feat: kata/java-generic-inheritance

docs/README.md

@@ -1,7 +1,7 @@
 # Codewars Handbook ☕️🚀
 
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@@ -25,7 +25,7 @@ slug.
 
 | [1 kyu](/kata/1-kyu/index.md) | [2 kyu](/kata/2-kyu/index.md) | [3 kyu](/kata/3-kyu/index.md) | [4 kyu](/kata/4-kyu/index.md) | [5 kyu](/kata/5-kyu/index.md) | [6 kyu](/kata/6-kyu/index.md) | [7 kyu](/kata/7-kyu/index.md) | [8 kyu](/kata/8-kyu/index.md) | [beta](/kata/beta/index.md) | [retired](/kata/retired/index.md) |
 |:-----------------------------:|:-----------------------------:|:-----------------------------:|:-----------------------------:|:-----------------------------:|:-----------------------------:|:-----------------------------:|:-----------------------------:|:---------------------------:|:---------------------------------:|
-| 0 | 1 | 2 | 26 | 48 | 431 | 584 | 218 | 56 | 79 |
+| 0 | 1 | 2 | 26 | 48 | 431 | 585 | 218 | 56 | 79 |
 
 **Note:** The source code is written in Java 17 and may use language features that are incompatible
 with Java 8, 11.

kata/7-kyu/index.md

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 - [Java Functional Programming (Part 1: The Beginning)](java-functional-programming-part-1-the-beginning)
 - [Java Functional Programming (Part 2: Multiline Functions)](java-functional-programming-part-2-multiline-functions)
 - [Java Functional Programming (Part 3: Closured for Business)](java-functional-programming-part-3-closured-for-business)
+- [Java: Generic Inheritance](java-generic-inheritance)
 - [Jumping Number (Special Numbers Series #4)](jumping-number-special-numbers-series-number-4)
 # K
 - [Keypad horror](keypad-horror)

kata/7-kyu/java-generic-inheritance/README.md

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+# [Java: Generic Inheritance](https://www.codewars.com/kata/java-generic-inheritance "https://www.codewars.com/kata/6705c60be3abf45ead76d052")
+
+I am creating a class hierarchy of geometrical `Shape`s. This is my base class:
+```java
+abstract class Shape {
+  public abstract double getArea();
+}
+```
+
+And this is a circle:
+
+```java
+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 `Rectangle`s, `Square`s, etc.
+
+___
+
+I also wrote a utility class `ShapeUtilities`. One of those utilities is a function that computes the total area of a list of shapes:
+```java
+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:
+```java
+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` :(
+
+```java
+List<Circle> circles = List.of(new Circle(2), new Circle(1), new Circle(5));
+double totalArea = ShapeUtilities.sumAllAreas(circles); // compilation error !
+```
+
+___
+
+Can you fix the code so that `sumAllAreas()` works with `Shape` and all its subtypes?

kata/7-kyu/java-generic-inheritance/main/Shape.java

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

kata/7-kyu/java-generic-inheritance/main/ShapeUtilities.java

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+import java.util.Collection;
+
+interface ShapeUtilities {
+  static double sumAllAreas(Collection<? extends Shape> shapes) {
+    return shapes.stream().mapToDouble(Shape::getArea).sum();
+  }
+}

kata/7-kyu/java-generic-inheritance/test/Circle.java

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+class Circle extends Shape {
+  private final double area;
+
+  Circle(double radius) {
+    this.area = Math.PI * radius * radius;
+  }
+
+  @Override
+  public double getArea() {
+    return area;
+  }
+}

kata/7-kyu/java-generic-inheritance/test/Rectangle.java

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+class Rectangle extends Shape {
+  private final double area;
+
+  Rectangle(double height, double width) {
+    this.area = height * width;
+  }
+
+  @Override
+  public double getArea() {
+    return area;
+  }
+}

kata/7-kyu/java-generic-inheritance/test/ShapeUtilitiesTest.java

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+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class ShapeUtilitiesTest {
+  @Test
+  void shouldWorkForShapes() {
+    List<Shape> shapes = List.of(new Circle(2), new Rectangle(3, 3), new Square(2));
+    assertEquals(25.567, ShapeUtilities.sumAllAreas(shapes), 1e-2);
+  }
+
+  @Test
+  void shouldWorkForCircles() {
+    List<Circle> circles = List.of(new Circle(2), new Circle(1), new Circle(5));
+    assertEquals(94.247, ShapeUtilities.sumAllAreas(circles), 1e-2);
+  }
+
+  @Test
+  void shouldWorkForRectangles() {
+    List<Shape> rectangles = List.of(new Rectangle(3, 1), new Square(2), new Rectangle(1, 2));
+    assertEquals(9.0, ShapeUtilities.sumAllAreas(rectangles), 1e-2);
+  }
+}

kata/7-kyu/java-generic-inheritance/test/Square.java

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+class Square extends Shape {
+  private final double area;
+
+  Square(double side) {
+    this.area = side * side;
+  }
+
+  @Override
+  public double getArea() {
+    return area;
+  }
+}