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!

Andreas Dolk · Accepted Answer · 2020-05-24 11:54:20Z

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.

Alex Rmcf · Accepted Answer · 2020-05-24 11:35:27Z

0

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.

answered May 24, 2020 at 11:35

Alex Rmcf

9248 silver badges14 bronze badges

Comments

Simon Kocurek · Accepted Answer · 2020-05-24 11:45:46Z

0

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;
}

edited May 24, 2020 at 11:45

answered May 24, 2020 at 11:40

Simon Kocurek

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