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Java Academy · Lesson

Building a Custom Collector

Implement the Collector interface to create a reusable aggregation for a domain-specific use case.

Why Build a Custom Collector?

When built-in collectors don't fit your domain, implement the Collector<T, A, R> interface to create reusable, composable aggregation logic. T=input, A=mutable accumulator, R=result.

The Collector Interface

A Collector has four components:

  • supplier() — creates the initial accumulator
  • accumulator() — folds one element into the accumulator
  • combiner() — merges two accumulators (for parallel streams)
  • finisher() — transforms the accumulator into the final result
  • characteristics() — optimization hints

All lessons in this course

  1. groupingBy: Classifying Elements
  2. partitioningBy and counting
  3. toMap, joining, and summarizing
  4. Building a Custom Collector
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