QLC-2) Find the Highest Number Extension

An organisation delivers several topics (subjects). Students are graded against each topic. You are required to store the top score for each topic.

We’ve designed the application so that it comprises of three core classes:

A class to find the highest number from an array of integers.
A class to find the highest score for a topic.
A class to write the topic and score to a file on the disk.

You are going to follow a TDD approach to find the highest score for a series of topics

Given the following specification

If the input is [{“Physics”, {56, 67, 45, 89}}], the result should be [{“Physics”, 89}]
If the input is [] the result should be []
If the input is [{“Physics”, {56, 67, 45, 89}}, {“Art”, {87, 66, 78}], the result should be [{“Physics”, 89}, {“Art”, 87}]
If the input is [{“Physics”, {56, 67, 45, 89}}, {“Art”, {87, 66, 78}}, {“Comp Sci”, {45, 88, 97, 56}}], the result should be [{“Physics”, 89}, {“Art”, 87}, {“Comp Sci”, 97}]

QLC-2.1) Setup and first test

Create a new Test Project in the FindHighestNumber Solution
Install the packages you want (MSTest already installed, or NUnit - we are going to be using NUnit)
Create a new setup the new test class as shown here (remember you will have errors)

namespace TopicManagerTests
{
    public class TopicManagerTests
    {
        [Test]
        public void find_heighest_score_in_empty_array_return_empty_array()
        {
            // Arrange
            int[] array = {};
            TopicManager cut = new TopicManager();
            int[] expectedResult = {};

            // Act
            Topic[] result = cut.findTopicHighScores(array);

            // Assert
            Assert.That(result, Is.EqualTo(result));
        }
    }
}

Create a new class called TopicManager in the FindHighestNumberService project
Clean the code up so that the class is in a namespace called TopicManagerService
Lines 9, 11, and 14 give us enough information to allow us to start thinking about what we are trying to design here. based on the requirements, we want to pass into the method findTopicHighScores an array of Topics and their accompanying scores. It should return the topic score of each Topic, we will call this TopicTopScore. Each item in the array being passed into the findTopicHighScores will be called TopicScores (plural I know, but it matches the context, you may want to lose the ‘s' at the end of the class name). Let’s refactor the code to reflect what we have outlined here.

    public class TopicManagerTests
    {
        [Test]
        public void find_heighest_score_in_empty_array_return_empty_array()
        {
            // Arrange
            TopicScores[] array = Array.Empty<TopicScores>();
            TopicManager cut = new TopicManager();
            TopicTopScore[] expectedResult = Array.Empty<TopicTopScore>();

            // Act
            TopicTopScore[] result = cut.findTopicHighScores(array);

            // Assert
            Assert.That(result, Is.EqualTo(result));
        }
    }

We are now in a good position to get VS to generate the findTopicHighScores with the correct signature

namespace TopicManagerService
{
    public class TopicManager // TopicManager.cs
    {
        public TopicTopScore[] findTopicHighScores(TopicScores[] array)
        {
            // This block oc code assumes that the input is an empty array, there is no need for an if statement
            return Array.Empty<TopicTopScore>();
        }
    }
}

Now we need to complete the first piece of implementation code to pass the test
So you will also need the following pieces of code

namespace TopicManagerService
{
    public class TopicTopScore // TopicTopSscore.cs
    {
    }
}

namespace TopicManagerService
{
    public class TopicScores // TopicScores.cs
    {
    }
}

Second test

Implement this second test

        public void find_heighest_score_with_array_of_one_return_array_of_one()
        {
            // Arrange
            int[] scores = { 56, 67, 45, 89 };
            string topicName = "Physics";
            TopicScores[] topicScores = { new TopicScores(topicName, scores) };
            TopicManager cut = new TopicManager();
            TopicTopScore[] expectedResult = {new TopicTopScore(topicName, 89)};

            // Act
            TopicTopScore[] result = cut.findTopicHighScores(topicScores);

            // Assert
            Assert.AreEqual(result[0].TopicName, expectedResult[0].TopicName);
            Assert.AreEqual(result[0].TopScore, expectedResult[0].TopScore);
        }

