Creating Tests for LCD Numbers Kata by Mocking Collaborators

This is an implementation of the LCD Numbers Kata in Java using outside-in development. There are no tests. In order to unit test its classes you need to create test doubles for the collaborators.

This is an Apache Maven project. Run mvnw test to run your tests. JUnit and Mockito are provided as dependencies.

For other languages see

Requirements (copied from Ruby Quiz)

The LcdDisplay creates an LCD string representation of an integer value using a 4x7 grid of space each, using minus and pipe characters for each digit. Each digit is shown below:

 --      --  --      --  --  --  --  --
|  |   |   |   ||  ||   |      ||  ||  |
|  |   |   |   ||  ||   |      ||  ||  |
         --  --  --  --  --      --  --
|  |   ||      |   |   ||  |   ||  |   |
|  |   ||      |   |   ||  |   ||  |   |
 --      --  --      --  --      --  --

The bar size should be adjustable. The default value is 2 - as shown above. Read more about it at RubyQuiz.

Description of Existing Solution

To make things easier for you, here is the description of the solution used in the code: The LcdDisplay is the entry point. The given number is split into digits according to the used NumeralSystem (which is the Decimal System in our case). Then the initial Digits are created by copying the template Lines from Patterns. Then the (LCD) digits are scaled by DigitScaler according to the Scaling value. The actual scaling is done inside the ScalingRepeater by repeating certain rows and columns of the template. Finally the list of digits is combined to a single String by the DigitPrinter.

UML Class Diagram

There is a class Main to see the LCD Numbers working. There you can see how all the classes are assembled and collaborators are combined with each other. You can run the Main with

java org.codecop.lcdnumbers.Main <number, e.g. 123> <size, e.g. 2>

Creating Unit Tests

Create unit tests for all classes. Make sure all units are tested in isolation. The point of this exercise is to go extreme to practise using test doubles. Create test doubles for everything! Create dummy objects for all used values and stub all collaborators, even the most simple classes like Scaling and Line.

  1. Start testing the simple units like Scaling, Line and Digit. These units have a single (interesting) method with a single collaborator.
  2. Work on DigitScaler and DigitPrinter. Both have more logic, but still only a single collaborator.
  3. Things are getting more complicated with DigitsSplitter.
  4. Finally go for LcdDisplay which has three collaborators.

There are sample LCD outputs in the src/test/resources folder to be used in tests. The size_1, size_2 and size_3 folders contain each digit in original size and scaled twice and three times.

Check your Coverage

To measure your progress you should use code coverage tools:

  • EclEmma inside of Eclipse.

  • Cobertura - type mvnw cobertura:cobertura or run cobertura.bat or ./

  • PIT - type mvnw test-compile org pitest:pitest-maven:mutationCoverage or run pit.bat or ./ Note that PIT does not work if there are any blanks in the path names leading to the current location.


Enforcing all calls to other classes in a test is not recommended in real unit tests because this leads to bad tests which are bound to the implementation. These over specified tests make it impossible to change the code under test without breaking. We want to be able to change the code without breaking the test as long as the required functionality is not changed. This exercise is for practising test doubles.


This work is licensed under a New BSD License, see license txt in repository.


Thanks to Ilke Zilci for trying out this exercise.