Good and Bad Tests

16. January, 2017

How do you distinguish good from bad tests in your code?

Check these criteria. Good tests

  • Nail down expectations
  • Monitor assumptions
  • Help to locate the cause of a failure
  • Document usage patterns
  • Allow to change code
  • Allow to verify changes
  • Are short (LOC + time)

Bad tests

  • Waste development time
  • Execute many, many lines of code
  • Prevent code changes
  • Need more time to write than the code they test
  • Need a lot of code to set up
  • Take ages to execute
  • Are hard to run


There are a lot of checks in your compiler. Those help to catch mistakes you make. Do the same with your tests. There are a lot of things that compilers don’t check: File encodings, existence of files, existence of config options, types of config options.

Use tests to nail down your expectations. Read config files and validate the odd option.

Create a test which collects the whole configuration of your program and checks it against a known state. Check that each config option is set only once (or at least that it has the same value in all places).

When you need to translate your texts, add tests which make sure that you have all the texts that you need, that texts are unique, etc.


Convention over configuration only works when everyone agrees what the convention is. Conventions are assumptions. Your brain has to know them since they are no longer in the code. If this approach fails for you, write a test that validates your assumptions.

Check that code throws the exceptions that you expect.

If you have found a bug in a framework and added a workaround, add a test which fails when the bug is fixed. Add a comment “If this test fails, you can remove the workaround.”


The world speeds up. No one can afford slow tests, tests that are hard to understand, hard to maintain, tests that get in the way of Get-Things-Done™. Make sure you can run your tests at the touch of a button. Make sure they never fail unless they should. Make sure they fail when they should. Make sure they are small (= execute fast, few lines to understand, little code to write, easy to change, cheap to delete). Ten tests, each asserting a single fact, are better than one test that asserts ten facts. If your tests run for more than ten seconds, you lose.


There is code rot. But long before that, there is documentation rot. Who has time to update the comments and documentation after a code change?

Why not document code usage in tests? Tests tell you the Truth™. Why give someone 100 pages of words they can’t trust when you can give them 100 unit tests they can execute?


Make your life easier. Stop wasting time in your debugger, begging for production log files, running code in your head. Write a good test today, it will watch your back for as long as the project lives. Write a thousand good tests and they will be like an army of angels, warding you from suffering, every time you press that button.

TNBT – Creating Tests from the Debugger

20. July, 2015

From my series “The Next Best Thing“:

Often, you will find yourself in a debugger, trying to follow some insanely complicated code to find the root cause of a bug.

I would like to see a button in my IDE which reads “New Test Case”. It should example the current program state, determine (with my help) what part of the code I want to test and then copy the current state into a unit test. In a second step, I could then trim down the unit tests but I would have all the input, all necessary dependencies would be there, correctly initialized.

It would also be great if the IDE would track the state of the code from which the unit test was generated. If the code changes too much (indicating that the unit test might become outdated), I’d like to see that. Or maybe the IDE could figure out by itself when code tested in such a way deviates “too much.”

Along the same lines, the IDE should be able to inject probes into the product code. As I click buttons and enter data in the UI, the IDE should generate a series of unit tests under the hood as described here. If you’re using frameworks like Spring, the tests should come with minimal (or mocked) dependencies.

Replacing Integration With Unit Tests

15. July, 2015

Google asks to “Just Say No to More End-to-End Tests” – just go and read it.

The suggestion in the document is to have a testing pyramid. A few (slow, expensive, dangerous) End-to-End (E2E) tests, more integration tests (IT) and a whole lot of unit tests.

My approach is to aim for 100% unit tests by breaking down the E2E and integration tests into chains of unit tests.

Example: You want to test whether you can save a UI field in a database.

The naive approach would be to create the UI, create a DB, simulate user input, click the save button, check the data in the database, update the UI afterwards.

My approach is to cut this into individual steps and use the output of test X as input for X+1. For unit tests, output is always a constant, so this doesn’t create dependencies between the tests except for the fact that some are the logical next steps. Tests can still be executed in any order.

But back to the example. The first set of tests would put various values in the UI, simulating user input. The tests would just read the values from the fields and compare them against expected values.

The next set of tests would be for the input validation. These tests would reuse some of the “expected output” values from the previous tests. No UI would be necessary (the code to test the display of validation messages would be in another set). We’re only interested in “do we get the correct error for input value FOO” where FOO is a constant.

The input validation tests can be grouped into two sets: One where the input value passes the validation and another (probably a much bigger) where the validation fails.

For all the inputs where the validation succeeds, we write a test that writes this value (again taken from a constant) into the database.

Lastly, to update the UI, we write a set of tests which examine the state of the database. And another set which tests that a certain state appears correctly in the UI.

As you can see, at no time we have a runtime dependency of tests. Tests work in any order. If the output of one test changes, only a small number of tests need to be updated (if at all). The IDE will help you locate all tests which are connected.

Instead of using the constant directly in an assert inside of a test, you can refer to it from an annotation. Now you can write tests that make sure that an output value is also used as an input somewhere.

If a test fails, only a single test fails. If the validation changes and a value isn’t valid anymore, the DB test might still pass. This is good: We don’t want code to fail unless it’s really broken. The DB interface might be able to write “illegal” values into the database but the validation layer is responsible to catch them. Don’t mix responsibilities.