And here is the implementation class

    public class TopicManager
    {
        private HighestNumberFinder highestNumberFinder = new HighestNumberFinder();

        public TopicTopScore[] findTopicHighScores(TopicScores[] array)
        {
            if(array.Length == 1)
            {
                List<TopicTopScore> topScores = new List<TopicTopScore>();

                TopicScores ts = array[0];
                int topScore = highestNumberFinder.findHighestNumber(ts.Scores);

                topScores.Add(new TopicTopScore(ts.TopicName, topScore));

                return topScores.ToArray();
            }
            else
                return Array.Empty<TopicTopScore>();
        }
    }

Line 3 represents the elephant in the code
1. We already have tests for the HighestNumberFinder class, and yes class TopicManager requires HighestNumberFinder. But the coupling between these two classes is such that we cannot test TopicManger independently of HighestNumberFinder. Actually, this is a code smell.
2. we can not substitute out HighestNumberFinder, this also is a code smell.
3. We can remove the code smell by injecting HighestNumberFinder into TopicManager, when TopicManager is created.
Modify TopicManager as follows
1. remove the creation of HighestNumberFinder at line 5
2. Pass an instance of a HighestNumberFinder into TopicManager as a constructor parameter

public class TopicManager
{
    private HighestNumberFinder highestNumberFinder;

    public  TopicManager( HighestNumberFinder hnf )
    {
        highestNumberFinder = hnf;
    }

    public TopicTopScore[] findTopicHighScores(TopicScores[] array)
    {
        if(array.Length == 1)
        {
            List<TopicTopScore> topScores = new List<TopicTopScore>();

            TopicScores ts = array[0];
            int topScore = highestNumberFinder.findHighestNumber(ts.Scores);

            topScores.Add(new TopicTopScore(ts.TopicName, topScore));

            return topScores.ToArray();
        }
        else
            return Array.Empty<TopicTopScore>();
    }
}

Refactor the tests so that they run. You should not have to change any of the test data.

Tests are a great way of identifying code smells. Highly coupled code leads to untestable code.

You want to test one class and one class only. The previous version of the TopicManager dragged in HighestNumberFinder. So inadvertently you were testing that class as well. This will become clearer as we continue to work through the TopicManager tests.

QLC-2.2) Working with Stubs

A Stub is part of the family of Test Doubles. They are used to ensure that tests focus on the behaviour of the CUT and not its dependents. Test environments should be controlled and predictable. Test Doubles give you that measure of stability and predictability.

A Stub method is one that returns canned results. A canned result is a predefined result. The result can be specific, a range of values, or any value. Also, the parameters into the method can be specific value, a range of values, or any value.

The HighestNumberFinder is designed to return an integer representing the number in a group of numbers. This can easily be stubbed out.

Here is the Stub

namespace TopicManagerService
{
    public class HighestNumberFinder
    {
        public int findHighestNumber(int[] values)
        {
            return 89;
        }
    }
}

But when you try and use this class in the tests, you should see type errors. This is because, in the implementation unit of TopicManager, it's typed against HighestNumberFinder in a namespace called FindHighestNumberService.v6. So namespaces do not offer us the solution we are looking for.

Here are the modified tests

using NUnit.Framework;

namespace TopicManagerService
{
    public class TopicManagerTests
    {
        [Test]
        public void find_heighest_score_in_empty_array_return_empty_array()
        {
            // Arrange
            TopicScores[] topicScores = Array.Empty<TopicScores>();
            HighestNumberFinder hnf = new HighestNumberFinder();
            TopicManager cut = new TopicManager(hnf);
            TopicTopScore[] expectedResult = Array.Empty<TopicTopScore>();

            // Act
            TopicTopScore[] result = cut.findTopicHighScores(topicScores);

