Move Sealed classes to 17

Author: Nicolai Parlog

README.md

@@ -72,7 +72,7 @@ Check out the [jpms](http://blog.codefx.org/tag/jpms/) tag on my blog, [this dem
 
 ## Language Changes
 
-* ⓧ [sealed classes](src/main/java/org/codefx/demo/java_next/lang/sealed) ([JEP 360](https://openjdk.java.net/jeps/360))
+* ⑰ [sealed classes](src/main/java/org/codefx/demo/java17/lang/sealed) (articles [1](https://www.infoq.com/articles/java-sealed-classes/), [2](https://nipafx.dev/java-visitor-pattern-pointless/), [JEP 409](https://openjdk.java.net/jeps/409))
 * ⑯ [`instanceof` pattern matching](src/main/java/org/codefx/demo/java16/lang/instanceof_) (articles [1](https://nipafx.dev/java-pattern-matching/), [2](https://nipafx.dev/java-type-pattern-matching/), [JEP 394](https://openjdk.java.net/jeps/394))
 * ⑮ [text blocks](src/main/java/org/codefx/demo/java15/lang/text_blocks/TextBlocks.java) ([article](https://blog.codefx.org/java/text-blocks), [JEP 378](https://openjdk.java.net/jeps/378))
 * ⑭ [switch expressions](src/main/java/org/codefx/demo/java14/lang/switch_/Switch.java) ([article](https://blog.codefx.org/java/switch-expressions/), [video](https://www.youtube.com/watch?v=1znHEf3oSNI), [JEP 361](https://openjdk.java.net/jeps/361))

src/main/java/org/codefx/demo/java_next/lang/sealed/shape/Circle.java renamed to src/main/java/org/codefx/demo/java17/lang/sealed/Circle.java

@@ -1,4 +1,4 @@
-package org.codefx.demo.java_next.lang.sealed.shape;
+package org.codefx.demo.java17.lang.sealed;
 
 public final class Circle implements Shape {
 

src/main/java/org/codefx/demo/java17/lang/sealed/Main.java

@@ -0,0 +1,27 @@
+package org.codefx.demo.java17.lang.sealed;
+
+public class Main {
+
+	public static void main(String[] args) {
+		Shape shape = new Circle(3);
+		System.out.println(areaMessage(shape));
+	}
+
+	private static String areaMessage(Shape shape) {
+		if (shape instanceof Circle)
+			return "Circle: " + shape.area();
+		else if (shape instanceof Rectangle)
+			return "Rectangle: " + shape.area();
+		else if (shape instanceof Triangle)
+			return "Triangle: " + shape.area();
+		else if (shape instanceof Unicorn)
+			return "Unicorn: " + shape.area();
+		// when not switching over a sealed type
+		// (but using an if-else chain like here),
+		// the compiler can't detect completeness,
+		// so this else branch is still needed
+		else
+			throw new IllegalArgumentException();
+	}
+
+}

src/main/java/org/codefx/demo/java_next/lang/sealed/shape/Rectangle.java renamed to src/main/java/org/codefx/demo/java17/lang/sealed/Rectangle.java

@@ -1,5 +1,9 @@
-package org.codefx.demo.java_next.lang.sealed.shape;
+package org.codefx.demo.java17.lang.sealed;
 
+/**
+ * Rectangle allows one further implementation, namely {@link Square}
+ * (because it's {@code sealed}).
+ */
 public sealed class Rectangle
 		implements Shape
 		permits Square {

src/main/java/org/codefx/demo/java_next/lang/sealed/shape/RightTriangle.java renamed to src/main/java/org/codefx/demo/java17/lang/sealed/RightTriangle.java

@@ -1,5 +1,10 @@
-package org.codefx.demo.java_next.lang.sealed.shape;
+package org.codefx.demo.java17.lang.sealed;
 
+/**
+ * Because {@link Triangle} isn't sealed, {@code RightTriangle} can implement
+ * it without having to be {@code final}, {@code sealed}, or {@code non-sealed}
+ * itself.
+ */
 public class RightTriangle implements Triangle {
 
 	private final double adjacent;
@@ -12,12 +17,6 @@ public RightTriangle(double adjacent, double opposite) {
 
 	@Override
 	public double area() {
-		interface People { String name(); }
-		record Person(String name) implements People { }
-		record Persons(String... names) { }
-
-		People p = new Person("John Doe");
-
 		return adjacent * opposite / 2;
 	}
 

src/main/java/org/codefx/demo/java17/lang/sealed/Shape.java

@@ -0,0 +1,20 @@
+package org.codefx.demo.java17.lang.sealed;
+
+/**
+ * A sealed type can only be (directly) implemented by the permitted
+ * types, in this case {@link Circle}, {@link Rectangle}, etc.
+ *
+ * These classes must explicitly state how they deal with further inheritance.
+ * Note that they're all either {@code final}, {@code sealed}, or
+ * {@code non-sealed}.
+ */
+public sealed interface Shape
+		permits Circle, Rectangle, Triangle, Unicorn {
+
+	double area();
+
+	default Shape rotate(double angle) {
+		return this;
+	}
+
+}

src/main/java/org/codefx/demo/java_next/lang/sealed/shape/Square.java renamed to src/main/java/org/codefx/demo/java17/lang/sealed/Square.java

@@ -1,4 +1,4 @@
-package org.codefx.demo.java_next.lang.sealed.shape;
+package org.codefx.demo.java17.lang.sealed;
 
 public final class Square extends Rectangle {
 

src/main/java/org/codefx/demo/java17/lang/sealed/Triangle.java

@@ -0,0 +1,9 @@
+package org.codefx.demo.java17.lang.sealed;
+
+/**
+ * {@code Triangle} allows unlimited further implementations
+ * (because it's {@code non-sealed}).
+ */
+public non-sealed interface Triangle extends Shape {
+
+}

src/main/java/org/codefx/demo/java17/lang/sealed/Unicorn.java

@@ -0,0 +1,9 @@
+package org.codefx.demo.java17.lang.sealed;
+
+/**
+ * Because records are implicitly final, they are a convenient way
+ * to implement/extend a sealed type.
+ */
+public record Unicorn(String name, double area) implements Shape {
+
+}

src/main/java/org/codefx/demo/java17/lang/sealed/self_contained/Shape.java

@@ -0,0 +1,13 @@
+package org.codefx.demo.java17.lang.sealed.self_contained;
+
+/**
+ * If a sealed type's source file contain all its intended
+ * implementations, no `permits` clause is needed
+ */
+sealed interface Shape { }
+
+non-sealed interface Circle extends Shape { }
+
+non-sealed interface Triangle extends Shape { }
+
+non-sealed interface Rectangle extends Shape { }