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Collector interface components

The java.util.stream.Collector interface defines a contract for mutable reduction operations, often used in conjunction with stream().collect() to transform input elements into a cumulative result.^[600-developer__java__java8__java8-collect.md]

Core Characteristics

Unlike reduction operations (such as reduce) which typically produce immutable results by combining elements iteratively, a Collector relies on a mutable result container^[600-developer__java__java8__java8-collect.md]. This approach allows for more efficient processing by accumulating values into a shared structure rather than creating new objects with every step^[600-developer__java__java8__java8-collect.md]. The standard library provides a utility class, java.util.stream.Collectors, which supplies common implementations of this interface^[600-developer__java__java8__java8-collect.md].

Functional Components

The Collector interface is structurally defined by four primary functional components that manage the lifecycle of the mutable result container^[600-developer__java__java8__java8-collect.md]:

  • Supplier (supplier()): Creates a new, empty result container to hold the accumulation process^[600-developer__java__java8__java8-collect.md].
  • Accumulator (accumulator()): Defines the operation to incorporate a new data element into the mutable result container^[600-developer__java__java8__java8-collect.md].
  • Combiner (combiner()): Defines how to merge two result containers into one, typically used in parallel processing to combine partial results^[600-developer__java__java8__java8-collect.md].
  • Finisher (finisher()): Performs an optional final transformation on the container, often converting the mutable accumulation structure into the final immutable result type^[600-developer__java__java8__java8-collect.md].
  • [[Java Streams]]
  • [[Mutable reduction]]
  • [[groupingBy]]

Sources

^[600-developer__java__java8__java8-collect.md]