            // Assert
            Assert.That(result, Is.EqualTo(expectedResult));
        }

        [Test]
        public void find_heighest_score_with_array_of_one_return_array_of_one()
        {
            // Arrange
            int[] scores = { 56, 67, 45, 89 };
            string topicName = "Physics";
            TopicScores[] topicScores = { new TopicScores(topicName, scores) };
            HighestNumberFinder hnf = new HighestNumberFinder();
            TopicManager cut = new TopicManager(hnf);
            TopicTopScore[] expectedResult = {new TopicTopScore(topicName, 89)};

            // Act
            TopicTopScore[] result = cut.findTopicHighScores(topicScores);

            // Assert
            Assert.AreEqual(result[0].TopicName, expectedResult[0].TopicName);
            Assert.AreEqual(result[0].TopScore, expectedResult[0].TopScore);
        }
    }
}

Observe errors on lines 14 and 32.

We need to use interfaces. And the interface needs to be in a place that is available to both production code and tests.

Create a new interface that is part of the HighestNumberFinder project

namespace FindHighestNumberService
{
    public interface IHighestNumberFinder
    {
        int findHighestNumber(int[] values);
    }
}

Copy HighestNumberFinder.v6 to HighestNumberFinderFinal and refactor the class so that it implements the interface above

using System;
using FindHighestNumberService;

namespace FindHighestNumberServiceFinal
{
    public class EmptyArrayException : Exception
    {
        public EmptyArrayException(string message):base(message)
        {
        }
    }
    public class HighestNumberFinder : IHighestNumberFinder
    {
        public int findHighestNumber(int[] values)
        {
            if (values.Length < 1)
                throw new EmptyArrayException("Array is empty");

            int result = Int32.MinValue;

            for (int i = 0; i < values.Length; i++)
            {
                if (values[i] > result)
                    result = values[i];
            }
            return result;
        }
    }
}

Refactor the tests so that the HighestNumberFinder is instantiated as follows

    IHighestNumberFinder hnf = new FindHighestNumberServiceFinal.HighestNumberFinder();

Refactor TopicManager so it now works with the interface IHighestNumberFinder and not directly with the implementation class
Rerun all your tests they should still be passing.

The tests have given us the confidence to refactor the code and begin to think more clearly about our implementation.

Now let’s substitute a stub in place of the real implementation of `HighestNumberFinder`.

Create a new called find_heighest_score_with_array_of_one_return_array_of_one_using_stub(), it's a copy of find_heighest_score_with_array_of_one_return_array_of_one()

        [Test]
        public void find_heighest_score_with_array_of_one_return_array_of_one_using_stub()
        {
            // Arrange
            int[] scores = { 56, 67, 45, 89 };
            string topicName = "Physics";
            TopicScores[] topicScores = { new TopicScores(topicName, scores) };
            IHighestNumberFinder hnf = new FindHighestNumberServiceFinal.HighestNumberFinder();
            TopicManager cut = new TopicManager(hnf);
            TopicTopScore[] expectedResult = { new TopicTopScore(topicName, 89) };

            // Act
            TopicTopScore[] result = cut.findTopicHighScores(topicScores);

            // Assert
            Assert.AreEqual(result[0].TopicName, expectedResult[0].TopicName);
            Assert.AreEqual(result[0].TopScore, expectedResult[0].TopScore);
        }
    }

Modify line 8 to

    IHighestNumberFinder hnf = new TopicManagerService.HighestNumberFinder();

The error is because of TopicManagerTests.HighestNumberFinder does not implement the IHighestNumberFinder interface. Modify it so that it does.
Rerun the tests, they should all pass.

We’ve now created a controlled environment for our tests

QLC-2.3 Stubs, complete this TopicManager requirement

Add more tests to handle the last two requirements

•If the input is [{“Physics”, {56, 67, 45, 89}}, {“Art”, {87, 66, 78}], the result should be [{“Physics”, 89}, {“Art”, 87}]

One of the results will fail. Why?

QLC-2.4 Mocks, complete the last TopicManager requirement

•If the input is [{“Physics”, {56, 67, 45, 89}}, {“Art”, {87, 66, 78}}, {“Comp Sci”, {45, 88, 97, 56}}], the result should be [{“Physics”, 89}, {“Art”, 87}, {“Comp Sci”, 97}]