shapes_Circle.java

package org.stepic.java.shapes;

/**
 * Created by vitaly on 24/09/16.
 */
public class Circle extends Shape {
    private final Point center;
    private final double radius;

    public Circle(Point center, double radius, Color color) {
        super(color);
        this.center = center;
        this.radius = radius;
    }

    public Point getCenter() { return center; }

    public double getRaius() { return radius; }

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

    @Override
    public String toString() {
        return "Circle {" +
                "center=" + center +
                ", radius=" + radius +
                ", color=" + getColor() +
                "}";
    }
}

shapes_Color.java

package org.stepic.java.shapes;

/**
 * Created by vitaly on 24/09/16.
 */
public enum Color {
    BLACK,
    WHITE,
    RED,
    GREEN,
    BLUE
}

shapes_Main.java

package org.stepic.java.shapes;

import java.util.DoubleSummaryStatistics;

/**
 * Created by vitaly on 24/09/16.
 */
public class Main {
    public static void main(String[] args) {
        Circle circle = new Circle(
                new Point(0, 0), 1, Color.BLACK);

        Triangle triangle = new Triangle(
                new Point(0, 0), new Point(1, 0), new Point(0, 1), Color.RED);

        Square square = new Square(
                new Point(5, 5), 2, Color.BLUE);

        Shape shape = triangle;
        Object object = triangle;
        triangle = (Triangle) object;

        Shape[] shapes = {circle, triangle, square};
        printArrayElements(shapes);

        Shape maxShape = findShapeWithMaxArea(shapes);
        System.out.println("Shape with max area: " + maxShape);
    }

    private static void printArrayElements(Object[] objects) {
        for (int i = 0; i < objects.length; i++) {
            System.out.println(i + ": " + objects[i]);
        }
    }

    private static Shape findShapeWithMaxArea(Shape[] shapes) {
        Shape maxShape = null;
        double maxArea = Double.NEGATIVE_INFINITY;
        for (Shape shape: shapes) {
            double area = shape.getArea();
            if (area > maxArea) {
                maxArea = area;
                maxShape = shape;
            }
        }
        return maxShape;
    }
}

shapes_Point.java

package org.stepic.java.shapes;

/**
 * Created by vitaly on 24/09/16.
 */
public class Point {
    private final double x;
    private final double y;

    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public double getX() { return x; }

    public double getY() { return y; }

    @Override
    public String toString() { return "(" + x + ", " + y + ")"; }
}

shapes_Shape.java

package org.stepic.java.shapes;

/**
 * Created by vitaly on 24/09/16.
 */


public class Shape {
    private final Color color;

    public Shape(Color color) { this.color = color; }

    public Color getColor() { return color; }

    public double getArea() { return Double.NaN; }

}

shapes_Square.java

package org.stepic.java.shapes;

/**
 * Created by vitaly on 24/09/16.
 */
public class Square extends Shape {
    private final Point corner;
    private final double size;

    public Square(Point corner, double size, Color color) {
        super(color);
        this.corner = corner;
        this.size = size;
    }

    public Point getCorner() { return corner;}

    public double getSize() { return size; }

    @Override
    public double getArea() { return size * size; }

    @Override
    public String toString() {
        return "Square{" +
                "corner=" + corner +
                ", size=" + size +
                ", color=" + getColor() +
                "}";
    }
}

shapes_Triangle.java

package org.stepic.java.shapes;

/**
 * Created by vitaly on 24/09/16.
 */
public class Triangle extends Shape {
    private final Point a;
    private final Point b;
    private final Point c;

    public Triangle(Point a, Point b, Point c, Color color) {
        super(color);
        this.a = a;
        this.b = b;
        this.c = c;
    }

    @Override
    public double getArea() {
        return Math.abs((a.getX() - c.getX()) * (b.getY() - c.getY())
                - (b.getX() - c.getX()) * (a.getY() - c.getY())) / 2;
    }

    @Override
    public String toString() {
        return "Triangle{" +
                "a=" + a +
                ", b=" + b +
                ", c=" + c +
                ", color=" + getColor() +
                "}";
    }
}