Part 5: deserialise a mixed list, and prove the lossy JSON cannot round-trip

src/main/java/com/ankurm/jackson3/part5polymorphic/F02DeserialiseMixedList.java

@@ -0,0 +1,56 @@
+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: "Deserialising a Mixed List"
+ *
+ * Deserialisation is the half that works exactly as the post describes — and it is
+ * also what proves the serialisation defect in F01: feed it the discriminator-less
+ * JSON and it fails outright.
+ */
+public class F02DeserialiseMixedList {
+
+    private static final String GOOD_JSON = "["
+            + "{\"paymentType\":\"credit_card\",\"paymentId\":1,\"amountDue\":99.99,"
+            + "\"cardNumberLastFour\":\"4242\",\"cardNetwork\":\"VISA\"},"
+            + "{\"paymentType\":\"bank_transfer\",\"paymentId\":2,\"amountDue\":250.0,"
+            + "\"bankAccountIban\":\"GB29NWBK60161331926819\",\"bankName\":\"National Bank\"}"
+            + "]";
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        List<PaymentMethod> payments =
+                mapper.readValue(GOOD_JSON, new TypeReference<List<PaymentMethod>>() { });
+
+        for (PaymentMethod payment : payments) {
+            if (payment instanceof CreditCardPayment cc) {
+                System.out.println("Card ending: " + cc.getCardNumberLastFour());
+            } else if (payment instanceof BankTransferPayment bt) {
+                System.out.println("Bank: " + bt.getBankName());
+            }
+        }
+
+        // Now prove the F01 defect matters: the lossy output cannot be read back.
+        String lossy = mapper.writeValueAsString(F01SerialiseMixedList.samplePayments());
+        try {
+            mapper.readValue(lossy, new TypeReference<List<PaymentMethod>>() { });
+            System.out.println("unreachable");
+        } catch (Exception e) {
+            System.out.println("lossy JSON round-trip -> " + e.getClass().getSimpleName());
+        }
+
+        // Whereas the correctly written output does round-trip.
+        String correct = mapper.writerFor(new TypeReference<List<PaymentMethod>>() { })
+                .writeValueAsString(F01SerialiseMixedList.samplePayments());
+        List<PaymentMethod> again =
+                mapper.readValue(correct, new TypeReference<List<PaymentMethod>>() { });
+        System.out.println("correct JSON round-trip -> " + again.size() + " payments, "
+                + again.get(0).getClass().getSimpleName() + " first");
+    }
+}