityRepository = entityRepository;
}
public string Name { get; set; }
public string GetNameWithPrefix(int id)
{
string name = string.Empty;
if (id > 0)
{
name = this._entityRepository.GetName(id);
}
return "Mr. " + name;
}
}
public interface IEntityRepository
{
string GetName(int id);
}
public class EntityRepository:IEntityRepository
{
public string GetName(int id)
{
// Code to connect to DB and get name based on Id
return "NameFromDb";
}
}
}
Related
I'm trying to figure out whether it's an API design flaw, it is actually OK, or the SRP is being violated.
I have 2 public methods initialize() and onListRefresh(). Both of them call the same private method updateList(). The only difference between both of them is that initialize() also check for a null argument to throw an exception.
The issue is that in order to test both public methods, I practically have to copy and paste the same mocks, stub, expectations and assertions, which are all for what happens on the private method, and it feels wrong. So which one is it:
Is there a flaw in the public API design?
It's all right, that's how it's supposed to be.
SRP is being violated by using initialize() to do both checking for an argument AND calling updateList()
I'd go with 2, and stick with the parallel tests. But here's something that may be calling to you from the test code: Extract helper methods.
This can happen anywhere in the Arrange, Act, Assert phases of the tests. You may extract helpers in all 3 phases. The trick is good names, so that the tests express, simply and legibly, what they are there for.
Most of what I've read about mocks, stubs (test doubles) involves some form of injection of the DOC either through the SUT method itself or constructor or setter methods. And injecting that breaks boundaries like InjectMock are frowned upon as a regular test strategy. But what if you are building a class that you do not want to expose those DOCs? Is there a way to 'unit' test such a module? Without AOP? Is such a test not a real 'unit' test anymore? Is the resistance I'm feeling really design smell and I should expose those DOCs somehow?
For example, lets say I have the following Class that I want to test (unit or otherwise):
public class RemoteRepository {
Properties props = null;
public RemoteRepository(Properties props) { this.props=props; }
public Item export (String itemName) {
JSch ssh = new JSch();
ssh.setIdentity(props.get("keyfile"));
ssh.connect();
ssh.execute("export "+itemName+" "+props.get("exportFilename"));
...
}
Here is a unit I'd like to write a unit test for, but I want to stub or mock out the JSch component. But the objects I create in the method to just do things that the method needs to accomplish are not exposed outside the method even. So I cannot inject a stub to replace them. I could change the export method signature to accept the stub, or add a constructor that does, but that changes my design just to suit a test.
Although the unit will connect to a real server to do the export in prod, when just testing the unit I either want to stub the DOC out completely, or simulate it with a real DOC that is simple and controlled.
This latter approach is like using an in memory db instead of a real one in that it acts and behaves like the eventual db that will be used, but can be confined to just what is needed for the test (eg. just the tables of interest, no heavy security, etc). So I could setup some kind of test double sshd in my test so that when the build runs the test, it has something to test against. This can be a lot of trouble to setup and maintain however and seems like overkill - sometimes trying to stub out a real DOC is harder than just using the real DOC somehow.
Am I stuck trying to setup a test framework that provides an sshd test double? Am I looking at this the wrong way? Do I just use AOP or mock library methods that break the class scope boundaries?
To restate the basic problem is that a lot of times I want to test a method that has complex DOCs (ie. those that interact with other systems: network, db, etc) and I don't want to change the design just to accommodate test double DOC injection. How do you approach testing in such a scenario?
My recommendation, based on personal experience, is to write integration tests where DOCs (Depended On Components) are not mocked.
However, if for whatever reason the teams insists on having unit tests instead, you would have to either use a suitable mocking tool (AOP tools are able, but not a good fit here), or change the design of SUT and DOCs in order to use "weaker" mocking tools.
I want to test my controller that depends on a hardware C# class, not an interface.
It's configured as a singleton and I just can't figure out how to RhinoMock it.
The hardware metadata (example) for the dependent class:
namespace Hardware.Client.Api
{
public class CHardwareManager
{
public static CHardwareManager GetInstance();
public string Connect(string clientId);
}
}
and in my code I want this something like this to return true, else I get an exception
if( !CHardwareManager.GetInstance().Connect("foo") )
I mock it using:
CHardwareManager mockHardwareMgr MockRepository.GenerateMock<CHardwareManager>();
But the Connect needs a GetInstance and the only combination I can get to "compile" is
mockHardwareMgr.Expect (x => x.Connected ).Return(true).Repeat.Any();
but it doesn't correctly mock, it throws an exception
but this complains about typing the GetInstance
mockHardwareMgr.Expect (x => x.GetInstance().Connected).Return(true).Repeat.Any();
So my problem - I think - is mocking a singleton. Then I have no idea how to make my controller use this mock since I don't pass the mock into the controller. It's a resource and namespace.
