PIC20A notes abstract class

Abstract class

You msut read Writing Abstract Classes and Methods in Java Tutorial.
Shape.java

public abstract class Shape {
    public abstract double area();
    public abstract double perimeter();
    
    /** return the ratio of area to perimeter */
    public double areaToPerimeterRatio() {
        return area()/perimeter();
    }
}

You can declare a method abstract. If you declare a method abstract, you don't have to implement the method. e.g. public abstract double area();
If a class has at least one abstract method (either in its class body or in its super class), then the class must be declared abstract. Fail to do this will cause a compiling error. e.g public abstract class Shape
You can use abstract methods as other methods, e.g. in public double areaToPerimeterRatio().
You can have constructors for Shape class. You can declare a Shape but you can't create an instance of Shape. e.g Shape shape is legal but Shape shape = new Shape() is not.

Shape2.java

public abstract class Shape2 extends Shape {
    /** return the ratio of area to the square of the perimeter */
    public double areaToPerimeterSquareRatio() {
        return area()/(perimeter()*perimeter());
    }
}

You can extends an abstract class
If the subclass does not implement all the abstract methods, it must be declared abstract

Rectangle.java

public class Rectangle extends Shape {
    private double width;
    private double height;
    
    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
    
    public double area() {
        return width*height;
    }
    
    public double perimeter() {
        return 2*(width+height);
    }
}

Circle.java

public class Circle extends Shape {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    public double area() {
        return Math.PI*radius*radius;
    }
    
    public double perimeter() {
        return 2*Math.PI*radius;
    }
}

both Rectangle and Circle classes extend Shape
they both overwrite area(), perimeter(), so they are not abstract classes.

ShapeTest.java

public class ShapeTest {
    public static void main(String[] args) {
        Rectangle r = new Rectangle(3,5);
        System.out.println("Area: " + r.area() + "  Perimeter: " + r.perimeter() + 
                                          "  Ratio: " + r.areaToPerimeterRatio());
        Circle c = new Circle(10.5);
        System.out.println("Area: " + c.area() + "  Perimeter: " + c.perimeter() + 
                                         "  Ratio: " + c.areaToPerimeterRatio());
    }
}

Output

Area: 15.0  Perimeter: 16.0  Ratio: 0.9375
Area: 346.36059005827474  Perimeter: 65.97344572538566  Ratio: 5.25

Both Circle and Rectangle can use areaToPerimeterRatio() method from the super class Shape