Question

Problem Description

I have an abstract Paper class that contains common properties of all papers and one or more child classes of paper that add additional information for that type of paper. I then have a HashMap<String, Paper> to store multiple papers.

My application allows the user to update a paper by providing a the pid and then supplying the attributes and values to update. The issue I am having is how do I update the properties on the sub classes when all I have is the super class.

What is the best way/practice to handle this situation?

Class Structure

public abstract class Paper {
    String pid;
    String title;
    String author;
}


public class Publication extends Paper {
    int pages;
}

public class PHDThesis extends Paper {
    String supervisor;
}

---

My Current Attempt

This is what I currently have** and it works by using instance of; but I feel there should be a better way to do this.

import java.util.*;

public class App {
    public static abstract class Paper {

        private String title;
        private String author;

        public Paper(String title, String author) {
            this.title = title;
            this.author = author;
        }

        public void update(String title, String author) {
            this.title = title;
            this.author = author;
        }
    }


    public static class Publication extends Paper {

        private int pages;

        public Publication(int pages, String title, String author) {
            super(title, author);
            this.pages = pages;
        }

        public void update(String title, String author, int pages) {
            super.update(title, author);
            this.pages = pages;
        }

    }

    public static class PHDThesis extends Paper {

        private String supervisor;

        public PHDThesis(String supervisor, String title, String author) {
            super(title, author);
            this.supervisor = supervisor;
        }

        public void update(String title, String author, String supervisor) {
            super.update(title, author);
            this.supervisor = supervisor;
        }
    }

    public static void main(String[] args) {
        HashMap<String, Paper> papers = new HashMap<String, Paper>();

        papers.put("P001", new PHDThesis("My Super", "My PHD Title", "My Author"));
        papers.put("P002", new Publication(22, "My Pub Title", "My Author"));

        Paper p = papers.get("P001");

        if (p instanceof PHDThesis) {
            ((PHDThesis)p).update("New Title", "New author", "New Super");
        } else if (p instanceof Publication) {
            ((Publication)p).update("New Title", "New author", 33);
        }
    }
}

** reduced test code, actual code is much more complex and better laid out.

Answer 1

You can create an object called UpdateBundle with getters for each attribute.

Then the Paper class will have a method update(UpdateBundle) which each child will implement differently.

All you have to do is call that method for each child and they will know how to handle it.

On a separate note, i don’t see why the paper class is abstract. You seem to have no abstract methods in it.

public abstract class Paper {
    String pid;
    String title;
    String author;

    public void update(PaperUpdateBundle bundle)
    {
        pid = bundle.getPID();
        title = budnle.getTitle();
        author = bundle.getAuthor();
    }
}


public class Publication extends Paper {
    int pages;

    public void update(PaperUpdateBundle bundle)
    {
       super.update(bundle);
       pages = bundle.getPages();
    }
}

public class PHDThesis {
    String supervisor;


    public void update(PaperUpdateBundle bundle)
    {
       super.update(bundle);
       supervisor = bundle.getSupervisor();
    }
}

public interface PaperUpdateBundle
{
    String getPID();
    String getTitle();
    String getAuthor();
    int getPages();
    String getSupervisor();
}