Part 2: sealed hierarchy with explicit @JsonSubTypes registry

src/main/java/com/ankurm/jackson3/part2modernjava/C03SealedWithSubTypes.java

@@ -0,0 +1,57 @@
+package com.ankurm.jackson3.part2modernjava;
+
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import tools.jackson.core.type.TypeReference;
+import tools.jackson.databind.json.JsonMapper;
+
+import java.util.List;
+
+/**
+ * Post: Jackson with Records, Optionals, Sealed Classes
+ *       https://ankurm.com/jackson-java-records-optionals/
+ * Section: "Jackson with Sealed Classes (Java 17+)"
+ *
+ * The explicit-registry form: @JsonTypeInfo plus a hand-maintained @JsonSubTypes.
+ * Compare with C04SealedAutoDiscovery, which drops the registry entirely.
+ */
+public class C03SealedWithSubTypes {
+
+    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "shapeType")
+    @JsonSubTypes({
+            @JsonSubTypes.Type(value = Circle.class,    name = "circle"),
+            @JsonSubTypes.Type(value = Rectangle.class, name = "rectangle")
+    })
+    public sealed interface Shape permits Circle, Rectangle { }
+
+    public record Circle(double radius) implements Shape { }
+    public record Rectangle(double width, double height) implements Shape { }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        String json = "["
+                + "{\"shapeType\":\"circle\",\"radius\":5.0},"
+                + "{\"shapeType\":\"rectangle\",\"width\":10.0,\"height\":4.0}"
+                + "]";
+
+        List<Shape> shapes = mapper.readValue(json, new TypeReference<List<Shape>>() { });
+
+        // Java 21 pattern matching for switch — exhaustive because Shape is sealed,
+        // so no default branch is needed and a new permitted type is a compile error.
+        for (Shape shape : shapes) {
+            String description = switch (shape) {
+                case Circle c    -> "Circle with radius: " + c.radius();
+                case Rectangle r -> "Rectangle " + r.width() + " x " + r.height();
+            };
+            System.out.println(description);
+        }
+
+        // CAREFUL: writeValueAsString(List<Shape>) loses the discriminator, because
+        // the runtime type of the list carries no element type for Jackson to read.
+        // The result does not round-trip. See part5polymorphic/F02 for the full story.
+        System.out.println("lossy   : " + mapper.writeValueAsString(shapes));
+        System.out.println("correct : " + mapper.writerFor(new TypeReference<List<Shape>>() { })
+                .writeValueAsString(shapes));
+    }
+}