Part 5: writeValueAsString(List) drops the discriminator - and the two fixes

src/main/java/com/ankurm/jackson3/part5polymorphic/F01SerialiseMixedList.java

@@ -0,0 +1,60 @@
+package com.ankurm.jackson3.part5polymorphic;
+
+import tools.jackson.core.type.TypeReference;
+import tools.jackson.databind.json.JsonMapper;
+
+import java.util.List;
+
+/**
+ * Post: Polymorphic Deserialisation — https://ankurm.com/jackson-polymorphic-deserialization/
+ * Section: "Serialising a Mixed List"
+ *
+ * CORRECTION TO THE POST. The post shows
+ *
+ *     mapper.writerWithDefaultPrettyPrinter().writeValueAsString(payments)
+ *
+ * producing JSON that contains "paymentType". It does not. Passing a List to
+ * writeValueAsString gives Jackson only the runtime class (ImmutableCollections.ListN),
+ * which carries no element type, so the polymorphic type serialiser is never engaged
+ * and the discriminator is silently omitted. The resulting JSON then fails to
+ * deserialise — see F02DeserialiseMixedList.
+ *
+ * Two things do work: a typed array, or writerFor(TypeReference).
+ */
+public class F01SerialiseMixedList {
+
+    static List<PaymentMethod> samplePayments() {
+        CreditCardPayment card = new CreditCardPayment();
+        card.setPaymentId(1L);
+        card.setAmountDue(99.99);
+        card.setCardNumberLastFour("4242");
+        card.setCardNetwork("VISA");
+
+        BankTransferPayment bank = new BankTransferPayment();
+        bank.setPaymentId(2L);
+        bank.setAmountDue(250.00);
+        bank.setBankAccountIban("GB29NWBK60161331926819");
+        bank.setBankName("National Bank");
+
+        return List.of(card, bank);
+    }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+        List<PaymentMethod> payments = samplePayments();
+
+        System.out.println("--- 1. single element: discriminator present ---");
+        System.out.println(mapper.writeValueAsString(payments.get(0)));
+
+        System.out.println("--- 2. BROKEN: writeValueAsString(List) drops paymentType ---");
+        System.out.println(mapper.writeValueAsString(payments));
+
+        System.out.println("--- 3. FIX A: writerFor(TypeReference) ---");
+        System.out.println(mapper.writerFor(new TypeReference<List<PaymentMethod>>() { })
+                .withDefaultPrettyPrinter()
+                .writeValueAsString(payments));
+
+        System.out.println("--- 4. FIX B: a typed array carries its component type ---");
+        System.out.println(mapper.writeValueAsString(payments.toArray(new PaymentMethod[0])));
+    }
+}