Jackson 3 series companion code

37 runnable examples covering the eight feature posts on ankurm.com, verified
against Jackson 3.2.1 on Temurin 21.0.5. Every output committed under docs/ was
produced by run-all.sh.

Also documents 11 places where the published snippets do not compile or do not
behave as printed against a real Jackson 3 build - most notably that
writeValueAsString(List<Base>) silently drops the polymorphic type discriminator,
so the post's serialised output cannot be read back.

src/main/java/com/ankurm/jackson3/part2modernjava/C02OptionalFields.java

@@ -0,0 +1,51 @@
+package com.ankurm.jackson3.part2modernjava;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import tools.jackson.databind.json.JsonMapper;
+
+import java.util.Optional;
+
+/**
+ * Post: Jackson with Records, Optionals, Sealed Classes
+ *       https://ankurm.com/jackson-java-records-optionals/
+ * Section: "Jackson with Optional<T>"
+ *
+ * Jackson 3 handles Optional natively. There is no jackson-datatype-jdk8
+ * dependency in the pom and no registerModule(new Jdk8Module()) call — those are
+ * Jackson 2 requirements, and the Jdk8Module class does not exist under
+ * tools.jackson at all.
+ */
+public class C02OptionalFields {
+
+    public record CustomerProfile(String customerName, Optional<String> middleName) { }
+
+    /** NON_ABSENT is the inclusion value that understands Optional.empty(). */
+    @JsonInclude(JsonInclude.Include.NON_ABSENT)
+    public record CompactProfile(String customerName, Optional<String> middleName) { }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        // Present value: the Optional is unwrapped, not wrapped in {"present":true}.
+        System.out.println("present : "
+                + mapper.writeValueAsString(new CustomerProfile("Alice", Optional.of("Marie"))));
+
+        // Empty Optional: serialises as null by default.
+        System.out.println("empty   : "
+                + mapper.writeValueAsString(new CustomerProfile("Bob", Optional.empty())));
+
+        // NON_ABSENT omits the property entirely instead of writing null.
+        System.out.println("absent  : "
+                + mapper.writeValueAsString(new CompactProfile("Bob", Optional.empty())));
+
+        // Deserialise back.
+        String json = "{\"customerName\":\"Alice\",\"middleName\":\"Marie\"}";
+        CustomerProfile restored = mapper.readValue(json, CustomerProfile.class);
+        System.out.println("isPresent: " + restored.middleName().isPresent());
+
+        // A missing property deserialises to Optional.empty(), never to null.
+        CustomerProfile missing = mapper.readValue("{\"customerName\":\"Carol\"}", CustomerProfile.class);
+        System.out.println("missing -> " + missing.middleName() + " (null? "
+                + (missing.middleName() == null) + ")");
+    }
+}