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8. Object-Oriented Programming: Method Overriding, Polymorphism

• The Java Tutorials - Overriding and Hiding Methods.
• The Java Tutorials - Polymorphism.
• w3schools - Java Polymorphism.
• w3schools - Java Abstraction.
• w3schools - Java Interface.

Note: The dictionary definition of polymorphism refers to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming and languages like the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class.

1) Create and test the following classes: Circle, Rectangle, Square. Implement the necessary methods: getXxx, setXxx, toString.

Class diagram of created classes:

Examples:

• Circle.java.
• Rectangle.java.
• Square.java.

2) Create ShapeUtils class, implement the ShapeUtils.printShapes method, as shown below.

public static void printShapes(Object[] shapes) {
	for (Object s : shapes) {
		System.out.println(s);
	}
}

3) Use the following Main class to test implemented functionality:

public class Main {
	public static void main(String[] args) {
		Object[] shapes = { new Circle(10), new Rectangle(20, 30), new Square(40) };
		ShapeUtils.printShapes(shapes);
	}
}

You should obtain the following result:

Circle: radius = 10.0
Rectangle: width = 20.0, height = 30.0
Square: width = 40.0

Note: We have: Circle, Rectangle, and Square classes.

Note: We want to be able to:

• Call getArea on an instance of any of three, even if we do not know which of the three types it is.
• Make an array of mixed shapes and calculate the sum of the areas.
• Make this array-summing method flexible enough to handle future types of shapes (Triangle, Ellipse, etc.).

4) Implement the ShapeUtils.sumAreas method, as shown below.

	public static double sumAreas(Object[] shapes) {
		double sum = 0;
		for (Object s : shapes) {
			// sum = sum + s.getArea(); // Illegal! Object does not have getArea
		}
		return (sum);
	}

5) Try to make this method workable. It should return the sum of the areas of the shapes. Modify Main class to test the ShapeUtils.sumAreas method:

Double sum = ShapeUtils.sumAreas(shapes);
System.out.format("%.2f", sum);

If you are using previously created shapes, you should obtain the following result:

2514.16

Hint: Use the super entity (e.g. Shape) with declared getArea() as the parent for your “geometrical” classes.

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