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Mutable vs immutable reduction in streams

In the context of Java Stream operations, data processing can be categorized into two distinct strategies: mutable reduction and immutable reduction^[600-developer__java__java8__java8-collect.md].

Immutable reduction

Immutable reduction is the standard reduction process typically associated with the reduce() operation. This approach combines stream elements iteratively to produce a single result without modifying any external state or accumulating structure. It generally involves taking an input value and combining it with the current accumulated value to produce a new accumulated value^[600-developer__java__java8__java8-collect.md].

Mutable reduction

Mutable reduction is a specific optimization process designed for collecting results into a mutable container (collection), such as a List, Map, or StringBuilder. This strategy allows for a more efficient accumulation of results by modifying the state of a specific container in place rather than creating new objects with every iteration^[600-developer__java__java8__java8-collect.md].

The implementation of a mutable reduction relies on the Collector interface (specifically java.util.stream.Collector), which provides the necessary mechanics to handle the mutable container. A Collector implementation defines four key components to facilitate this process^[600-developer__java__java8__java8-collect.md]:

  • Supplier: Creates a new result container.
  • Accumulator: Incorporates a new data element into the result container.
  • Combiner: Combines two result containers into one.
  • Finisher: Performs an optional final transform on the container^[600-developer__java__java8__java8-collect.md].

In Java Streams, mutable reduction is most commonly invoked via the stream().collect() method^[600-developer__java__java8__java8-collect.md].

  • [[Java Streams]]
  • [[Collector]]

Sources

^[600-developer__java__java8__java8-collect.md]