How to Compare Two JSON Files in Java

Learn how to effectively compare two JSON files in Java, detect differences, and utilize POJOs for structured data representation.
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How to Compare Two JSON Files in Java: A Comprehensive Guide

In today's digital world, JSON (JavaScript Object Notation) has become a prevalent format for data exchange. As developers, we often face the challenge of comparing two JSON files to ensure their integrity and consistency. This guide walks you through a specific scenario where two JSON files need to be compared using Java, showing you how to identify differences and rectify potential issues, especially when using POJOs (Plain Old Java Objects).

The Problem Statement

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The POJO Classes

To represent the JSON structure in Java, you've created two POJO classes: PickEvent and Activation. Here are their definitions:

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The Initial Attempt to Compare

You wrote a test method to read the JSON files and compare the objects. However, the method you implemented was flawed, as you were using assertSame, which checks if two references point to the same object. Here's your original compare method:

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The Issue

You encountered an assertion error indicating that the values for OrderId, lineNbr, pick, and activations were null, despite expecting them to reflect the contents of the JSON files. This happened because the parser hit an exception during the parsing process and ended up returning null objects. Setting FAIL_ON_UNKNOWN_PROPERTIES to false prevented an error from being thrown.

The Solution

Updated Code Implementation

Instead of relying on equality checks, we need to ensure we're correctly parsing and comparing the JSON files. Here's how you can structure your compare method:

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Key Changes Explained

Use Wrapper Class: By wrapping the PickEvent activities in another class (Wrapper), you can represent the JSON structure more accurately.

Readability of Comparison: Using equals allows you to check if the contents of the two objects are equivalent rather than their references in memory.

Conclusion

Comparing JSON files in Java can be tricky, especially if you don't have the structures properly represented or you use incorrect assertion methods. By following the guidelines outlined in this post, you can effectively compare JSON files and pinpoint discrepancies, thereby maintaining data integrity in your applications.

Feel free to adapt this method to meet your specific needs, and happy coding!