IT 114: Introduction to Java

IT 114: Introduction to Java
Class 35

Topics

Review

Creating Superclasses
The Movie Subclass

New Material

The Lectures Class
Lectures Constructor
Accessors and Mutators for Lectures
The VideoLibrary Class
VideoLibrary Version 1
Adding a Movie Object
Adding a Lectures Object
Listing Library Objects
Polymorphism

Homework 11

Homework 11 is due this Sunday at 11:59 PM.

You will find the assignment here.

This will be the last homework assignment.

If you have a problem or a question, make a post on the Class Discussion Area.

Review

Creating Superclasses

In practice, you almost never create an object of the superclass
The superclass is just a place to store attributes and methods the subclasses will use
The superclass contains methods and fields that are shared by all subclasses
You can count on having certain methods and attributes in each subclass you create

The Movie Subclass

A movie entry will have the four attributes of the Video class
- collection_no
- title
- length
- format
But it will also have the following additional attributes
- director
- studio
- actors
The fields director and studio will be set by the constructor
But actors can have many entries
So we'll make it an ArrayList of strings
The constructor will create the object
But adding individual actors will be left to a mutator method we will define later

Here are the fields for Movie

    private String director;
    private String studio;
    private ArrayList actors;

New Material

The Lectures Class

Now we need another subclass to handle videos from The Great Courses
Each course comes on a number of DVDs
And each DVD has a number of lectures
So we need the following new fields
- course
- disc_no
- instructor
- lectures
lectures will be an ArrayList since there are many lectures on each disc

Lectures Constructor

The title for each disc in a series presents a challenge
Since there are more than one disc for each lecture series the we can't just use the course name
Instead we will use a combination of the course name and the disc number

Here is our first attempt at the constructor

import java.util.ArrayList;

public class Lectures1 extends Video2 {
    private String course;
    private int disc_no;
    private String instructor;
    private ArrayList<String> lectures;

    public Lectures1 (int collection_no, String title, int length, 
        String format, String course, int disc_no, String instructor){
        super(collection_no, title, length, format);
        this.course     = course;
        this.disc_no    = disc_no;
        this.instructor = instructor;
        this.lectures   = new ArrayList<>();
    }
}

Here is the driver

public class Lectures1Driver {
    public static void main (String[] args){
        Lectures1 l = new Lectures1(2, "Origins of Great Ancient Civilizations - Disc 1", 
            180, "DVD", "Origins of Great Ancient Civilizations", 1, "Kenneth W. Harl");
        System.out.println(l);
    }
}

$ java Lectures1Driver
#1: Origins of Great Ancient Civilizations - Disc 1, 180 minutes, DVD

This code works, but there is a problem
The user has to enter the course name and the disc number twice
Once in the title
And again in the field values
This is unacceptable
To fix this we will calculate the title inside the call to super
And remove title from the parameter list

Here is an improve constructor

    public Lectures2 (int collection_no, int length, 
        String format, String course, int disc_no, String instructor){
        super(collection_no, course + " - Disc " + disc_no, length, format);
        this.course     = course;
        this.disc_no    = disc_no;
        this.instructor = instructor;
        this.lectures   = new ArrayList<>();
    }
}

And a new driver

public class Lectures2Driver {
    public static void main (String[] args){
        Lectures2 l = new Lectures2(2, 180, "DVD", "Origins of Great Ancient Civilizations", 
            1, "Kenneth W. Harl");
        System.out.println(l);
    }
}

$ java Lectures2Driver 
#1: Origins of Great Ancient Civilizations - Disc 1, 180 minutes, DVD

Accessors and Mutators for Lectures

Adding getters is easy

	public String course(){
        return course;
    }

    public int disc_no() {
        return disc_no;
    }

    public String instructor(){
        return instructor;
    }

And toString only has to add instructor

Because course and disc_no are included in the title

    public String toString() {
        return super.toString() + ", " + instructor;
    }

Now we need a setter and a getter for lectures

    public void add_lecture(String lecture){
        lectures.add(lecture);
    }

    public String lectures(){
        String value = "";
        for (int i = 0; i < lectures.size(); i++){
            value += lectures.get(i) + "\n";
        }
        return value;
    }

