java-generic-inheritance

Java: Generic Inheritance

I am creating a class hierarchy of geometrical Shapes. This is my base class:

abstract class Shape {
  public abstract double getArea();
}

And this is a circle:

class Circle extends Shape {
  public final double radius;
  public Circle(double radius) {
    this.radius = radius;
  }

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

There are other shapes, like Rectangles, Squares, etc.

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I also wrote a utility class ShapeUtilities. One of those utilities is a function that computes the total area of a list of shapes:

public static double sumAllAreas(List<Shape> shapes) {
  double totalArea = 0.0;
  for (Shape shape : shapes)
    totalArea += shape.getArea();
  return totalArea;
}

This seems to work fine:

List<Shape> shapes = List.of(new Circle(2), new Rectangle(3, 3), new Square(2));
double totalArea = ShapeUtilities.sumAllAreas(shapes); // 25.567

But when I try to call it with a List<Circle>, the code does not compile, even though Circle inherits from Shape :(

List<Circle> circles = List.of(new Circle(2), new Circle(1), new Circle(5));
double totalArea = ShapeUtilities.sumAllAreas(circles); // compilation error !

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Can you fix the code so that sumAllAreas() works with Shape and all its subtypes?