Full complete versions of the code

package com.celestial.mockito.filetodb;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import org.junit.Test;
import static org.junit.Assert.*;

/**
 *
 * @author selvy
 */
public class LiveFileLoaderTest 
{
    // The inital design is described in this test
    /*
     The weakness should be obvious?  The file to be loaded and it's location
     I use a shared network drive to run this code from different machines, normally
     dev-ing from a PC.  When I ran the code on the laptop from a cafe it 
     immediately failed because the C: on the laptop was completely different
     to the PC, so the original file C:/tmp/KeyboardHandler.txt did not exist
    
     THIS IS A GREAT EXAMPLE OF WHY THE UNIT TEST AND CUT SHOULD NOT BE STRONGLY
     LINKED TO ANY IO - NETWORK, DB, AND FILE SYSTEM
    */
     @Test
     public void load_all_of_file_using_inbuilt_Files_type() 
     {
         // arrange
         String fileToLoad = "c:/tmp/kipstor-settings.js";
         FileLoader cut = new FileLoader(fileToLoad);
         int expectedBytesRead = 301;
         
         // act
         int bytesRead = cut.loadFile(fileToLoad);
         
         // assert
         assertEquals(expectedBytesRead, bytesRead);
     }
}

package com.celestial.mockito.filetodb;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.*;


/**
 *
 * @author selvy
 */
public class FileLoader 
{
    String fileToLoad;
    List<String> lines = Collections.emptyList();

    public FileLoader(String fileToLoad) 
    {
        this.fileToLoad = fileToLoad;
    }

    int loadFile(String fname) 
    {
        try
        {
            lines = Files.readAllLines(Paths.get(fname), StandardCharsets.UTF_8);
        }
        catch (IOException e){}

        return calculateFileSize();
    }    
    
    private int calculateFileSize()
    {
        IntWrapper result = new IntWrapper();
        
        lines.forEach(line -> {
                result.value += line.length();
        });
        
        return result.value;
    }
}

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package com.celestial.mockito.filetodb;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.junit.Test;
import static org.junit.Assert.*;
import org.mockito.MockedStatic;
import org.mockito.Mockito;

/**
 *
 * @author selvy
 */
public class FileLoaderTest 
{
    
    // To use a different type of file system loader, pass a lambda to loadFile()
    // as shown here
    /*
        int bytesRead = cut.loadFile((fname) ->
        {
           List<String> result = null;
           try
           {
               result = Files.readAllLines(Paths.get(fname), StandardCharsets.UTF_8);
           }
           catch (IOException e){}
           return result;
        });
    */

     // Redesign the FileLoader so that the machenism to load files up can be 
     // passed in as a lambda - still titghtly coupled the file system
     @Test
     public void load_all_of_file_using_inbuilt_Files_type_as_lambda() 
     {
         // arrange
         String fileToLoad = "c:/tmp/KeyboardHandler.txt";
         FileLoader cut = new FileLoader(fileToLoad);
         int expectedBytesRead = 10;    //1371;
         List<String> pretendFileContent = new ArrayList<>();
         pretendFileContent.add("Hello");
         pretendFileContent.add("world");
         MockedStatic<Files> ff = Mockito.mockStatic(Files.class);
         ff.when(() -> Files.readAllLines(Paths.get(fileToLoad), StandardCharsets.UTF_8)).thenReturn(pretendFileContent);
         
         // act
         int bytesRead = cut.loadFile((fname) ->
         {
            List<String> result = null;
            try
            {
                result = Files.readAllLines(Paths.get(fname), StandardCharsets.UTF_8);
            }
            catch (IOException e){}
            return result;
         });
         
         // assert
         assertEquals(expectedBytesRead, bytesRead);
     }

     // Redesign has worked so this test uses a stub to simulate a file being
     // loaded, it is passed in as lambda - We've decoupled ourselves from the 
     // filesystem
     @Test
     public void load_all_of_file_via_stub() 
     {
         // arrange
         String fileToLoad = "c:/tmp/KeyboardHandler.txt";
         FileLoader cut = new FileLoader(fileToLoad);
         int expectedBytesRead = 10;
         
         // act
         int bytesRead = cut.loadFile((fname) ->
         {
            List<String> result = new ArrayList<>();
            
            result.add("Hello");
            result.add("world");

            return result;
         });
         
         // assert
         assertEquals(expectedBytesRead, bytesRead);
     }
     
     
}

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package com.celestial.mockito.filetodb;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.*;


/**
 *
 * @author selvy
 */
public class FileLoader 
{
    class   IntWrapper
    {
        int value;
    }
    
    String fileToLoad;
    List<String> lines = Collections.emptyList();

    public FileLoader(String fileToLoad) 
    {
        this.fileToLoad = fileToLoad;
    }

    public List<String> getLines() {
        return lines;
    }
    
    int loadFile(String fname) 
    {
        try
        {
            lines = Files.readAllLines(Paths.get(fname), StandardCharsets.UTF_8);
        }
        catch (IOException e)
        {
        }

        return calculateFileSize();
    }    
    
    int loadFile(ILoader func) 
    {
        lines = func.loadFile(fileToLoad);
        return calculateFileSize();
    }    
    
    private int calculateFileSize()
    {
        IntWrapper result = new IntWrapper();
        
        lines.forEach(line -> {
                result.value += line.length();
        });
        
        return result.value;
    }
}