How can I implement a shape class Java?

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.java
public 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.java
public 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 = π r^2
        return pi * Math.pow(radius, 2);
    }
    public double perimeter() {
        // P = 2πr
        return 2 * pi * radius;
    }
}
Triangle.java
public 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.java
public 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");
    }
}