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.

docs/part2-modern-java.md

@@ -0,0 +1,70 @@
+# Part 2 — Records, Optionals and sealed types
+
+Post: <https://ankurm.com/jackson-java-records-optionals/>
+
+The single biggest simplification in Jackson 3 lands here. All three features are in
+core: the project's [`pom.xml`](../pom.xml) has one dependency, no
+`jackson-datatype-jdk8`, no `jackson-module-parameter-names`, and no `-parameters`
+compiler argument.
+
+## C01 — records
+
+[`C01RecordRoundTrip.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C01RecordRoundTrip.java)
+
+```
+{"productId":101,"productName":"Wireless Keyboard","unitPrice":49.99}
+Wireless Keyboard
+round-trip equal: true
+```
+
+The post's own dependency block here is wrong twice over: it suggests
+`com.fasterxml.jackson.module:jackson-module-parameter-names:3.1.2`, which is both an
+artifact Jackson 3 does not need and a version that does not exist at that coordinate.
+
+## C02 — Optional
+
+[`C02OptionalFields.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C02OptionalFields.java)
+
+```
+present : {"customerName":"Alice","middleName":"Marie"}
+empty   : {"customerName":"Bob","middleName":null}
+absent  : {"customerName":"Bob"}
+isPresent: true
+missing -> Optional.empty (null? false)
+```
+
+Two behaviours worth committing to memory: an empty `Optional` serialises as `null`
+unless you ask for `NON_ABSENT`, and a *missing* property deserialises to
+`Optional.empty()` rather than to `null`, so the field is never null.
+
+## C03 — sealed types with an explicit registry
+
+[`C03SealedWithSubTypes.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C03SealedWithSubTypes.java)
+
+```
+Circle with radius: 5.0
+Rectangle 10.0 x 4.0
+lossy   : [{"radius":5.0},{"width":10.0,"height":4.0}]
+correct : [{"shapeType":"circle","radius":5.0},{"shapeType":"rectangle","width":10.0,"height":4.0}]
+```
+
+Note the `lossy` line. Serialising a `List<Shape>` drops the `shapeType` discriminator,
+because a `List` gives Jackson no element type to dispatch on. This is the same defect
+covered at length in [Part 5](part5-polymorphic.md).
+
+## C04 — sealed auto-discovery
+
+[`C04SealedAutoDiscovery.java`](../src/main/java/com/ankurm/jackson3/part2modernjava/C04SealedAutoDiscovery.java)
+
+Beyond the post: Jackson 3 reads the `permits` clause, so `@JsonSubTypes` can be dropped
+entirely when each permitted type carries `@JsonTypeName`. There is no `@JsonSubTypes`
+anywhere in that file, and a third subtype added later just works.
+
+```
+Circle                   -> {"shapeType":"circle","radius":5.0}                  -> Circle[radius=5.0]
+Rectangle                -> {"shapeType":"rectangle","width":10.0,"height":4.0}  -> Rectangle[width=10.0, height=4.0]
+Triangle                 -> {"shapeType":"triangle","base":3.0,"height":6.0}     -> Triangle[base=3.0, height=6.0]
+```
+
+In Jackson 2 the `@JsonSubTypes` registry had to be maintained in parallel with
+`permits`, and drifted whenever someone added a case.