QLC-3) Guided Solution.

Create a new test project in the FindHighestNumber solution
We’ll draft out a test defining what we think we want the production code to do

public class TopicScoreWriterTest
{
   @Test
   public   void  verify_topic_score_details_written_out_once()
   {
      // arrange
      String physics = "Physics";

      ArrayList<TopicTopScore> topTopScores = new ArrayList<>();
      topTopScores.add( new TopicTopScore(physics, 89)); 
      
      TopicScoreWriter cut = new TopicScoreWriter();
      
      // act
      cut.writeScores( topTopScores );
      
      // assert
   }
}

Line 10 represents the implementation class. We will call a method called writeScores() passing an array TopicTopScores. Since this method will not return a result that is testable in this context, how are we going to test the result of the writeScrores(), it's going to make a call to a Java API. We are not going to test the Java API call, many millions of developers have already done this. We can use the mocking libraries verify capability. But what are we going to verify?
We’ll begin by decoupling the Java API call to write to the file stream from the TopicScorWriter. To do this could pass into the constructor a java.io.FileWriter object or something similar. But this would tie the production too tightly to java.io.FileWriter. Instead, let’s pass an object that encapsulates the file writer functionality. This object needs to support two capabilities 1) open a text file to write to, and 2) write the formatted string to the opened text file. With this design in mind, we can update our test and then dig a little deeper into the implementation code.
Our updated test class now looks like this

namespace TopicWriterTests
{
    interface IFileWriter
    {
        void WriteLine(String lineToWrite);
    }
    public class TopicWriterTests
    {
        [Test]
        public void Verify_topic_score_details_written_out_once()
        {
            // arrange
            String physics = "Physics";
            String art = "Art";
            String compSci = "Comp Sci";
            String expectedResult = "Physics, 89";
            TopicTopScore[] topTopScores = { new TopicTopScore(physics, 89) };
            IFileWriter fileWriter = Substitute.For<IFileWriter>();
      
            TopicScoreWriter cut = new TopicScoreWriter(fileWriter);
            
            // act
            cut.WriteScores(topTopScores);

            // assert
            fileWriter.Received(1).WriteLine(expectedResult);
        }
    }
}

Notice the use of the interface at line 3. We then use this interface to create Mock object at line 18. At line 26 we are going to verify that the method WriteLine() is called once with a string formatted as shown at line 16.
Move the IFileWriter interfaces into the same file as TopicScoreWriter

namespace TopicManagerService
{
    public interface IFileWriter
    {
        void WriteLine(string lineToWrite);
    }

Let’s write the implementation code for TopicScoreWriter. We’ve used a canned result (line 20) to test our theory

   public class TopicScoreWriter
    {
        private IFileWriter fileWriter;

        public TopicScoreWriter(IFileWriter fileWriter)
        {
            this.fileWriter = fileWriter;
        }

        public void WriteScores(TopicTopScore[] topTopScores)
        {
            fileWriter.WriteLine("Physics, 89");
        }
    }
}

Run the test and it should pass
We’ll now update WriteScores() so that it writes the data being passed

        public void WriteScores(TopicTopScore[] topTopScores)
        {
            if (topTopScores.Length > 0)
            {
                TopicTopScore tts = topTopScores[0];
                string dataToWrite = tts.TopicName + ", " + tts.TopScore;
                fileWriter.WriteLine(dataToWrite);
            }
        }

The if statement introduces a new logic path in the code, we should have test for this

        [Test]
        public void Verify_empty_array_topic_score_not_written()
        {
            // arrange
            String physics = "Physics";
            String art = "Art";
            String compSci = "Comp Sci";
            String expectedResult = "Physics, 89";
            TopicTopScore[] topTopScores = Array.Empty<TopicTopScore>();
            IFileWriter fileWriter = Substitute.For<IFileWriter>();

            TopicScoreWriter cut = new TopicScoreWriter(fileWriter);
            // act
            cut.WriteScores(topTopScores);

            // assert
            fileWriter.DidNotReceive().WriteLine(expectedResult);
        }

Finally, add another test that writes more top scores for different topics

        [Test]
        public void Verify_topic_score_details_written_out_multiple_times()
        {
            // arrange
            string physics = "Physics";
            string art = "Art";
            string compSci = "Comp Sci";
            string physicsResult = "Physics, 89";
            string artsResult = "Art, 87";
            string comSciResult = "Comp Sci, 97";

            TopicTopScore[] topTopScores = { new TopicTopScore(physics, 89),
                                             new TopicTopScore(art, 87),
                                             new TopicTopScore(compSci, 97)};

            String fileToWrite = "testfile.txt";
            IFileWriter fileWriter = Substitute.For<IFileWriter>();
      
            TopicScoreWriter cut = new TopicScoreWriter(fileWriter);

            // act
            cut.WriteScores(topTopScores );

            // assert
            fileWriter.Received(1).WriteLine(physicsResult);
            fileWriter.Received(1).WriteLine(artsResult);
            fileWriter.Received(1).WriteLine(comSciResult);
        }

And the implementation code has been updated to

namespace TopicManagerService
{
    public interface IFileWriter
    {
        void WriteLine(string lineToWrite);
    }

    public class TopicScoreWriter
    {
        private IFileWriter fileWriter;

        public TopicScoreWriter(IFileWriter fileWriter)
        {
            this.fileWriter = fileWriter;
        }

        public void WriteScores(TopicTopScore[] topTopScores)
        {
            foreach( TopicTopScore tts in topTopScores )
            {
                string dataToWrite = tts.TopicName + ", " + tts.TopScore;
                fileWriter.WriteLine(dataToWrite);
            }
        }
    }
}