90% of my work requires external components I need to mock, most times I don't write the classes or interfaces, and I'm struggling to get them mocked and my code tested.
Any pointers would be welcome.
Thanks in advance (yes, I've been searching through SO and have not seen something like this. But then, maybe my search was not good.
The usual way to avoid problems with mocking external components is not to use them directly in your code. Instead, define an anti-corruption layer (usually through an interface that looks like your external component) and test your code using mocked implementation of this interface. After all, you're testing your own code, not the external one.
Even better way is to adjust this interface to your needs so it only exposes stuff that you actually need, not the whole API the external component provides (so it's actually an Adapter pattern).
External components are tested using different approaches: system testing, in which case you don't really mock them, you use the actual implementation.
Usually when you try to get Rhino Mocks to do something which feels unnatural and Rhino growls, this is a good sign that your approach is not the right one. Almost everything can be done using simple interface mocking.
As Igor said RhinoMocks (and most other free mocking frameworks, e.g. Moq) can only mock interfaces.
For mocking classes try (and pay) TypeMock.
For mocking singletons see my answer to:
How to Mock a Static Singleton?
Yes, I'm somewhat undermining the common understanding of what's deemed testable and thus "good" code. However I'm starting to resent answers like "You're doing it wrong. Make everything anew." for those answers don't solve the problem at hand.
No, this is not pointing at Igor, but at many others in similar threads, who answered "Singletons are unmockable. (Make everything anew.)".
I have been learning about TDD (using JUnit) and I have a doubt about how to go about testing void methods, in which case I can't directly use something like an assertTrue() on the return value of a method.. For example, say I have a simple console based application, and a part of it prints a menu on screen, say using this method:
public void printMenu()
{
System.out.println("Menu:");
System.out.println("1. Option ONE");
System.out.println("2. Option TWO");
System.out.println("3. Exit");
}
My question is, do I actually have to test this method?? And if so, how should I do it?
It is difficult to unit test a method which relies on static method calls. It is not a matter of returning something or void. What you could do is abstract the printing into an interface and have your class depend on this interface (using constructor injection for example):
private SomePrinterInterface _printer;
public void printMenu()
{
_printer.println("Menu:");
_printer.println("1. Option ONE");
_printer.println("2. Option TWO");
_printer.println("3. Exit");
}
In your unit test you could mock the interface and verify if correct methods have been called on it. This way you could test the printMenu in independently.
First: testing UI is hard. Some people don't bother testing things like this because it is very difficult to write meaningful tests that aren't fragile to the point of uselessness. I wouldn't bother testing this method.
But:
If you want to test menu generation, because your menu code is complex and you need ways to ensure that it works, you have a couple choices.
Refactor your method so that it accepts the output stream as a parameter, then pass in an output stream whose contents you can inspect. You might also be able to redirect System.out to achieve this.
Refactor your method so that the menu is generated as a bunch of objects, then printed separately. You can inspect and verify those objects.
capture the console output and compare to expectations
You can not unit test this method.
This method does no logic or processing that you need to unit test.
If you need to unit test the Print Menu, you can consider outputing the result to a text file.
Then read the text file and compare the menu texts..
I decided to add unit tests to existing project (quite big one).
I am using "google toolbox for mac" for various type of STAssert... and OCMock framework.
But I think I'm testing wrong. For example, I have public function saveData, which doesn't return anything and only change internal state of the object. Should I test it? Due to encapsulation principle - I don't have to worry much about object implementation and I mustn't depend on private variables (because they can change/be deleted in the future)
#implementation Foo
-(void) saveData {
internalData_ = 88;
}
In real project this function saveData is 100 lines long and it's change a lot of private variables of the class.
So, should I test it or not? I have a little previous experience in unit testing and cannot make decision by my own.
Does the internal state that gets changed affect any later calls on that object? If so, you should include it in a unit test like
Test a()
Do saveData()
Test a() again
Even if not, it might be a good idea to unit test it. Not for determining whether other code will break by using this method, but for automatically testing the correct implementation of the method. Even though the method doesn't return anything, it probably still has some kind of contract ("If I call it, this must happen") and you should check if what should've happened, happened (e.g. a line added in a log file, or something).
Now, how to check that if the method doesn't return anything, is another question entirely. Ironically enough, that's an implementation detail of the unit test.