Use inheritance when creating objects

Question

I need a bit of a hand, I'm writing a program that's fundamentally a little shop. I have different items here under the class Content, which looks like this:

public abstract class Content {

    // Content Attributes
    private String contentID;
    private String contentName;
    private double applicationPrice;
    private int downloadCount;

    //private Comments[] comments = new Comments[100];
    //private int commentCount= 0;


    // Constructor
    public Content(String ID, String Name, double price) {
        this.contentID = ID;
        this.contentName = Name;
        this.applicationPrice= price;

    }

    public Content(String ID, String Name) {
        this.contentID = ID;
        this.contentName = Name;
        this.applicationPrice= 0;

    }


    // Accessor Methods
    public String getID() {
        return contentID;
    }
    public double getApplicationPrice() {
        return applicationPrice;
    }


    public String getContentName() {
        return contentName;
    }

    public void download() {
        //code to make a download happen
        //++ download

        return;
    }

    public void addComment() {
        //scanner input comment

        //this.comments[commentCount]= my new comment;
    }




}

Under content I have other classes that extend it such as Application and Publication (Publication is further extended by Book and Magazine). As an example Application looks like this:

import java.util.Scanner;

public class Application extends Content {
    // attributes

    private String osType;

    //constructor

    public Application(String ID, String name, double price, String os) {

        super(ID, name, price);

        this.osType = os;
    }

    public Application(String ID, String name, String os) {
        super(ID, name);
        this.osType = os;

    }


    //accessor methods


    public String getOsType() {
        return this.osType;
    }




    }

My issue is that I think there's a problem with my fundamental understanding of inheritance in practice. I want to understand how best I can allow my user to create these objects. So far I have a MyShop class where they can create an application:

import java.util.Scanner;

public class MyShop {
    // instance variables that you need (marked as private)

    // declare a private array to store content here
    private Content[] contentList = new Content[100];
    private int contentCount = 0;

    // declare a private array to store users here

    public MyShop() {

    }



    public void addApplication(){

        Scanner console = new Scanner(System.in);
        String appID;
        String appName;
        double appPrice;
        String appOS;
        int control = 0;

        while(control== 0) {
            try {
                System.out.println("What is the ID of the Application?");
                appID = console.next();

                System.out.println("What is the name of the Application?");
                appName = console.next();

                System.out.println("What is the price of the Application?");
                appPrice = console.nextDouble();

                System.out.println("What is the minimum OS of the Application?");
                appOS = console.next();

                Application newApplication= new Application(appID, appName, appPrice, appOS);
                System.out.println(newApplication.getApplicationPrice());
                contentList[contentCount] = newApplication;
                contentCount++;
                console.close();
                control = 1;
            }catch(Exception e) {
                System.out.println("invalid input try again!"); 
            }

        }


    }




    public void showContent() {
        System.out.println(contentList);
    }


//  public void addUser(....) {
//      // code here
//  }


    // space for other possible methods here
}

Is there a way I can use inheritance to avoid having to have a addBook() and addMagazine() in addition to addApplication() and instead have something generic like addContent()?

Please let me know if my question is unclear or if more information is needed!

Answer 1

Rethink your model, it should match how it works in the real world. Your shop sells items, it's not a manufactorer. So the shop class will never create an application. It will have a pool of sellable things. Products. A stock. And some products are applications, some are books, some are magazines, cars, shampoos, whatever.

Rename Content to StockItem. contentId would be itemId, contentName itemName or just name. A model is easier (far easier!) to understand if it matches the real world.

Then, add a Stock class, which has all the items (a List<StockItem> items for a very simple first approach). The Stock will have some methods, like

public StockItem findById(String stockItemId);
public List<StockItem> findAllByName(String name);
public void addItem(StockItem newItem);
public void itemsSold(String stockItemID, int quantity);
//...

Now if you want to add a book to the stock, simply call

addItem(new Book("Clean Code", "Robert Martin"));

while this book is a stockitem:

public class Publication extends StockItem { ... }
public class Book extends Publication { ... }

Later, you'd learn that this simplified model is not production ready, there are better solutions for real shops, but the message would be: start with a good model. Look at the real world, describe what you see and use exactly that to build your classes and add the methods.

Answer 2

Factory pattern solves exactly this problem.
You can create a method:

<T extends Content> void addContent(Class<T> contentClass) {
  if (Application.class.equals(contentClass)) {
  ... 
  } else if ...

  } else {
    throw new RuntimeException("Unknown Content type");
  }
}

Or

void addContent(ContentType contentType) {
  switch (contentType) {
    case APPLICATION:
       ...
    case ...
    default:
      throw new RuntimeException("Unknown Content type")
  }
}

public enum ContentType {
  APPLICATION, BOOK, MAGAZINE;
}

Answer 3

I think you can make interface with method addContent(int mode) with 3 integers inside with names APPLICATION, BOOK, MAGAZINE and then implement it to your myShop class. According to int mode you make realization of method addContent.