Populatron

Objectives

Practice outside-in testing with TDD
Refactor code into testable units
Isolate and mock hard-to-test final JVM classes
Isolate and mock dependencies that use IO
Isolate application from entry point
Use mockito to mock classes
Use behavior-based and state-based testing techniques

Rationale

Real-life applications are complex. Naive programmers programmers will create complex monoliths which are hard to test.

Adherents to TDD will create complex applications composed of simple modules.

This exercise gives students an isolated practice problem to hone their skills.

Background

In this repository you will find a csv file containing information on the cities in the world.

You will also find a gradle project with a single entry point and a single integration test.

The test follows the out-side in paradigm, and the current state is green.

Now it is time for you to refactor this monolith into smaller classes, writing unit tests for each one as you go.

TODO

Isolate the entry point from the application class. No notion command line arguments or console output should leak into the application class itself. If this was turned into a web-based app tomorrow, it should be easy as pie.
Isolate file IO from the app. Disk access is brittle (incorrect paths, disk full, access denied). Refactor file IO into it's own class so it can be mocked.
Use mockito to mock and inject your file IO class.
Isolate your CSV parsing logic from your app by refactoring it into a parser that returns an generic list of POJOs. Use reflection to initialize the POJOs.
Isolate your CSV parser from reflection logic and file IO. The parser should only be concerned with parsing.

Verification

Below are some questions you can ask yourself to determine if your code is TDD compliant:

Do your classes communicate by passing messages? Or are there deep seated dependencies leaking between them? A good way to determine this is to look at namespaces. Is java.lang.reflect imported into any files outside of your reflection helpers? Does java.nio appear outside of you file helpers? If so, you probably need to refactor.
Are any of your methods longer than 5-10 lines max? Do they do more than 1 thing? If so, you probably need to break them up into smaller methods.
Do you use the new keyword outside of dedicated, injectable factory classes (eg. inside your constructor or when initializing fields)? If so, you probably can't inject them and you need to refactor.
Are your tests reliant on data that was in the file? If so, you need to alter your code to mock the appropriate classes so that your tests are in charge of the data being processed. Pure unit tests of a CSV reader should involve just a few lines with a few columns.
Do your tests use real models, or did you make deliberately simplified models to test with? Unit tests should use simplified data and models.
If you set a breakpoint in the constructor of all objects, and you run your unit test, does more than one breakpoint get hit? If so, you aren't mocking sufficiantly.
Do your methods mutate any variables? Can you refactor them to be immutable?

Remember these questions, because they apply to all code, not just the code you have written in for this exercise.

Stretch

Write a pure unit test for your file IO module using PowerMock to mock the JDK IO classes.
Refactor all your tests to use behavior based testing and mock all dependencies.
Re-wire your app and tests to use Google Guice to inject dependencies. (Follow their getting started guide for help)

Final note

The focus of this exercise is on unit tests and mocking because these are often where developers go wrong. However if you only have unit tests, you face other problems:

Make sure you don't fall into the mockist trap and skip your integration testing.

gSchool/populatron_java