In E2E or integration tests, a single failure somewhere in the whole chain of operations breaks the whole test. With the approach outlined above, a single failure just breaks very few tests. Each test still runs only a small number of code lines, making it easier to locate the cause of the problem. Tests are fast, you can run tests for a subset of the chain – why test the database when you’re improving validation? Or start the whole UI? You can run fast tests all the time and mark slow ones (UI, DB) for “run only on CI server“.

SoCraTes Day: Testing the Impossible

21. June, 2015

I’m back from SoCraTes Day Switzerland where I help a Code&Hack session called “Testing the Impossible”.

The session is based on this Mercurial repository de.pdark.testing.


  • 20150619_105103-socrates-day-testing-the-impossible-page1Space Shuttle – They Write the Right Stuff
    • Why wasn’t the bug caught by QA?
    • Why did the bug escape dev?
  • Personal Bug Diary
  • Team Culture: Strengths va. Blame Game
  • Miscommunication with Customers
  • Anyone can press the “Red Button
  • Make failures cheap. Fail fast.
  • I’m wrong. I have learned something.
  • Bug-.hunting takes time and mindshare.
  • If you found a bug, write a test for it

How do I test randomness?

  1. Test small pieces
  2. Fix things that varies (IDs, timestamps, etc.)

How do I test a DB?

  • 20150619_105103-socrates-day-testing-the-impossible-page2Layer between DB and App code which allows to switch between real DB and mock
  • Verify generated SQL instead of executing it
    • Fast
    • Allows to test thousands of combinations easily
    • Useful for code that builds search queries
  • Run SQL against real DB during the night (CI server)
  • H2 embedded DB can emulate Oracle, MySQL, PostgreSQL, …
  • Test at various depths (speed vs. accuracy)
  • Testing Walrus dives deep!

Eclipse Finance Day 2014: Testing business applications with RCPTT

3. November, 2014

RCP Testing Tool (RCPTT) is an Open Source tool for UI testing of Eclipse-based applications. During the demo by Ivan Inozemtsev, I got the impression that they thought of everything:

  • There is a recorder so you can quickly create a test case by clicking thought the application
  • There is an assertion builder where you can say “this element should be red”, “this checkbox needs to be checked”, “this image must be visible”
  • There is special support for all kinds of widgets like text editors where “position” is row/column (i.e. cursor position) instead of mouse coordinates or character offsets. The test API allows for checks like “is styled like a keyword”.
  • If the UI is resized after the tests have been recorded, the tool will try to calculate the relative click point. Unless the UI element is special (like a graph editor) where it has different heuristics (like when you clicked in an empty area but now, there is something below at the coordinate). If everything else fails, the testing tool will try to resize the UI element to the recorded size.
  • There is a compact, extensible DSL to describe the test cases. The DSL supports procedures so you can reuse test code.

The tool was fast and stable during the demo. It could reset Eclipse’s workspace to a certain state (open perspective, open editors, open projects) – I’m currently thinking about using it to create the default workspace for our development team.

One more things from the demo: Black box testing is a myth or at least a dream. A testing tool for anything complex needs to know internal details (like Eclipse’s background jobs). Otherwise the tests will either fail randomly when some background job didn’t complete in time or they will be slow (since each of them will be sprinkled with “WAIT 1 MINUTE” instructions) or they will be sprinkled with application specific “wait” instructions.

Eclipse Finance Day 2014: Automating user interface tests with behavior-driven development (BDD)

3. November, 2014

The last two talks were about testing.

“Automating user interface tests with behavior-driven development (BDD)” by Jose Badeau and Dietmar Stoll used an Xtext-based DSL to connect requirements, wireframes and test descriptions so they can validate each other. In the example, they removed a field from the wireframe mockup and the Eclipse editor for the test cases showed errors where they were used in the DSL.

The demo showed once again how much power lies in connecting information from different sources. If you remember, then this was the key feature which set Eclipse apart from all the other Java tools in 2001: It could bring you to the place where something was defined by pressing F3 or Ctrl+Click. If you changed one file, it would instantly compile all the other files and show you any problems it would find. Having a tool which actively searches for problems saved and still saves a lot of time.


Agile For Prudes

12. November, 2013

The article “WORKING IN A WANNABE-AGILE TEAM” points out a common problem in agile: It really exposes you and most people simply are prude.

Unlike many people want to make you believe, they are aware of their own flaws and how much a certain process humiliates them – it’s a skill everyone adopts at an early age, and therefore almost completely subconscious.

Since there is no way to be agile without looking at the team’s issues, the solution is to offer them something else instead.

In my experience, the easiest foot to get into the door is testing. When customers ask for features, ask “How would you know that it’s working correctly? Can you give me an example?” Yay, acceptance tests for free.

People struggling with a piece of code that fails all the time in interesting ways? “How do you know it’s wrong? What would be right? Maybe we could write a piece of code that makes sure it stays right from now on?” Yay, unit testing.

“Can you write a test?” “You can’t test that!” “…. You wrote software that can’t be tested? Seriously?” “… No, of course you could test it but …”

The best part: It focuses on solutions. When suggesting tests, no one can get into the blame game. Everyone can get involved. Customers, managers, developers, everyone understands tests. And they offer the most value for the least investment.

When people have started testing, they become interested in other things as well: Agile planning. Listening to the customer. That’s when you can start to change the culture – you now have some trust that you can spend.