POJO vs JSON: What's the Difference?

New to Java? Confused about POJOs and JSON? This guide explains both concepts, their differences, and how they work together.

TL;DR Quick Answer

POJO (Plain Old Java Object)

A Java class in memory. It has fields, getters, setters, and methods. Lives in your code.

JSON (JavaScript Object Notation)

A text format for data. It's a string that represents data. Used for APIs, files, storage.

Side-by-Side Comparison

POJO (Java Class)

public class User {
    private String name;
    private int age;
    
    // Getters
    public String getName() {
        return name;
    }
    
    public int getAge() {
        return age;
    }
    
    // Setters
    public void setName(String name) {
        this.name = name;
    }
}

JSON (Text Format)

{
    "name": "John Doe",
    "age": 30
}

Just a string! Can be saved to a file, sent over HTTP, or stored in a database.

Key Differences

Aspect	POJO	JSON
What is it?	Java class (code)	Text format (data)
Where it lives	In JVM memory	Files, APIs, databases
Can have methods?	Yes	No (data only)
Type safety	Compile-time	Runtime only
Language	Java/Kotlin	Language-agnostic

How They Work Together

In real applications, you constantly convert between POJO and JSON:

POJO

In-memory object

→ serialize← deserialize

JSON

Text for transfer

API Response: Server sends JSON → You deserialize to POJO
API Request: You serialize POJO → Send JSON to server
File Storage: Serialize POJO → Save JSON to file

Converting Between POJO and JSON

JSON → POJO (Deserialization)

ObjectMapper mapper = new ObjectMapper();
String json = "{\"name\":\"John\"}";

// Parse JSON to POJO
User user = mapper.readValue(
    json, User.class);

System.out.println(user.getName());
// Output: John

POJO → JSON (Serialization)

ObjectMapper mapper = new ObjectMapper();
User user = new User();
user.setName("Jane");

// Convert POJO to JSON
String json = mapper
    .writeValueAsString(user);

System.out.println(json);
// Output: {"name":"Jane"}

When to Use Each

Use POJO When:

• Working with data inside your Java app
• Need methods and business logic
• Want compile-time type safety
• Building domain models

Use JSON When:

• Sending data over HTTP/APIs
• Storing data in files/databases
• Communicating with other languages
• Configuration files

Convert JSON to POJO Instantly

Skip the manual class creation. Paste your JSON and get Java POJOs in seconds.

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