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In the //Act part of the test, we want to write something like this

Code Block

language	java

        // 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;
        });

Reading the API guides we see that File.readAllLines() returns a List<string>, so let's change the code to match this

Code Block

language	java

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 static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;
import org.mockito.MockedStatic;
import org.mockito.Mockito;

public class FileLoaderTest 
{
    // 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);
    }
}

At line 3228, we are using Mockito’s mockStatic() to mock a Static class
At line 3329, we setup the expectation
So we need to design a loadFile() method that takes as a parameter a block of code that can read the file from any source.
public int LoadFile(string fname, ILoadFile func)
Create a new interface called

Code Block

language	java

package com.celestial.mockito.filetodb;

import java.util.List;

/**
 *
 * @author selvy
 */
@FunctionalInterface
public interface ILoader 
{
    List<String>    loadFile(String fname);
}

Modify FileLoader so it has this additional functionality

Code Block

language	java

    public List<String> getLines() {
        return lines;
    }
    
    int loadFile(ILoader func) 
    {
        lines = func.loadFile(fileToLoad);
        return calculateFileSize();
    }    
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In this implementation of the test System.IO.File we want to use java.nio.file.Files object. But wait, Systemjava.nio.IOfile.FileFiles.ReadLines() is a class-level operation and not an object-level operation. Most mocking frameworks struggle with mocking class-level methods.

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