Writing better tests with assertThat

by Christoph on 18.03.2015

To me writing tests and developing software are not two different things. They belong together and best are part of one easy and quick process. It can make life much more pleasant to have well-written tests in your project. They usually increase my joy and productivity at work.

However they do not write and maintain themselves. To avoid spending hours in fixing (and reading) test-code and let tests become great obstacles during development, they need to be written with care. I want to be honest: Tests I write are hardly as accurate as the productive code but still I consider them far from being hard-to-read copy-pasted spaghetti code. You know what I mean.

Here I want to show one approach I use to validate complex objects and object graphs without messing up my test code.

  • If you are interested in more background of hamcrest (which this article is based on), check out the tutorial and their Github page
  • the code of this article is available on Github 

Validating primitive variables

Like other test frameworks jUnit offers a handful of assertions to validate some actual data against the programmers assertions.

I like to use them since they

  • make tests more easy to read and
  • produce failure message which help narrow down the problem.
assertEquals "incorrect nickname", 'alice', 'alice-wrong'
           // we get:
           // incorrect nickname expected:<alice[]> but was:<alice[-wrong]>

In this example comparing the two strings works just fine. The error message even emphasizes the difference - just great.

Now I want this to work just as good for much more complex data structures with nested objects as well.

Validating complex objects

One way to get there is to use assertThat. I want to show how and why in the follow example.

Assume you have a simple entity Person.

class Person {
           String nickname
           String name

Lazy approach

assertEquals expectedPerson, actualPerson

This might actually work depending on the implementation of equals() and hashCode() but in our example it would compare the persons by object reference.

Even if the content would be validated the way I want in this test, the failure message would only contain the toString() results

  • possibly only the nickname.

Eager approach

assertEquals "incorrect nickname", expected.nickname, actual.nickname
           assertEquals "incorrect name", expected.name, actual.name

This is slightly better than the approach before. It compares the persons by content and the failure messages are really helpful.

Things get more complex

For now the eager approach may just work out. One could even put the two lines into an extra method assertPerson.

That is why I want to make things a bit more tricky by adding a list of children to my person.

class Person { String nickname String name List<Person> children = [] }

Eager approach including children

Now my eager approach starts to loose attractiveness.

assertEquals "incorrect nickname", expected.nickname, actual.nickname
           assertEquals "incorrect name", expected.name, actual.name
           assertEquals "incorrect number of children", expected.children.size(), actual.children.size()
           for (def actualFriend in actual.children) {
           def expectedFriend = expected.children.find { it.nickname == actualFriend.nickname }
           assertNotNull "unexpected friend '$actualFriend.nickname'", expectedFriend

           assertEquals "incorrect name", actualFriend.name, expectedFriend.name
           // TODO: compare the children of this child as well

Just to understanding this I need more time than to understand the Person and it is most definitely something I do not want to spread throughout all my person-validating tests. Since the order of the children does not matter to me the validation becomes even more tedious.

I do not consider this validating code nice any more. It is error-prone and delivers not helpful failure message and it does not even test the content of grand-children.

Hamcrest matcher approach

Using Hamcrest matchers can give the test code a much better shape. This example contains two Person matchers.

Simple person matcher

The PersonMatchers.PersonMatcher contains a straight forward implementation of a matcher validating a Person. The matcher does already validate all children, grand-children, great-grand-children and so on. It does not support reference cycles though.

With the comparison code hidden in the PersonMatcher the test code looks very neat.

assertThat "unexpected Alice", actual, isPersonLike(expected)
           // in case of a failure we get
           // unexpected Alice
           // Expected: <alice>
           //     but: was <alice>

I can very well use isPersonLike in several test classes avoiding copy-paste code.

This is already very cool except for the failure message.

Better person matcher

As for strings I want to see exactly why the test fails. I expect the incorrect value somewhere in the actual Person emphasized which causes my test to fail. So to say I want to see the path through all children and grand-children to the property with the wrong content.

assertThat "unexpected Alice", actual, isPersonLike(expected)
           // in case of error we get
           // MisMatchException: incorrect child: expected little_bob but is little_bob
           // cause:   incorrect child: expected tiny_dave but is tiny_dave
           // cause:   name incorrect: expected 'Dave' but is 'David'

The matcher PersonMatchers.BetterPersonMatcher contains an implementation using a special MismatchException to generate the failure message.

This kind of test code and failure message gives me a nice feeling. I think that if one of my fellow team members needs to fix broken tests or write some new ones, she or he starts right away without the need to decrypt long failure messages and tests. More time for features and more fun for us!

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