When to Use toMap() Instead of GroupingBy and Reducing in Java Streams?

Java Streams: Using toMap Instead of GroupingBy and Reducing

When working with Java Streams, developers often face situations where they use both the groupingBy() method and the reducing() collector to manipulate their data. However, there is a more efficient approach using the toMap() method in certain cases.

What is the Pattern?

As pointed out by @Holger in a previous discussion, it has been noticed that when using the reducing collector with groupingBy, it's often more appropriate to use the toMap collector instead. This pattern is evident through experience and observations from numerous Stackoverflow discussions.

Why toMap() is Preferred

The groupingBy collector expects a second Collector as an argument, making it suitable for applying Mutable Reduction to groups of data. In contrast, the toMap collector with a merge function is ideal for classical Reduction. The merge function in toMap has a similar purpose and shape to a Reduction function, even though it is not labeled as such.

In practice, the collectors that perform Reduction return an Optional, which is not always desirable when using groupingBy. This is where toMap comes in, as it handles such cases more seamlessly.

Additional Patterns

While this specific pattern has been identified, there are likely more patterns that become apparent while using these APIs. However, collecting them all in a single answer is beyond the scope of Stackoverflow. Developers are encouraged to explore these APIs further and share their experiences to uncover additional patterns and best practices.

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