Why am I able to access the list of videos that is private?

Question

My question is why can I access and alter the private member property mVideos from the "Course" class? My original understanding was that since it's been declared as "private" I shouldn't be able to change it unless there is a "setter" method. In this challenge there is only the getVideos() method, but I can still add a video using "course.getVideos().add(1 , video). Will I also be able to remove an element from this list using "coures.getVideos().remove(someIndex)"? I guess I need a bit more explanation on this. Could somebody explain it to me?

Code:

import com.example.model.Course;
import com.example.model.Video;

import java.util.Map;

public class QuickFix {
  public void addForgottenVideo(Course course) {
    List<Video> videoList = course.getVideos(); //Formal step. Can be solved without this.
    // TODO(1):  Create a new video called "The Beginning Bits"
    Video vid = new Video("The Beginning Bits");

    // TODO(2):  Add the newly created video to the course videos as the second video.
    videoList.add(1, vid); //first solution
    course.getVideos().add(1, vid); //second solution
  }

  public void fixVideoTitle(Course course, String oldTitle, String newTitle) {

  }

  public Map<String, Video> videosByTitle(Course course) {
    return null;
  }
}

package com.example.model;

public class Video {
  private String mTitle;

  public Video(String title) {
    mTitle = title;
  }

  public String getTitle() {
    return mTitle;
  }

  public void setTitle(String title) {
    mTitle = title;
  }

}

package com.example.model;

import java.util.List;

public class Course {
  private String mName;
  private List<Video> mVideos; 

  public Course(String name, List<Video> videos) {
    mName = name;
    mVideos = videos;
  }

  public String getName() {
    return mName;
  }

  public List<Video> getVideos() {
    return mVideos;
  }

}

Answer 1 · 2017-07-31T14:26:09Z

July 31, 2017 2:26pm

Hi Vladislav,

You ask a good question. I would imagine that this has to do with the fact that the "List" interface is a public interface with its own default modifier methods.

To make it truly "read-only" you will need to use the "Collections.unModifiableList()" method to return an unmodifiable list.

The "private" modifier just means you can not Directly access the variables. But through other methods you might be able too.

example:

//Setting up a list of videos and a course.
List<Video> videos = new List<>();
videos.add(new Video("This is a new video");
Course newCourse = new Course("course", videos);

You would not be able to do:

newCourse.getVideos().getIndex(0).mTitle = "Trying change title on chosen video";

Because you are not allowed to access the mTitle directly. But you could to it through a setter-method.

That is what is being done in the case of adding to the list. you are accessing the list variables through the "List"-interfaces methods.

This is atleast how i understand it to work and it makes sense in my head.

If someone finds this information to be complete BS, please let me know so i can remove it from the forum.

Please "Up-vote" if this helped you in any way.

Chris.

Answer 2 · 2017-07-31T17:09:10Z

July 31, 2017 5:09pm

Hi again Vladislav,

If you try running the following code to test your classes

public class Testing    {
    public static void main(String[] args)  {
        //Setting up a list of videos and a course.
        List<Video> videos = new ArrayList<>();
        videos.add(new Video("This is a new video");
        Course newCourse = new Course("course", videos);

        //this does not work with mTitle being private.
        newCourse.getVideos().get(0).mTitle = "Trying change title on chosen video";
        //but if you now change mTitle to be public, you can edit it directly. Thats the purpose of the private modifier.
    }
}

You will now see that if you change the modifier on mTitle to public, you can change the title directly, if its private, you have to go through another method, a setter or through a method in the List-class.

A modifier is just so that you can or can not access a variable/method directly, but you might be able to in-directly, through another method. Like setters and alike.

Did this make it clearer? :-)

Chris

Answer 3 · 2017-07-31T08:37:02Z

July 31, 2017 8:37am

I think you are mixing the "private"modifier with the "final" keyword. Final makes a variable "un-editable". The "private" just makes it so you cannot access the variable outside the class.

private: can be altered (if not final) but cannot be accessed directly outside its own class.
public: can be altered (if not final) and can be accessed outside its own class.
protected: can be altered (if not final) and can be accesses outside its own class if
           the classes are in the same package. or if it is a subclass of the protected class.

Hope this made things a little clearer. Please up-vote if they did :-)

Chris

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Vladislav Kobyakov

Vladislav Kobyakov

Why am I able to access the list of videos that is private?

3 Answers

Chris Tvedt

Chris Tvedt

Vladislav Kobyakov

Vladislav Kobyakov

Chris Tvedt

Chris Tvedt

Vladislav Kobyakov

Vladislav Kobyakov

Chris Tvedt

Chris Tvedt

Chris Tvedt

Chris Tvedt

Vladislav Kobyakov

Vladislav Kobyakov

Chris Tvedt

Chris Tvedt