Using toMap Instead of GroupingBy and Reducing in Java Streams

When working with Java Streams, it's common to encounter situations where groupingBy and reducing collectors are used in combination. However, it's worth exploring an alternative pattern suggested by Holger in a previous discussion on StackOverflow.

Holger remarked that whenever groupingBy and reducing are employed, considering toMap as a more appropriate option is recommended. This pattern has proven to be effective in enhancing code efficiency and readability.

The Rationale for Using toMap

This pattern has emerged through practical experience with stream collectors. Both groupingBy and toMap have their uses, but toMap exhibits certain advantages in certain scenarios.

Firstly, toMap mimics a traditional reduction operation more closely than groupingBy combined with reducing. The merge function employed in toMap functions similarly to a reduction function, despite its different nomenclature.

Furthermore, groupingBy typically returns an Optional value, which can be inconvenient when used with groupingBy. In contrast, toMap seamlessly integrates with groupingBy and avoids this issue.

In summary, toMap offers a concise and elegant approach to grouping and reducing data in Java Streams. While both collectors have their place, it's essential to recognize the advantages of toMap and consider its use as an alternative to groupingBy and reducing combinations.