To learn more about OOP, @nhgrif challenged me to implement a Shape abstract class (more details in code remarks below. Here is how I did it. Any and all advice appreciated!Shape.java/* nhgrif says: * Phrancis ready for inheritance/polymorphism? * Given the following abstract class: * *  public abstract class Shape { *      public abstract double area(); *      public abstract double perimeter(); *  } * * Implement a Circle, Triangle, and Rectangle class which extend the class Shape. * Ex: public class Circle extends Shape ... etc */public abstract class Shape {    public abstract double area();    public abstract double perimeter();}Rectangle.javapublic class Rectangle extends Shape {    private final double width, length; //sides    public Rectangle() {        this(1,1);    }    public Rectangle(double width, double length) {        this.width = width;        this.length = length;    }    @Override    public double area() {        // A = w * l        return width * length;    }    @Override    public double perimeter() {        // P = 2(w + l)        return 2 * (width + length);    }}Circle.javapublic class Circle extends Shape {    private final double radius;    final double pi = Math.PI;    public Circle() {        this(1);    }       public Circle(double radius) {        this.radius = radius;    }    @Override    public double area() {        // A =  r^2        return pi * Math.pow(radius, 2);    }    public double perimeter() {        // P = 2r        return 2 * pi * radius;    }}Triangle.javapublic class Triangle extends Shape {    private final double a, b, c; // sides    public Triangle() {        this(1,1,1);    }    public Triangle(double a, double b, double c) {        this.a = a;        this.b = b;        this.c = c;    }    @Override    public double area() {        // Heron's formula:        // A = SquareRoot(s * (s - a) * (s - b) * (s - c))         // where s = (a + b + c) / 2, or 1/2 of the perimeter of the triangle         double s = (a + b + c) / 2;        return Math.sqrt(s * (s - a) * (s - b) * (s - c));    }    @Override    public double perimeter() {        // P = a + b + c        return a + b + c;    }}And this is what I used to test it all, which it all works as intended:TestShape.javapublic class TestShape {    public static void main(String[] args) {        // Rectangle test        double width = 5, length = 7;        Shape rectangle = new Rectangle(width, length);        System.out.println(Rectangle width:  + width +  and length:  + length                + \nResulting area:  + rectangle.area()                + \nResulting perimeter:  + rectangle.perimeter() + \n);        // Circle test        double radius = 5;        Shape circle = new Circle(radius);        System.out.println(Circle radius:  + radius            + \nResulting Area:  + circle.area()            + \nResulting Perimeter:  + circle.perimeter() + \n);        // Triangle test        double a = 5, b = 3, c = 4;        Shape triangle = new Triangle(a,b,c);        System.out.println(Triangle sides lengths:  + a + ,  + b + ,  + c                + \nResulting Area:  + triangle.area()                + \nResulting Perimeter:  + triangle.perimeter() + \n);    }}
Implement a Shape abstract class
java;beginner;inheritance
I am generally impressed with the consistency of the implementations, neatness, etc.I have a comment about the basic premise. The Shape should not be an abstract class, but an interface. It has no concrete implementations for any methods, and making it an abstract class makes it hard to inherit from other places too. Consider this instead:public interface Shape {    public double area();    public double perimeter();}In addition to that concern, the following are also things you should consider:The pi field on your Circle class should be private. There's no need to re-expose an already public constant in another way.I would avoid the 'unit' default constructors. They don't help with anything. When would someone want to call new Triangle() and not want to have the dimensions?Your classes are missing a toString() method. These are useful for many reasons, especially debugging.You don't have a plan for out-of-bound dimensions. What will you do with negative, NaN, or infinite input?