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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 12 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 we 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

public class TopicScoreWriterTest
{
   interface IFileWriter
   {
      void  writeLine( String lineToWrite );
   }
   
   @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";

      ArrayList<TopicTopScore> topTopScores = new ArrayList<>();
      topTopScores.add( new TopicTopScore(physics, 89)); 
      
      String fileToWrite = "testfile.txt";
      IFileWriter fileWriter = mock(IFileWriter.class);
      
      TopicScoreWriter cut = new TopicScoreWriter( fileWriter );
      
      // act
      cut.writeScores( topTopScores );
      
      // assert
      verify( fileWriter, times(1)).writeLine(expectedResult);
   }   
}

Notice the use of the interface at line 3. We then use this interface to create Mock object at line 21. At line 29 wr are going to verify that the method writeLine() is called once with a string formatted as shown at line 15.
Move the IFileWriter interfaces into its own file

Code Block
package com.s2s.demos.topicmanager; 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

package com.s2s.demos.topicmanager;

import java.util.ArrayList;

/**
 *
 * @author selvy
 */
public class TopicScoreWriter
{
   private IFileWriter itsFileWriter;
   
   public TopicScoreWriter(IFileWriter fileWriter )
   {
      this.itsFileWriter = fileWriter;
   }

   void writeScores(ArrayList<TopicTopScore> topTopScores)
   {
      itsFileWriter.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

   void writeScores(ArrayList<TopicTopScore> topTopScores)
   {
      if( topTopScores.size() > 0 )
      {
         TopicTopScore tts = topTopScores.get(0);
         String dataToWrite = tts.getTopicName() + ", " + tts.getTopScore();
         itsFileWriter.writeLine(dataToWrite);
      }
   }

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

Code Block

language	java

   @Test
   public   void  verify_topic_score_details_not_written()
   {
      // arrange

      ArrayList<TopicTopScore> topTopScores = new ArrayList<>();
      
      IFileWriter fileWriter = mock(IFileWriter.class);
      
      TopicScoreWriter cut = new TopicScoreWriter( fileWriter );
      
      // act
      cut.writeScores( topTopScores );
      
      // assert
      verify( fileWriter, times(0)).writeLine(Mockito.any());
   }

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

Code Block

   @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";

      ArrayList<TopicTopScore> topTopScores = new ArrayList<>();
      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.class);
      
      TopicScoreWriter cut = new TopicScoreWriter( fileWriter );
      
      // act
      cut.writeScores( topTopScores );
      
      // assert
      verify( fileWriter, times(1)).writeLine(physicsResult);
      verify( fileWriter, times(1)).writeLine(artsResult);
      verify( fileWriter, times(1)).writeLine(comSciResult);
   }

And the implementation code has been updated to

Code Block

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

   void writeScores(ArrayList<TopicTopScore> topTopScores)
   {
      for(TopicTopScore tts : topTopScores)
      {
         String dataToWrite = tts.getTopicName() + ", " + tts.getTopScore();
         itsFileWriter.writeLine(dataToWrite);
      }
   }
}

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