Circle.java

public class Circle extends Shape {

    public double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double calcArea() {
        return Math.PI * this.radius * this.radius;
    }

    @Override
    public double calcCircumference() {
        return 2 * Math.PI * this.radius;
    }
}

Main.java

import java.util.ArrayList;

public class Main {

    public static void main(String[] args) {
        ArrayList<Shape> shapes = new ArrayList<>();
        shapes.add(new Rectangle(10, 5.25));
        shapes.add(new Circle(10));
        shapes.add(new Circle(11.153));
        shapes.add(new RATriangle(10, 10));
        shapes.add(new RATriangle(3.5, 7.5));

        double totalArea = 0;
        double totalCirc = 0;
        for (int i = 0; i < shapes.size(); i++) {
            Shape shape = shapes.get(i);
            totalArea += shape.calcArea();
            totalCirc += shape.calcCircumference();
        }
        System.out.println("Average area: " + totalArea / shapes.size());
        System.out.println("Average circ: " + totalCirc / shapes.size());
    }
}

RATriangle.java

public class RATriangle extends Shape {

    public double a;
    public double b;

    public RATriangle(double a, double b) {
        this.a = a;
        this.b = b;
    }

    @Override
    public double calcArea() {
        return (this.a * this.b) / 2;
    }

    @Override
    public double calcCircumference() {
        return this.a + this.b + Math.sqrt(this.a * this.a + this.b * this.b);
    }
}

Rectangle.java

public class Rectangle extends Shape {

    public double width;
    public double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double calcArea() {
        return this.width * this.height;
    }

    @Override
    public double calcCircumference() {
        return 2 * (this.width + this.height);
    }
}

Shape.java

public class Shape {

    public double calcArea() {
        return 0;
    }

    public double calcCircumference() {
        return 0;
    }
}