We also need a driver

public class Lectures3Driver {
    public static void main (String[] args){
        Lectures3 l = new Lectures3(2, 180, "DVD", "Origins of Great Ancient Civilizations", 
            1, "Kenneth W. Harl");
        System.out.println(l);
        l.add_lecture("Cradles of Civilization");
        l.add_lecture("First Cities of Sumer");
        l.add_lecture("Mesopotamian Kings and Scribes");
        System.out.println("Course:      " + l.course());
        System.out.println("Disc number: " + l.disc_no());
        System.out.println("Instructor:  " + l.instructor());
        System.out.println("Lectures:\n" +  l.lectures());
    }
}

$ java Lectures3Driver
#2: Origins of Great Ancient Civilizations - Disc 1, 180 minutes, DVD, Kenneth W. Harl
Course:      Origins of Great Ancient Civilizations
Disc number: 1
Instructor:  Kenneth W. Harl
Lectures:
Cradles of Civilization
First Cities of Sumer
Mesopotamian Kings and Scribes

You can see the final version of this class here

The VideoLibrary Class

In order to create an application that uses the Video subclasses, we need another class with a main method
So this will be
We will call this class VideoLibrary
It will allow the user to add new videos to the collection
And list them
It will store the Video objects in an ArrayList

VideoLibrary Version 1

For a first crack at this application let
- Prompt the user for input
- Check that the input is valid
- Quit when the user says they are done
The main method has a while loop asking the user to say what they want to do
The user input is checked against a list of possible responses
The loop ends when the user enters "D" or "d"

Here is the code

import java.util.ArrayList;
import java.util.Scanner;

public class VideoLibrary1 {
    private static        ArrayList<Video> library = new ArrayList<>();
    private static final  String [] action_values  = {"m", "l", "s", "d"};
    private static final  String prompt            = "M(ovie), L(ecture), S(ee entries), D(one): ";

    public static void main (String[] args){
        Scanner console = new Scanner(System.in);
        boolean done = false;
        while (! done){
            System.out.print(prompt);
            String action = console.next();
            action        = action.trim();        // remove any leading or trailing whitespace
            action        = action.toLowerCase(); // only lowercase letters
            if (! action_valid(action)){
                System.out.println(action + " is not a valid choice");
                continue;
            } else if (action.equals("d")) {
                System.out.println("Exiting");
                done = true;
            }
        }
    }

    private static boolean action_valid(String action){
        action = action.trim();
        boolean result = false;
        for (int i = 0; i < action_values.length; i++){
            if (action.equals(action_values[i])) {
                return true;
            }
        }
        return result;
    }

}

When we run it we get

$ java VideoLibrary1.java 
M(ovie), L(ecture), S(ee entries), D(one): m
M(ovie), L(ecture), S(ee entries), D(one): M
M(ovie), L(ecture), S(ee entries), D(one): l
M(ovie), L(ecture), S(ee entries), D(one): L
M(ovie), L(ecture), S(ee entries), D(one): s
M(ovie), L(ecture), S(ee entries), D(one): S
M(ovie), L(ecture), S(ee entries), D(one): D
Exiting

Adding a Movie Object

Next we need to create the method add_movie
Getting input from the command line can be complicated in Java
So this method will instead ask the user for a filename that contains the data
Then extract the information it needs from that file

Here is the method

    private static Movie add_movie(Scanner console)throws FileNotFoundException{
        System.out.print("Filename: ");
        String filename   = console.next();
        File file         = new File(filename);
        Scanner input     = new Scanner(file);
        int collection_no = Integer.parseInt(input.nextLine());
        String title      = input.nextLine();
        int length        = Integer.parseInt(input.nextLine());
        String format     = input.nextLine();
        String director   = input.nextLine();
        String studio     = input.nextLine();
        Movie movie       = new Movie(collection_no, title, length, format, director, studio);
        System.out.println(movie);
        return movie
    }

Now we have to add an else if clause to our while loop

            } else if (action.equals("m")) {
                Video2 movie = add_movie(console);
                library.add(movie);
            }

The to test this we create the file forbidden_planet.txt
```
1
Forbidden Planet
98
DVD
Fred McLeod Wilcox
MGM
```

