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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[Test]
   public   void  verify_topic_score_details_written_out_once()
   {
      // arrange
      String physics = "Physics";

      ArrayList<TopicTopScore>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

publicnamespace classTopicWriterTests
TopicScoreWriterTest{
{    interface IFileWriter
    {
        void  writeLineWriteLine( String lineToWrite );
    }
    public class  @TestTopicWriterTests
    {
        [Test]
      public  public void  verifyVerify_topic_score_details_written_out_once()
        {
            // arrange
            String physics = "Physics";
            String art = "Art";
            String compSci = "Comp Sci";
            String expectedResult = "Physics, 89";
       ArrayList<TopicTopScore> topTopScores = new ArrayList<>();
      topTopScores.add(  TopicTopScore[] topTopScores = { new TopicTopScore(physics, 89)) };
            IFileWriter  String fileToWritefileWriter = "testfile.txt";
Substitute.For<IFileWriter>();
     IFileWriter fileWriter = mock(IFileWriter.class); 
            TopicScoreWriter cut = new TopicScoreWriter( fileWriter );
            
            // act
            cut.writeScoresWriteScores( topTopScores );

            // assert
           verify( fileWriter, times.Received(1)).writeLineWriteLine(expectedResult);
        }
    }
}

Notice the use of the interface at line 3. We then use this interface to create Mock object at line 2118. At line 29 wr 26 we are going to verify that the method writeLineWriteLine() is called once with a string formatted as shown at line 1516.
Move the IFileWriter interfaces into its own the same file as TopicScoreWriter

Code Block
package com.s2s.demos.topicmanager;namespace TopicManagerService { public interface IFileWriter { void writeLineWriteLine( Stringstring 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

package com.s2s.demos.topicmanager;  import java.util.ArrayList;

/**
 *
 * @author selvy
 */
public class TopicScoreWriter
{ public class TopicScoreWriter
    {
        private IFileWriter itsFileWriterfileWriter;

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

        public void writeScoresWriteScores(ArrayList<TopicTopScore>TopicTopScore[] topTopScores)
        {
      itsFileWriter.writeLine      fileWriter.WriteLine("Physics, 89");
        }
    }
}

Run the test and it should pass
We’ll now update writeScoresWriteScores() 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);];
                Stringstring dataToWrite = tts.getTopicName()TopicName + ", " + tts.getTopScore();TopScore;
                itsFileWriterfileWriter.writeLineWriteLine(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
            String physics = "Physics";
            String art = "Art";
            String compSci = "Comp Sci";
            String expectedResult = "Physics, 89";
            ArrayList<TopicTopScore>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(0).DidNotReceive().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";
       String     string artsResult = "Art, 87";
      String      string 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;
         itsFileWriter.writeLine       fileWriter.WriteLine(dataToWrite);
            }
        }
    }
}

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