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Discover how to efficiently use Java Stream Collectors to create a map that summarizes totals and counts from a list of objects.
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Mastering Java Stream Collectors: Creating Totals with Counting

When working with collections in Java, particularly with streams, consolidating data to generate meaningful summaries can often pose a challenge. One common scenario involves needing to create a summary map from a list of objects that not only aggregates values but also counts occurrences. In this guide, we will explore how to implement this using Java Streams and Collectors, ensuring both efficiency and clarity.

The Problem at Hand

Imagine you have a list of Result objects, each containing a code, a description, and a duration value. Alongside this, you also have a Product class defined by the same code and description. Your goal is to produce a Map<Product, Total>, where:

Total includes:

The aggregated duration of all Result objects with the same code and description.

The count of such Result objects.

To clarify, we have the following class definitions:

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

An Initial Attempt

Typically, you might think of converting the Result list into a Map by first grouping the entries based on the associated Product. One initial approach could look like this:

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

While this solution works, it’s not the most efficient approach.

The Efficient Solution

Instead of first collecting the elements into a List, we can perform the reduction directly within the grouping phase itself. This will save unnecessary memory usage and enhance performance. Here’s how to do it properly:

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

Breakdown of the Efficient Approach

Grouping: The groupingBy operator creates groups based on the Product created from the Result.

Tees: The teeing collector combines two results:

Reducing: This collects the duration values with a start of Duration.ZERO and combines them using the plus method.

Count: This uses counting() to simply count the number of elements in each group.

Combining Results: The Total::new method reference creates a new Total object by combining the results of the two collectors.

Conclusion

Using Java Stream collectors can open up powerful ways to manipulate and aggregate collections effectively. By following the efficient approach highlighted above, you'll not only be able to create aggregations of values but also count occurrences in a streamlined manner.

Next time you face a similar challenge, consider applying these techniques to simplify your code and enhance performance. We hope this helps you in mastering your data processing tasks in Java!