TD3 — Inheritance, Interfaces & Polymorphism

← Part 1 Overview

S7 Inf A3 — Java for Graphical and Mobile Programming Stéphane Derrode — Centrale Lyon
Part 1 — Java Fundamentals
Duration: 2h · Continue in your Maven project (td1 or create td3)

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Objectives

• Build a class hierarchy using extends and abstract
• Implement multiple interfaces on a single class
• Exploit polymorphism to write generic algorithms
• Use Comparable<T> to sort objects
• Practice with static fields and utility classes

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Part 1 — Shape Hierarchy (50 min)

1.1 Abstract base class Shape

Create src/main/java/com/s7infa3/shapes/Shape.java.

package com.s7infa3.shapes;

public abstract class Shape implements Comparable<Shape> {
    protected String color;

    public Shape(String color) {
        this.color = color;
    }

    public String getColor() { return color; }

    // Abstract — every concrete shape must implement these
    public abstract double area();
    public abstract double perimeter();

    // Concrete — available to all subclasses
    public String describe() {
        return String.format("%s %s: area=%.2f, perimeter=%.2f",
            color, getClass().getSimpleName(), area(), perimeter());
    }

    // Natural ordering by area (ascending)
    @Override
    public int compareTo(Shape other) {
        return Double.compare(this.area(), other.area());
    }

    @Override
    public String toString() {
        return describe();
    }
}

1.2 Interface Drawable

Create src/main/java/com/s7infa3/shapes/Drawable.java.

package com.s7infa3.shapes;

public interface Drawable {
    void draw();                        // print a text representation
    default String getDisplayInfo() {   // default method
        return "Drawable shape";
    }
}

1.3 Concrete classes

Create the following three classes in the same package. Each must: - Extend Shape - Implement Drawable - Provide a constructor, area(), perimeter(), and draw()

Circle

Member	Formula / value
field	double radius
area()	π × r²
perimeter()	2 × π × r
draw()	prints ○ Circle(r=3.00, color=red)

Rectangle

Member	Formula / value
fields	double width, double height
area()	width × height
perimeter()	2 × (width + height)
draw()	prints ▭ Rectangle(4.00×2.00, color=blue)

Triangle

Member	Formula / value
fields	double a, double b, double c (three sides)
area()	Heron's formula: √(s(s-a)(s-b)(s-c)) where s = (a+b+c)/2
perimeter()	a + b + c
draw()	prints △ Triangle(3.00, 4.00, 5.00, color=green)
constructor	throw IllegalArgumentException if the three sides do not form a valid triangle (triangle inequality: each side must be < sum of the other two)

1.4 Test in main()

import com.s7infa3.shapes.*;
import java.util.*;

List<Shape> shapes = new ArrayList<>();
shapes.add(new Circle("red", 3.0));
shapes.add(new Rectangle("blue", 4.0, 2.0));
shapes.add(new Triangle("green", 3.0, 4.0, 5.0));
shapes.add(new Circle("yellow", 1.5));

// Polymorphism: describe() calls the right area/perimeter
for (Shape s : shapes) {
    System.out.println(s.describe());
}

// Sort by area (uses compareTo)
Collections.sort(shapes);
System.out.println("\n--- Sorted by area (ascending) ---");
for (Shape s : shapes) {
    System.out.printf("  %.2f  %s%n", s.area(), s.getClass().getSimpleName());
}

// Draw all Drawable shapes
System.out.println("\n--- Drawing ---");
for (Shape s : shapes) {
    if (s instanceof Drawable d) {   // pattern matching (Java 16+)
        d.draw();
    }
}

// Sort descending by perimeter using lambda
shapes.sort((a, b) -> Double.compare(b.perimeter(), a.perimeter()));
System.out.println("\n--- Sorted by perimeter (descending) ---");
shapes.forEach(s -> System.out.printf("  %.2f  %s%n",
    s.perimeter(), s.getClass().getSimpleName()));

Expected output (approximate):

red Circle: area=28.27, perimeter=18.85
blue Rectangle: area=8.00, perimeter=12.00
green Triangle: area=6.00, perimeter=12.00
yellow Circle: area=7.07, perimeter=9.42

--- Sorted by area (ascending) ---
  6.00  Triangle
  7.07  Circle
  8.00  Rectangle
  28.27  Circle

--- Drawing ---
○ Circle(r=3.00, color=red)
▭ Rectangle(4.00×2.00, color=blue)
△ Triangle(3.00, 4.00, 5.00, color=green)
○ Circle(r=1.50, color=yellow)

1.5 Test the triangle guard

try {
    new Triangle("red", 1.0, 2.0, 10.0); // invalid: 1+2 < 10
} catch (IllegalArgumentException e) {
    System.out.println("Caught: " + e.getMessage()); // expected
}

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Part 2 — Static Utility Class ShapeStats (20 min)

Create src/main/java/com/s7infa3/shapes/ShapeStats.java with only static methods — no instance fields, no constructor (or a private no-arg constructor to prevent instantiation).

