Parsing Deeply Nested JSON in Java: Lists, Static Classes, and Sanity

Learning how to convert nested JSON to Java is a rite of passage for backend developers.

We have all been there. You look at the API documentation for a new integration. The “GET User” endpoint looks simple enough. But then you curl the request, and the terminal floods with text.

It isn’t just a flat object. It’s a Russian Nesting Doll of curly braces. To convert nested JSON to Java manually, you have to navigate five levels of nesting with Arrays inside Objects inside Arrays. If you miss one bracket or mistype a generic type, Jackson throws a MismatchedInputException.

It’s a User. Who has a list of Orders. And every Order has a list of LineItems. And every LineItem has a ProductDetails object nested inside it.

It’s a Russian Nesting Doll of curly braces.

Mapping a simple key-value pair in Java is trivial. But when you are staring at five levels of nesting with Arrays inside Objects inside Arrays, things get messy fast. You miss one bracket, or you mistype one generic type, and Jackson throws a MismatchedInputException.

In this post, we are going to look at the cleanest ways to handle complex nested JSON arrays in Java, discuss the debate between “Inner Classes” vs. “Separate Files,” and show you how to automate the entire structural generation.

The Challenge: How to Convert Nested JSON to Java Correctly

Let’s look at the enemy. Here is a simplified version of a nested structure I had to deal with recently for an e-commerce integration:

{
  "customer_id": "C-9921",
  "history": [
    {
      "order_id": 551,
      "items": [
        {
          "sku": "TECH-11",
          "meta": { "color": "black", "warranty": true }
        },
        {
          "sku": "TECH-12",
          "meta": { "color": "white", "warranty": false }
        }
      ]
    }
  ]
}

To map this to Java manually, you have to mentally compile a tree structure. You need a Customer class. Then you need an Order class. Then an Item class. Then a Meta class.

If you are writing this by hand, you have two architectural choices to make, and both come with trade-offs.

Architecture: Separate Files vs. Static Inner Classes

When defining DTOs (Data Transfer Objects) for an API response, where should these classes live?

Approach 1: The “Separate File” Method

You create CustomerResponse.java, OrderDTO.java, ItemDTO.java, and MetaDTO.java.

The Good:

• Reusability: If ItemDTO is used in five different API calls, you only define it once.

The Bad:

• Project Pollution: Your project explorer gets cluttered with dozens of tiny classes that are only used for data transfer.
• Context Loss: MetaDTO is a terrible name. Meta for what? A product? A user? A transaction? You end up naming things ProductItemMetaDTO just to avoid collisions.

Approach 2: The Static Inner Class Method (Recommended)

You create one main file: CustomerResponse.java. All the nested objects live inside it as static inner classes.

The Good:

• Encapsulation: Everything related to that API response lives in one file. If you delete the endpoint code, you delete this one file, and everything is gone. No “orphan classes” left behind.
• Readability: You can see the entire data structure in one scroll.

The Senior Dev Take: For API integrations, always lean toward Static Inner Classes initially. It keeps your domain model clean. Do not pollute your core business logic with “an object that only exists to parse a JSON response from Stripe.”

How to Code Nested Lists Correctly

If you go with the Inner Class approach (which you should), here is how you tackle the List mapping.

The most common mistake beginners make is forgetting the static keyword. If your inner class isn’t static, Jackson (the JSON library) cannot instantiate it without an instance of the parent class, leading to cryptic instantiation errors.

Here is the correct structure for the JSON above:

import com.fasterxml.jackson.annotation.JsonProperty;
import java.util.List;

public class CustomerHistoryResponse {

    @JsonProperty("customer_id")
    private String customerId;

    // Direct mapping of the JSON Array to a List
    @JsonProperty("history")
    private List<Order> history;

    // --- Level 1 Nesting ---
    public static class Order {
        @JsonProperty("order_id")
        private Long orderId;

        // Nested List inside the Order object
        @JsonProperty("items")
        private List<LineItem> items;
        
        // Getters/Setters
    }

    // --- Level 2 Nesting ---
    public static class LineItem {
        @JsonProperty("sku")
        private String sku;

        // Object inside the List object
        @JsonProperty("meta")
        private MetaDetails meta;
        
        // Getters/Setters
    }

    // --- Level 3 Nesting ---
    public static class MetaDetails {
        @JsonProperty("color")
        private String color;

        @JsonProperty("warranty")
        private boolean warranty;
        
        // Getters/Setters
    }
}

Why usage of List matters

Notice I used List<Order> and not ArrayList<Order>.

In Java, always program to the interface, not the implementation. Jackson usually returns an ArrayList by default, but if you define your field as LinkedList or ArrayList, you are tightening the coupling unnecessarily. Stick to List<>.

The Maintenance Nightmare

Writing the code above isn’t difficult—it’s just tedious.

The problem arises when the API changes. Or when you have 15 different endpoints, each returning slightly different variations of “Order.”

If you type these out manually, you risk:

1. Typo Fatigue: Misspelling @JsonProperty("historry") and getting a null list.
2. Generic Type Erasure: Accidentally typing List<String> instead of List<Order> and crashing at runtime.
3. Time Waste: Spending 30 minutes writing boilerplate code before you’ve even written a single line of business logic.

This is where automation stops being a luxury and starts being a requirement for efficient engineering.

Integrating the Tool: Stop Writing Boilerplate

I built the JSON to Java Converter because I got tired of writing static inner classes by hand.

The tool is specifically tuned to handle Deep Nesting. It parses the indentation and hierarchy of your JSON array and reconstructs the class structure for you.

How it handles the “Heavy Lifting”

When you paste that deep e-commerce JSON into the tool:

1. Auto-Detection of Arrays: It sees [ and immediately knows to generate a List<T>.
2. Inner Class Generation: It looks at the object inside the array, determines its fields, and creates a public static class for it.
3. Variable Naming: It takes order_id and converts it to orderId automatically, applying the correct @JsonProperty annotation.

The Workflow

Instead of creating 4 files and typing 50 lines of code:

1. Copy your JSON.
2. Paste it into the Toolshref Converter.
3. Copy the single generated class.
4. Paste it into your IDE.

You now have a fully typed, compilation-ready structure. If the API has 10 levels of nesting, the tool generates 10 levels of static classes. You don’t have to track which bracket belongs to which object.

Final Thoughts

Handling nested objects is less about “knowing Java” and more about staying organized.

While it is crucial to understand how static inner classes work (so you can debug them), you shouldn’t be writing them manually in 2025. It is error-prone and low-value work.

Keep your DTOs encapsulated, use List interfaces, and let tools handle the structure generation. Your brain power is better spent on the logic that happens after the data is parsed.