SWEN20003
Object Oriented Software Development
Workshop 5
Semester 1, 2021
Workshop
This week, we are learning all about inheritance.
• Inheritance allows many classes (the subclasses, derived, or child classes) to share attributes and
methods from one class (the parent, base, or superclass).
• Subclasses automatically inherit all attributes and methods from their parent.
• Subclasses can call the parent’s constructor with super().
• Inheritance represents an is-a relationship; if Circle extends Shape, then Circle is a shape.
– This means subclasses can be upcasted to their parent class type:
Circle circle = new Circle();
Shape shape = circle; // upcast happens here
– A reference to a parent class can only be safely downcasted to the subclass type using instanceof.
if (shape instanceof Circle) {
circle = (Circle) shape;
}
– Upcasting is an example of polymorphism.
• Methods in a parent class can be overridden by subclasses. This replaces their functionality, even if
upcasting is used.
Questions
1. Run the chess example introduced in the lecture in IntelliJ. Make sure you understand how it works.
Implement the following additional chess pieces:
• Pawn, which can move two spaces vertically if it hasn’t yet moved, otherwise one space vertically
• Bishop, which can move any number of spaces diagonally
2. Consider the below class.
public class Shape {
public final double x;
public final double y;
public Shape(double x, double y) {
this.x = x;
this.y = y;
}
public double getArea() { return 0; }
public double getPerimeter() { return 0; }
public String toString() { return “Plain Shape”; }
}
1
(a) A shape has a position (x, y).
(b) Create the following subclasses:
• Rectangle, which has a width and height
• Circle, which has a radius
(c) Override the getArea() and getPerimeter() methods in your subclasses.
(d) Write appropriate toString() methods in your subclasses using the overridden methods.
(e) Make the getArea() and getPerimeter() methods abstract (in the Shape class).
(f) Write a main class. You should create a Shape array with 10 shapes (randomly chosen between
Circle and Rectangle), where their parameters are chosen randomly between 0 and 5. Loop over
this array and print the result of calling toString() on them.
3. Consider the below classes.
public class HttpRequest {
private static final short PORT = 80;
public final String address;
public final String file;
public final String method;
public HttpRequest(String address, String file, String method) {
this.address = address;
this.file = file;
this.method = method;
}
public short getPort() {
return PORT;
}
public String getAddress() {
return address;
}
public String getFullRequest() {
return String.format(“%s %s HTTP/1.1\r\n\r\n”, method, file);
}
}
public class FtpRequest {
private static final short PORT = 21;
public final String address;
public final String file;
public FtpRequest(String address, String file) {
this.address = address;
this.file = file;
}
public short getPort() {
return PORT;
}
public String getAddress() {
return address;
}
public String getFullRequest() {
return String.format(“RETR %s\u0015\u0012”, file);
}
}
2
(a) Create a Request base class with appropriate methods, including at least one abstract method.
Change the above classes to inherit from Request.
(b) Write a main method to test your code. It should accept command-line arguments (via the args
parameter of main). To test these in IntelliJ, click the main class name in the top-right (next to the
Play button), click Edit Configurations…, and add data to the Program arguments field.
Figure 1: Accessing the Edit Configurations dialog.
Figure 2: Adding command-line arguments to your project.
Your program should accept each of the following formats:
• http google.com GET /, in which case it should create an instance of HttpRequest with ad-
dress google.com, method GET, and file /
• ftp eleanorm.info swen20003.txt, in which case it should create an instance of FtpRequest
with address eleanorm.info and file swen20003.txt
It should create an appropriate instance and upcast it to Request, then print the result of calling
getFullRequest().
4. We will now use inheritance with data structures.
(a) Define a Node class, containing a public final integer (its value—) and two Nodes (the next
and previous nodes). Add a getter for each node. (The node may be null, meaning there is no
next/previous node.)
(b) Define a LinkedList class that contains a node (the root). It should have the following methods:
• Node root(): return the root node
• int length(): return the length (i.e. the number of nodes in the list)1
• void append(int value): create a new node with the appropriate value, and add it to the end
of the list
1Bonus challenge: make this O(1).
3
• void insert(int index, int value): create a new node with the appropriate value, and
insert it at the index (starting from 0)
• void remove(int n): remove the nth node (starting from 0)
(c) Define a Stack class inheriting from LinkedList. Add the following methods:
• void push(int value): add a node with the appropriate value at the end of the list
• Integer pop(): remove the last node and return its value (or null if there is no such node)
(d) Define a Queue class inheriting from LinkedList. Add the following methods:
• void enqueue(int value): add a node with the appropriate value to the end of the list
• Integer take(): remove the first node and return its value (or null if there is no such node)
5. We are ambitious Java enthusiasts and are already ready to begin creating our own small ‘graphics’
library. Our first task is to design a system to render simple shapes onto the screen. For now, we are
concerned about two types of shapes in particular: squares and triangles. A shape has a specific area
associated with it, and it can also be rendered to the screen. You are not required to implement
any rendering logic. For simplicity, you are to print a description of the shape to be rendered to the
console instead of rendering anything to the screen.
A shape also has a colour associated with it. We will be using the the RGB colour system which specifies
a colour through three values: red, green, blue. The red, green, and blue values of a colour must be within
the range of 0-255 (inclusive) at all times. If a colour is not specified, a shape’s default colour is black
(red = 0, green = 0, blue = 0). Your Colour implementation should be immutable. The system should
be compatible with the following driver class:
public class Driver {
private static final int MAX_SHAPES = 4;
public static void main(String[] args) {
Shape[] shapes = new Shape[MAX_SHAPES];
// Black rectangle (red=0, green=0, blue=0) with width 20.52px and height 50px
shapes[0] = new Rectangle(20.52, 50);
// Crimson-coloured triangle (r=220, g=20, b=60) with base 392.2px and height 0.01px
shapes[1] = new Triangle(392.2, 0.01, new Colour(220, 20, 60));
// White (r=255, g=255, b=255) rectangle with width 50px and height 50.3px
shapes[2] = new Rectangle(50, 50.3, Colour.WHITE);
// Black triangle (red=0, green=0, blue=0) with base 10px and height 20.12px
shapes[3] = new Triangle(10, 20.12);
for (Shape shape : shapes) {
shape.render();
}
double average = 0;
for (int i = 0; i < MAX_SHAPES; i++) { average = (average * i + shapes[i].getArea()) / (i + 1); } System.out.format("Average area of rendered shapes: %.2f\n", average); } } Example output: Drawing a Rectangle with colour:(0, 0, 0) and area:1026.00px2 Drawing a Triangle with colour:(220, 20, 60) and area:1.96px2 Drawing a Rectangle with colour:(255, 255, 255) and area:2515.00px2 Drawing a Triangle with colour:(0, 0, 0) and area:100.60px2 Average area of rendered shapes: 910.89px2 4