Method	Description
totalArea(List<Shape> shapes)	Returns the sum of all areas
totalPerimeter(List<Shape> shapes)	Returns the sum of all perimeters
largest(List<Shape> shapes)	Returns the shape with the largest area
smallest(List<Shape> shapes)	Returns the shape with the smallest area
countByType(List<Shape> shapes, Class<?> type)	Returns the number of shapes of a given type

Test:

System.out.printf("Total area: %.2f%n", ShapeStats.totalArea(shapes));
System.out.println("Largest:  " + ShapeStats.largest(shapes).describe());
System.out.println("Smallest: " + ShapeStats.smallest(shapes).describe());
System.out.println("Circles: " + ShapeStats.countByType(shapes, Circle.class));

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Part 3 — Zoo Polymorphism (20 min)

3.1 Class hierarchy

Create in package com.s7infa3.zoo:

Animal (abstract)
├── field: String name
├── abstract: String speak()
├── concrete: String describe() → "Rex (Dog) says: Woof!"
│
├── Dog    extends Animal → speak() = name + " says: Woof!"
├── Cat    extends Animal → speak() = name + " says: Meow!"
└── Bird   extends Animal → speak() = name + " says: Tweet!"

Also create a Trainable interface:

public interface Trainable {
    boolean learn(String command);  // returns true if the animal can learn it
    List<String> getKnownCommands();
}

Make Dog implement Trainable: - Stores learned commands in an ArrayList<String> - learn(command) adds the command and returns true - A Cat is NOT Trainable

3.2 Test polymorphism

List<Animal> animals = new ArrayList<>();
animals.add(new Dog("Rex"));
animals.add(new Cat("Luna"));
animals.add(new Dog("Max"));
animals.add(new Bird("Tweety"));

// Polymorphic loop
for (Animal a : animals) {
    System.out.println(a.describe());
}

// Train only Trainable animals
for (Animal a : animals) {
    if (a instanceof Trainable t) {
        t.learn("sit");
        t.learn("fetch");
        System.out.println(a.getName() + " knows: " + t.getKnownCommands());
    }
}

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Part 4 — Exploration (remaining time)

4.1 Add a Square class that extends Rectangle — the constructor takes only one side length. Override toString() to print "Square(side=3.00)". Does it need to override area() and perimeter()?

4.2 Add a static int instanceCount field to Shape that counts the total number of shapes ever created. Increment it in the Shape constructor. Add Shape.getInstanceCount().

4.3 Make Triangle implement Comparable<Triangle> independently of Shape, sorting by perimeter instead of area. What problem do you encounter? How would you solve it?

4.4 (Modern Java — optional) Rewrite ShapeStats with the Streams API instead of explicit loops — the Java counterpart of Python comprehensions / sum(...):

import java.util.Comparator;

public static double totalArea(List<Shape> shapes) {
    return shapes.stream().mapToDouble(Shape::area).sum();
}

public static Shape largest(List<Shape> shapes) {
    return shapes.stream()
                 .max(Comparator.comparingDouble(Shape::area))
                 .orElse(null);
}

public static int countByType(List<Shape> shapes, Class<?> type) {
    return (int) shapes.stream().filter(type::isInstance).count();
}

Same results as your loop version, less code. See the Python → Java reference.

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Deliverable

No formal submission. Verify all main() outputs match the expected values before the end of the session.

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Common Errors

Error	Cause	Fix
Class is not abstract and does not override abstract method	Forgot to implement area() or perimeter()	Add the missing @Override methods
cannot find symbol on getClass().getSimpleName()	Called on wrong object	Check you call it on this inside the class
instanceof pattern not recognized	Java < 16	Use instanceof Drawable && ((Drawable)s).draw()
NaN for triangle area	Negative value under sqrt (invalid triangle not guarded)	Add the triangle inequality check
Collections.sort() fails	Shape doesn't implement Comparable<Shape>	Check implements Comparable<Shape> on the abstract class