I’ve circled in red all the areas I want to address.

The road to Generics

1. Start by checking out to git tag v1.5-tests_completed_code_refactored with the command git tag v1.5-tests_completed_code_refactored

2. We’ll begin by modifying the TextFileSource, replacing references of Iterable<String> with ArrayList<String>

3. Cut ArrayList<String> lines = new ArrayList<>(); in loadData( String name, ICollectionLoader func ) (left code line 36) and move it to the top of loadData( String name ); (right code line 25)

4. In the loadData( String fname ) method add an extra parameter to loadData(...) (right code line 31), and modify the method signature to line 35 of the right-hand code fragment

5. Once completed make sure the tests still pass - test_count_chars_in_BasicDataProcessor_no_mocking

6. So your code should look like this

7. 

   You should be getting errors in App and TextFileSource if you’ve started from the git tag v1.5-tests_completed_code_refactored. We need to fix the method loadData() in IDataSource so it now matches the revised method signature in TextFileSource, so it should look like this now

   1. public interface IDataSource
      {
          public  ArrayList<String>  loadData(  String fname );
          public  ArrayList<String>  loadData(  String fname, ArrayList<String> storage, ICollectionLoader func );
      }

   2. Notice that we’ve updated the return type of each method so they match the methods in TextFileSource

8. And App should now look like this

   1. 

9. We now need to templatize the class, methods, and parameters so the type is consistent across all three points

10. 

    You will get an error because the interface is out of sync with what we what the implementation class looks like, so update IDataSource to the following

11. public interface IDataSource<T>
    {
        public  <T> T  loadData(  String fname );
        public  <T> T  loadData(  String fname, T storage, ICollectionLoader func );
    }

12. You will still be getting an error in TextFileSource

    1. 

13. It is to do with the functor. ICollectionLoader is a Generic and it’s bound to ArrayList<String>, if we don’t specify a generic parameter type for ICollectionLoader, the Java compiler widens it to Object. Also in the line where we create the functor, we cannot use the generic param T because the Java compiler is unable to determine if the c.add() is a method on the type T. So in both cases we need to help the compiler. We need to update IDataSource again to the following

    1. public interface IDataSource<T>
       {
           public  <T> T  loadData(  String fname );
           public  <T> T  loadData(  String fname, T storage, ICollectionLoader<T> func );
       }

14. We are now telling ICollectionLoader to expect a type T not a type Object

15. Also update TextFileSource as shown here

    1. 

16. In both cases, we are helping the Java compiler

17. Now complete the process by updating the App class

    1. 

TextFileSource test file

Now is a good time to add this test file, we should have added this at the beginning

package com.celestial.mocking_explored;

import org.junit.jupiter.api.Test;

import java.util.ArrayList;
import java.util.PriorityQueue;
import java.util.Stack;


class TextFileSourceTest
{
    @Test
    void howto_use_loadData_method_with_single_param()
    {
        TextFileSource<ArrayList<String>> tfl = new TextFileSource<>();

        String fname = "C:\\tmp\\KeyboardHandler.java.txt";
        ArrayList<String> lines = tfl.loadData(fname);

        lines.forEach((element) ->{
            System.out.println(">> " + element);
        });
    }

    @Test
    void howto_use_LoadData_method_with_3_params()
    {
        TextFileSource<ArrayList<String>> tfl = new TextFileSource<>();

        // We create a lambda expression to do the work in the TextFileLoader
        ICollectionLoader<ArrayList<String>> functor = (c, l) -> {
            c.add(l);
            return c;
        };

        String fname = "C:\\tmp\\KeyboardHandler.java.txt";
        ArrayList<String> data = new ArrayList<>();

        ArrayList<String> lines = tfl.loadData(fname, data, functor);

        lines.forEach((element) ->{
            System.out.println(">> " + element);
        });
    }

    @Test
    void howto_use_LoadData_method_with_3_params_using_queue()
    {
        TextFileSource<ArrayList<String>> tfl = new TextFileSource<>();

        // We create a lambda expression to do the work in the TextFileLoader
        ICollectionLoader<PriorityQueue<String>> functor = (c, l) -> {
            c.add(l);
            return c;
        };

        String fname = "C:\\tmp\\KeyboardHandler.java.txt";
        PriorityQueue<String> data = new PriorityQueue<>();

        PriorityQueue<String> lines = tfl.loadData(fname, data, functor);

        lines.forEach((element) ->{
            System.out.println(">> " + element);
        });
    }

    @Test
    void howto_use_LoadData_method_with_3_params_using_Stack()
    {
        TextFileSource<ArrayList<String>> tfl = new TextFileSource<>();

        // We create a lambda expression to do the work in the TextFileLoader
        ICollectionLoader<Stack<String>> functor = (c, l) -> {
            c.push(l);
            return c;
        };

        String fname = "C:\\tmp\\KeyboardHandler.java.txt";
        Stack<String> data = new Stack<>();

        Stack<String> lines = tfl.loadData(fname, data, functor);

        lines.forEach((element) ->{
            System.out.println(">> " + element);
        });
    }
}