How to Group By Multiple Attributes and Sum Values in Java 8 Streams

Learn how to efficiently group by multiple attributes and sum values in Java 8 streams using an example with DTO objects.
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This video is based on the question stackoverflow.com/q/65668829/ asked by the user 'Meow' ( stackoverflow.com/u/3097279/ ) and on the answer stackoverflow.com/a/65669161/ provided by the user 'Youcef LAIDANI' ( stackoverflow.com/u/5558072/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Grouping by Multiple Attributes and Summing in Java 8 Streams

Java 8 introduced a powerful new feature called Streams, which allows developers to process collections of objects in a functional style. However, when it comes to more complex operations like grouping by multiple attributes and calculating sums, it can be a bit tricky. In this guide, we'll explore how to tackle this problem effectively.

The Problem

Imagine you have a list of objects called DTO, which contain several attributes, including multiple strings and a numeric value. You want to group these objects by three of their attributes and then sum their amounts. Here’s what the DTO class looks like:

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You might start grouping the objects using Java 8 streams like this:

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However, this approach results in a highly nested map structure that can be cumbersome to work with, particularly when you also want to sum up the amounts. So, how can you achieve this effectively?

The Solution

To simplify your approach, you'll want to do the following:

Step 1: Create a New Grouping Constructor

You can start by creating a constructor that only holds the attributes you want to group by. This will help facilitate the grouping process:

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Step 2: Group and Sum Using Streams

Next, you can utilize the Collectors.groupingBy and Collectors.reducing methods to group by the attributes and calculate the sum of the amounts in a more intuitive way. Here’s how you can do it:

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Step 3: Ensure Proper Equality

For this to work, it's essential that you override the equals and hashCode methods in your DTO class so that the grouping logic can function properly.

Example

Let’s consider an example to illustrate this:

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When you apply the grouping and summing logic, you will get the following output:

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Conclusion

In this guide, we explored how to group objects by multiple attributes and sum their values using Java 8 streams. By creating a specialized constructor for grouping, using the right collectors, and ensuring equality logic is in place, you can efficiently manage complex data processing scenarios.

Happy coding!