AP Computer Science Exam Question

Consider the following Java code:

public class Shape {
    protected double area;
    
    public Shape(double area) {
        this.area = area;
    }
    
    public double getArea() {
        return area;
    }
    
    public String displayShape() {
        return "This is a shape";
    }
}

public class Circle extends Shape {
    private double radius;
    
    public Circle(double radius) {
        super(Math.PI * radius * radius);
        this.radius = radius;
    }
    
    @Override
    public double getArea() {
        return super.getArea();
    }
    
    public double getRadius() {
        return radius;
    }
    
    @Override
    public String displayShape() {
        return "This is a circle";
    }
}

Explain the concepts of inheritance and polymorphism demonstrated in the given code.

Answer

Inheritance: Inheritance is a key concept in object-oriented programming where a new class inherits properties and behaviors (methods) from an existing class. In the given code, the class Circle is inheriting from the class Shape. This means that the Circle class has access to the properties and methods defined in the Shape class, and it can also override these methods with its own implementation, allowing for code reuse and extension.

Polymorphism: Polymorphism is the ability of a subclass to redefine a method that is already defined in its superclass. In the given code, the Circle class overrides the getArea method from the Shape class. This allows instances of Circle to use the overridden getArea method instead of the one defined in Shape, demonstrating polymorphism.

Additionally, the displayShape method is also overridden in the Circle class. This allows instances of Circle to use the overridden displayShape method instead of the one defined in Shape, further demonstrating polymorphism.

Overall, the code demonstrates how inheritance and polymorphism work together to create a more flexible and reusable object-oriented design.