Now we can add a Movie object

$ java VideoLibrary
M(ovie), L(ecture), S(ee entries), D(one): m
Filename: forbidden_planet.txt
Forbidden Planet
MGM
#1: Forbidden Planet, 98 minutes, DVD, Fred McLeod Wilcox, MGM
M(ovie), L(ecture), S(ee entries), D(one): d
Exiting

Adding a Lectures Object

We will create a Lectures object in the same way

Here is add_lectures

    private static Lectures add_lectures(Scanner console)throws FileNotFoundException{
        System.out.print("Filename: ");
        String filename   = console.next();
        File file         = new File(filename);
        Scanner input     = new Scanner(file);
        int collection_no = Integer.parseInt(input.nextLine());
        int length        = Integer.parseInt(input.nextLine());
        String format     = input.nextLine();
        String course     = input.nextLine();
        int disc_no       = Integer.parseInt(input.nextLine());
        String instructor = input.nextLine();
        Lectures lectures = new Lectures(collection_no, length, format, course, disc_no, instructor);
        System.out.println(lectures);
        return lectures;
    }

Then we add the following else if clause to the while loop in the main method

            } else if (action.equals("l")) {
                Video2 lectures = add_lectures(console);
                library.add(lectures);
            }

We use the file ancient_civilizations_1.txt to test this addition

2
180
DVD
Origins of Great Ancient Civilizations
1
Kenneth W. Harl

And get the following when we test the code

$ java VideoLibrary4
M(ovie), L(ecture), S(ee entries), D(one): l
Filename: ancient_civilizations_1.txt 
#2: Origins of Great Ancient Civilizations - Disc 1, 180 minutes, DVD, Kenneth W. Harl
M(ovie), L(ecture), S(ee entries), D(one): d
Exiting

Listing Library Objects

The last method we need to add is list_videos

    private static void list_videos(){
        for (int i = 0; i < library.size(); i++){
            System.out.println(library.get(i));
        }
    }

Then we add the else if clause to the while loop in the main method

            } else if (action.equals("s")) {
                list_videos();
            }

Running the code we get

$ java VideoLibrary5
M(ovie), L(ecture), S(ee entries), D(one): m
Filename: forbidden_planet.txt
#1: Forbidden Planet, 98 minutes, DVD, Fred McLeod Wilcox, MGM
M(ovie), L(ecture), S(ee entries), D(one): l
Filename: ancient_civilizations_1.txt
#2: Origins of Great Ancient Civilizations - Disc 1, 180 minutes, DVD, Kenneth W. Harl
M(ovie), L(ecture), S(ee entries), D(one): s
#1: Forbidden Planet, 98 minutes, DVD, Fred McLeod Wilcox, MGM
#2: Origins of Great Ancient Civilizations - Disc 1, 180 minutes, DVD, Kenneth W. Harl
M(ovie), L(ecture), S(ee entries), D(one): d
Exiting

Polymorphism

If you look carefully at the code above you will see something interesting

Look at the following lines

    private static ArrayList<Video2> library       = new ArrayList<>();
    ...
            } else if (action.equals("m")) {
                Video2 movie = add_movie(console);
                library.add(movie);
            } else if (action.equals("l")) {
                Video2 lectures = add_lectures(console);
                library.add(lectures);
            }

library is an ArrayList of Video2 objects
Yet we can add both Movie and Lectures objects to it
That's not too surprising since both classes are subclasses of Video2

But there is a hidden subtlety in the list_videos method

    private static void list_videos(){
        for (int i = 0; i < library.size(); i++){
            System.out.println(library.get(i));
        }
    }

In order to print both objects you need to call the toString method of two different classes
This is an example of a feature of inheritance called polymorphism
Polymorphism occurs when we call a method in the base class when the object we are using is actually an instance of a derived class
Instead of calling the method in the base class Java will use a method of the same name in the subclass
So in list_videos we are looping through an ArrayList of Video2 objects
And calling the toString method on each object
But instead of calling the toString method of Video2 on each object
We call the toString method of Movie on Movie objects
and the toString method of Lectures on Lectures objects