Lab: a streaming catalog built on inheritance

☕ Java 17 · Building Classes on Top of Other Classes · 3/3

Goal: build a four-level hierarchy, from Object down to Documentary, and see one object use members from every level.

Setup: in an empty folder, create one file per class: Content.java, Series.java, Movie.java, Documentary.java, and StreamingCatalog.java for main. No package.

Step 1 — The parent

public class Content {
    String title;

    void play() {
        System.out.println("Now playing: " + title);
    }
}

title and play() live here, and only here.

Step 2 — A first child

public class Series extends Content {
    int seasons;
}

Series inherits title and play() and adds only seasons.

Step 3 — Use it

public class StreamingCatalog {
    public static void main(String[] args) {
        Series dark = new Series();
        dark.title = "Dark";       // from Content
        dark.seasons = 3;          // from Series
        dark.play();               // from Content
        System.out.println(dark.title + " has " + dark.seasons + " seasons");
        System.out.println(dark.toString());   // from Object
    }
}

$ javac *.java
$ java StreamingCatalog
Now playing: Dark
Dark has 3 seasons
Series@4e50df2e

The last line is Object's default toString(): the class name, @, and a hash code that will differ on your machine.

Step 4 — Grow the tree

public class Movie extends Content {
    int durationMinutes;
}

public class Documentary extends Movie {
    String topic;
}

Add to main:

        Movie inception = new Movie();
        inception.title = "Inception";
        inception.durationMinutes = 148;
        inception.play();

        Documentary ourPlanet = new Documentary();
        ourPlanet.title = "Our Planet";        // Content
        ourPlanet.durationMinutes = 50;        // Movie
        ourPlanet.topic = "Nature";            // Documentary
        ourPlanet.play();                      // Content
        System.out.println(ourPlanet.title + " is about " + ourPlanet.topic
                + " and lasts " + ourPlanet.durationMinutes + " minutes");

...
Now playing: Inception
Now playing: Our Planet
Our Planet is about Nature and lasts 50 minutes

One Documentary object holds data from Content, Movie and Documentary.

Step 5 — One place to fix

In Content.java, change the message:

        System.out.println("Now streaming: " + title);

Recompile and run: Dark, Inception and Our Planet all say Now streaming. One line changed, in one file, and every kind of content picks it up.

Full code on GitHub

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