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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

Code Block

language	java

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

Code Block

language	java

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

Code Block
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

Code Block

language	java

   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

Code Block

language	java

        public void writeScoresWriteScores(ArrayList<TopicTopScore>TopicTopScore[] topTopScores)
        {
            if ( topTopScores.size()Length > 0)
    )        {
                TopicTopScore tts = topTopScores.get([0)];
              String  string dataToWrite = tts.getTopicName()TopicName + ", " + tts.getTopScore()TopScore;
         itsFileWriter.writeLine       fileWriter.WriteLine(dataToWrite);
            }
        }

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

Code Block

language	java

   @Test     [Test]
     public   public void  verifyVerify_empty_array_topic_score_details_not_written()
        {
            // arrange
            ArrayList<TopicTopScore>String physics = "Physics";
            String art = "Art";
            String compSci = "Comp Sci";
            String expectedResult = "Physics, 89";
            TopicTopScore[] topTopScores = new ArrayList<>Array.Empty<TopicTopScore>();

            IFileWriter fileWriter = mock(IFileWriter.classSubstitute.For<IFileWriter>();

            TopicScoreWriter cut = new TopicScoreWriter( fileWriter );

            // act
            cut.writeScoresWriteScores( topTopScores );

            // assert
           verify( fileWriter, times.DidNotReceive(0)).writeLine(Mockito.any()WriteLine(expectedResult);
        }

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

Code Block

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

           ArrayList<TopicTopScore> TopicTopScore[] topTopScores = { new ArrayList<>TopicTopScore();physics, 89),
                                 topTopScores.add( new TopicTopScore(physics, 89));        topTopScores.add( new TopicTopScore(art, 87));,
                                            topTopScores.add( new TopicTopScore(compSci, 97))};

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

            // act
            cut.writeScoresWriteScores( topTopScores );

            // assert
          verify(  fileWriter, times.Received(1)).writeLineWriteLine(physicsResult);
         verify(   fileWriter, times.Received(1)).writeLineWriteLine(artsResult);
      verify(      fileWriter, times.Received(1)).writeLineWriteLine(comSciResult);
        }

And the implementation code has been updated to

Code Block

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

    public class TopicScoreWriter
    {
        private IFileWriter itsFileWriterfileWriter;

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

        public void writeScoresWriteScores(ArrayList<TopicTopScore>TopicTopScore[] topTopScores)
        {
       for(     foreach( TopicTopScore tts :in topTopScores )
            {
         String       string dataToWrite = tts.getTopicName()TopicName + ", " + tts.getTopScore();TopScore;
                itsFileWriterfileWriter.writeLineWriteLine(dataToWrite);
            }
        }
    }
}

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