How to Perform a Filter on a Nested List in Java Using Streams

Discover how to efficiently filter a nested list within another Java list using streams. This guide will walk you through transforming JSON data in Spring Boot for desired outputs.
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How to Filter a Nested List in Java Using Streams

Java's powerful stream API allows developers to perform complex operations on collections, including nested lists. If you're new to Java or Spring Boot and you're trying to filter a nested list based on specific criteria, you might find yourself stuck. In this post, we'll tackle the problem of filtering a nested list of brands from a list of years based on the brand name "Toyota."

Understanding the Problem

You have a JSON-like structure consisting of a list of years, each containing an associated list of car brands. Your goal is to filter those brands based on their names and transform the data into a new structure (DTO) while maintaining a clean and efficient coding practice in Java.

Here's a simplified structure of the JSON data you are working with:

[[See Video to Reveal this Text or Code Snippet]]

When a user filters by the brand name "Toyota", the expected output is a reduced list of years, each only showing the relevant brands.

Steps to Filter the Nested List

Step 1: Review Your Current Code

Here's a snippet of your initial approach:

[[See Video to Reveal this Text or Code Snippet]]

While the intent here is right, the filtering logic and mapping need adjustments.

Step 2: Adjust the Filtering and Mapping Logic

Instead of trying to map the filtered brands directly into the YearBrandDTO, we should first filter the brands and then create a new YearBrandDTO object. Here is a refined version of your code:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Adjust Your DTO Class

Your YearBrandDTO class must reflect the type for the brands correctly. Instead of using an Object, define it as a list of BrandDTO:

[[See Video to Reveal this Text or Code Snippet]]

Step 4: Create the BrandDTO Class

To represent brand details accurately, you need a BrandDTO class:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By restructuring the way you filter and map your data, you can achieve the desired output efficiently using Java Streams. Always remember to ensure that your DTO classes correctly reflect the data they are designed to communicate. With these adjustments, you should now be able to filter a nested list in Java seamlessly.

Happy coding!