JSON & Serialization — Jackson, Gson

9. Practice Problems

1. Write a Product record with name, price, stock, then manually print its JSON.
   একটি Product record লিখুন — name, price, stock — তারপর manually JSON print করুন।
   Show Answer (উত্তর দেখুন)

   Main.java

   class Main {
       record Product(String name, double price, int stock) {}
       public static void main(String[] args) {
           Product p = new Product("Keyboard", 1499.0, 7);
           String json = "{\"name\":\"" + p.name() + "\",\"price\":" + p.price() + ",\"stock\":" + p.stock() + "}";
           System.out.println(json);
       }
   }

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2. Explain in 2 sentences when you would use @JsonIgnore.
   দুই বাক্যে বলুন — কখন @JsonIgnore ব্যবহার করবেন।
   Show Answer (উত্তর দেখুন)

   Answer: Use @JsonIgnore to keep sensitive or internal fields out of the JSON you ship to clients — passwords, hashes, database IDs, internal audit timestamps, lazy-loaded ORM relationships. It prevents accidental leaks and avoids infinite recursion in bidirectional entity graphs.

   password, hash, internal DB id, lazy-loaded ORM relationship — এসব field JSON-এ না পাঠানোর জন্য @JsonIgnore লাগান। security leak এড়ায়, এবং bidirectional entity-তে infinite recursion থামায়।

3. Produce a JSON array of three student records using streams.
   stream দিয়ে তিনজন ছাত্রের JSON array তৈরি করুন।
   Show Answer (উত্তর দেখুন)

   Main.java

   import java.util.*;
   import java.util.stream.*;
   class Main {
       record Student(String id, String name) {
           String j() { return "{\"id\":\""+id+"\",\"name\":\""+name+"\"}"; }
       }
       public static void main(String[] args) {
           String json = Stream.of(
                   new Student("22-1", "Raihan"),
                   new Student("22-2", "Nusrat"),
                   new Student("22-3", "Tariq"))
               .map(Student::j).collect(Collectors.joining(",", "[", "]"));
           System.out.println(json);
       }
   }

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4. In 3 sentences, say why native Java Serialization is discouraged today.
   তিন বাক্যে বলুন — native Java Serialization কেন আজ discourage করা হয়।
   Show Answer (উত্তর দেখুন)

   Answer: Native Java Serialization has been the root cause of many high-severity CVEs — crafted bytes can trigger arbitrary code execution during deserialization. The binary format is unreadable outside Java, so no other language on the other end of an API can parse it. It also tightly couples the serialized bytes to the exact class structure, making version evolution painful.

   Native Java Serialization বহু high-severity CVE-র উৎস — crafted bytes deserialization-এ arbitrary code run করাতে পারে। Binary format Java-র বাইরে কেউ পড়তে পারে না। Class structure সামান্য বদলালেই বহু আগের serialized data আর read হয় না।

5. Produce JSON for a nested object — an Order containing a list of Items.
   একটি nested JSON তৈরি করুন — Order-এর ভেতরে Item-এর list।
   Show Answer (উত্তর দেখুন)

   Main.java

   import java.util.*;
   import java.util.stream.*;
   class Main {
       record Item(String name, int qty) {
           String j() { return "{\"name\":\""+name+"\",\"qty\":"+qty+"}"; }
       }
       record Order(String id, List<Item> items) {
           String j() {
               String arr = items.stream().map(Item::j)
                   .collect(Collectors.joining(",", "[", "]"));
               return "{\"id\":\""+id+"\",\"items\":"+arr+"}";
           }
       }
       public static void main(String[] args) {
           Order o = new Order("A-1", List.of(
               new Item("Rice", 3), new Item("Oil", 1)));
           System.out.println(o.j());
       }
   }

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