Stream Gatherers: The Missing Link in Java Streams

Ever wished you could do more with Java Streams? While adding custom terminal operations through Collectors is straightforward, creating new intermediate operations has always been challenging. This talk introduces Stream Gatherers, the feature that elegantly solves this limitation. Drawing from extensive experience developing the open source Gatherers4J library, we'll examine how Gatherers enable developers to create custom intermediate operations that seamlessly integrate with the existing Stream API. Through live coding and practical examples, you'll discover how to write custom Gatherers, understand their internal mechanics, and learn when they're the right tool for the job. This session is perfect for developers who want to level up their Java Stream expertise and expand their stream processing capabilities beyond what collectors alone can provide.

Transcript

1. Stream Gatherers TriJUG 2025-09-22 Todd Ginsberg The Missing Link In Java Streams @ToddGinsberg Lead Engineer - Payments Deutsche Bank
2. Hello! Todd Ginsberg Raleigh, NC TriJUG organizer 30 years of professional experience Currently: Director, Lead Engineer - Payments Deutsche Bank Cary, NC Photo Credit: Andrew Byala @todd.ginsberg.com
3. @todd.ginsberg.com Why Are We Here? Enhance the Stream API to support custom intermediate operations. This will allow stream pipelines to transform data in ways that are not easily achievable with the existing built-in intermediate operations. -- JEP-485: Stream Gatherers
4. @todd.ginsberg.com Why Are We Here? Enhance the Stream API to support custom intermediate operations. This will allow stream pipelines to transform data in ways that are not easily achievable with the existing built-in intermediate operations. -- JEP-485: Stream Gatherers
5. @todd.ginsberg.com Intermediate Operations? Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
6. @todd.ginsberg.com Intermediate Operations? Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
7. @todd.ginsberg.com Intermediate Operations? Stream.of("A", "B", "C") .filter(it -> it.length() < 3 ).toList();
8. Streams Review
9. @todd.ginsberg.com Anatomy Of A Stream Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
10. @todd.ginsberg.com Source Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
11. @todd.ginsberg.com Intermediate Operation Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
12. @todd.ginsberg.com Terminal Operation Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
13. Demo
14. @todd.ginsberg.com Source Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
15. @todd.ginsberg.com Anatomy Of A Stream: Sources list.stream() set.stream() Arrays.stream(...) Stream.of(...) Files.lines(path) Stream.generate() Stream.iterate()
16. Demo
17. @todd.ginsberg.com Terminal Operation Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
18. @todd.ginsberg.com Anatomy Of A Stream: Terminal Operations forEach() forEachOrdered() toArray() reduce() collect() toList() min() max() count()
19. @todd.ginsberg.com Anatomy Of A Stream: Terminal Operations forEach() forEachOrdered() toArray() reduce() collect() toList() min() max() count()
20. @todd.ginsberg.com Collectors Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
21. @todd.ginsberg.com Collectors Stream.of("A", "B", "C") .map(String::toLowerCase) .collect( Collectors.toList() );
22. @todd.ginsberg.com Built-in Collectors averagingDouble() averagingInt() averagingLong() groupingBy() joining() minBy() maxBy() counting() partitioningBy() toList() toUnmodifiableList() toMap() toUnmodifiableMap() toSet() toUnmodifiableSet()
23. @todd.ginsberg.com public interface Collector<T, A, R> { } Collector Interface
24. @todd.ginsberg.com public interface Collector<T, A, R> { } Collector Interface
25. @todd.ginsberg.com public interface Collector<IN, A, R> { } Collector Interface
26. @todd.ginsberg.com public interface Collector<IN, A, R> { } Collector Interface
27. @todd.ginsberg.com public interface Collector<IN, STATE, R> { } Collector Interface
28. @todd.ginsberg.com public interface Collector<IN, STATE, R> { } Collector Interface
29. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { } Collector Interface
30. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); } Collector Interface
31. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); } Collector Interface
32. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); } Collector Interface
33. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); } Collector Interface
34. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Collector Interface
35. Demo
36. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ]
37. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ]
38. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ]
39. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ]
40. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ] [ A, B ] [ C, D ] [ E, F ] [ G, H ]
41. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ] [ A, B ] [ C, D ] [ E, F ] [ G, H ] State State State State
42. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ] [ 2 ] [ 2 ] [ 2 ] [ 2 ] State State State State
43. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ 4 ] [ E, F, G, H ] [ 2 ] [ 2 ] State State State
44. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ 4 ] [ 4 ] State State
45. @todd.ginsberg.com Parallel Evaluation [ 8 ]
46. Demo
47. @todd.ginsberg.com Intermediate Operations Stream.of("A", "B", "C") .map(String::toLowerCase) .toList();
48. @todd.ginsberg.com Anatomy Of A Stream: Intermediate Operations map() mapToInt() mapToLong() mapToDouble() flatMap() flatMapToInt() flatMapToLong() flatMapToDouble() mapMulti() mapMultiToInt() mapMultiToLong() mapMultiToDouble() filter() distinct() sorted() peek() limit() skip() takeWhile() dropWhile()
49. @todd.ginsberg.com Anatomy Of A Stream: Intermediate Operations map() mapToInt() mapToLong() mapToDouble() flatMap() flatMapToInt() flatMapToLong() flatMapToDouble() mapMulti() mapMultiToInt() mapMultiToLong() mapMultiToDouble() filter() distinct() sorted() peek() limit() skip() takeWhile() dropWhile()
50. @todd.ginsberg.com Anatomy Of A Stream: Intermediate Operations map() flatMap() mapMulti() filter() distinct() sorted() peek() limit() skip() takeWhile() dropWhile()
51. @todd.ginsberg.com Where is dedupeConsecutive()? Stream.of("A","A","B","B","C","C") .dedupeConsecutive() .toList();
52. @todd.ginsberg.com Where is dedupeConsecutive()? Stream.of("A","A","B","B","C","C") .dedupeConsecutive() .toList();
53. @todd.ginsberg.com Where is dedupeConsecutive()? Stream.of("A","A","B","B","C","C") .dedupeConsecutive() .toList(); // [ "A", "B", "C" ]
54. @todd.ginsberg.com How about fold()? Stream.of(1, 2, 3, 4, 5) .fold( () -> 1, (acc, next) -> acc * next ).findFirst()
55. @todd.ginsberg.com How about fold()? Stream.of(1, 2, 3, 4, 5) .fold( () -> 1, (acc, next) -> acc * next ).findFirst()
56. @todd.ginsberg.com How about fold()? Stream.of(1, 2, 3, 4, 5) .fold( () -> 1, (acc, next) -> acc * next ).findFirst()
57. @todd.ginsberg.com How about fold()? Stream.of(1, 2, 3, 4, 5) .fold( () -> 1, (acc, next) -> acc * next ).findFirst() // Optional(120)
58. @todd.ginsberg.com How about Kotlin’s mapIndexed()? Stream.of("A", "B", "C") .mapIndexed( (idx, elem) -> idx + "::" + elem ).toList()
59. @todd.ginsberg.com How about Kotlin’s mapIndexed()? Stream.of("A", "B", "C") .mapIndexed( (idx, elem) -> idx + "::" + elem ).toList()
60. @todd.ginsberg.com How about Kotlin’s mapIndexed()? Stream.of("A", "B", "C") .mapIndexed( (idx, elem) -> idx + "::" + elem ).toList() // ["0::A", "1::B", "2::C"]
61. @todd.ginsberg.com Clearly, We Are Missing Some Operations…
62. @todd.ginsberg.com Let’s Plunder Ideas From Other Languages!
63. @todd.ginsberg.com Ahoy, Mateys!
64. @todd.ginsberg.com Yo-ho-ho ye Bilge Rats!
65. @todd.ginsberg.com Avast there! Be careful!
66. @todd.ginsberg.com Why Not? Over the years, many new intermediate operations have been suggested for the Stream API. Most of them make sense when considered in isolation, but adding all of them would make the (already large) Stream API more difficult to learn because its operations would be less discoverable. -- JEP-485: Stream Gatherers
67. @todd.ginsberg.com Why Not? Over the years, many new intermediate operations have been suggested for the Stream API. Most of them make sense when considered in isolation, but adding all of them would make the (already large) Stream API more difficult to learn because its operations would be less discoverable. -- JEP-485: Stream Gatherers
68. @todd.ginsberg.com Why Not? Over the years, many new intermediate operations have been suggested for the Stream API. Most of them make sense when considered in isolation, but adding all of them would make the (already large) Stream API more difficult to learn because its operations would be less discoverable. -- JEP-485: Stream Gatherers
69. @todd.ginsberg.com We Can Only Add One Function To The Stream API…
70. @todd.ginsberg.com That Allows Any Intermediate Operation
71. @todd.ginsberg.com What Do We Mean By “Any”?
72. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state
73. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state Short Circuit
74. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state End of Stream Signal Short Circuit
75. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state End of Stream Signal Short Circuit Parallel Optional
76. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state End of Stream Signal Finite Streams Short Circuit Parallel Optional
77. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state End of Stream Signal Finite Streams Short Circuit Parallel Optional Infinite Streams
78. @todd.ginsberg.com What Do We Mean By “Any”? Maintain state End of Stream Signal Finite Streams Short Circuit Parallel Optional Infinite Streams Any input/output ratio
79. @todd.ginsberg.com Can’t We Just Use Collector? Maintain state End of Stream Signal Finite Streams Short Circuit Parallel Optional Infinite Streams Any input/output ratio
80. @todd.ginsberg.com Can’t We Just Use Collector? YES Maintain state End of Stream Signal Finite Streams NO Short Circuit Parallel Optional Infinite Streams Any input/output ratio
81. @todd.ginsberg.com Gather Function Stream.of("A", "B", "C") .collect(...);
82. @todd.ginsberg.com Gather Function Stream.of("A", "B", "C") .collect(<Collector>);
83. @todd.ginsberg.com Gather Function Stream.of("A", "B", "C") .gather(...) .collect(<Collector>);
84. @todd.ginsberg.com Gather Function Stream.of("A", "B", "C") .gather(...) .collect(<Collector>);
85. @todd.ginsberg.com Gather Function Stream.of("A", "B", "C") .gather(<Gatherer>) .collect(<Collector>);
86. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Start With Something We Know…
87. @todd.ginsberg.com public interface Collector<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
88. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
89. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
90. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> supplier(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
91. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
92. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); BiConsumer<STATE, IN> accumulator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
93. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); BiConsumer<STATE, IN> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
94. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); BiConsumer<STATE, IN> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
95. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
96. @todd.ginsberg.com public interface Integrator<STATE, IN, OUT> { boolean integrate( STATE state, IN element, Downstream<OUT> downstream ); } Gatherer Interface: Integrator
97. @todd.ginsberg.com public interface Downstream<OUT> { boolean push(OUT element); boolean isRejecting(); } Gatherer Interface: Downstream
98. @todd.ginsberg.com public interface Downstream<OUT> { boolean push(OUT element); boolean isRejecting(); } Gatherer Interface: Downstream
99. @todd.ginsberg.com public interface Integrator<STATE, IN, OUT> { boolean integrate( STATE state, IN element, Downstream<OUT> downstream ); } Gatherer Interface: Integrator
100. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
101. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
102. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
103. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); Function<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
104. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
105. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, OUT> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
106. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, Downstream<OUT>> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
107. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, Downstream<OUT>> finisher(); Set<Characteristics> characteristics(); } Gatherer Interface: Under Construction
108. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, Downstream<OUT>> finisher(); Gatherer<> andThen(Gatherer<>); } Gatherer Interface: Under Construction
109. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, Downstream<OUT>> finisher(); Gatherer<> andThen(Gatherer<>); } Gatherer Interface
110. @todd.ginsberg.com public interface Gatherer<IN, STATE, OUT> { Supplier<STATE> initializer(); Integrator<STATE, IN, OUT> integrator(); BinaryOperator<STATE> combiner(); BiConsumer<STATE, Downstream<OUT>> finisher(); Gatherer<> andThen(Gatherer<>); } Gatherer Interface
111. Demo
112. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ] [ A, B ] [ C, D ] [ E, F ] [ G, H ]
113. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ] [ A, B ] [ C, D ] [ E, F ] [ G, H ] S S S S
114. @todd.ginsberg.com Parallel Evaluation [ A, B, C, D, E, F, G, H ] [ A, B, C, D ] [ E, F, G, H ] [ A, B ] [ C, D ] [ E, F ] [ G, H ] S D S D S D S D
115. Demo
116. @todd.ginsberg.com Built-In Gatherers • Fold • Scan • Fixed and Sliding window • Concurrent mapping
117. Demo
118. @todd.ginsberg.com Gatherers4J https://tginsberg.github.io/gatherers4j/ Gatherers4J Documentation Site An Open Source library of 60+ stream gatherers
119. @todd.ginsberg.com Gatherers4j Sequence Operations Filtering and Selection Grouping and Windowing Validation and Constraints Mathematical Operations
120. @todd.ginsberg.com Gatherers4j: 60+ Implementations! crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
121. @todd.ginsberg.com Gatherers4j: Indexing crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
122. @todd.ginsberg.com Gatherers4j: Time crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
123. @todd.ginsberg.com Gatherers4j: Window crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
124. @todd.ginsberg.com Gatherers4j: Math and Statistics crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
125. @todd.ginsberg.com Gatherers4j: Enforce Constraints crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
126. @todd.ginsberg.com Gatherers4j: ..By crossWith debounce dedupeConsecutive dedupeConsecutiveBy distinctBy dropEveryNth dropLast ensureOrdered ensureOrderedBy ensureSize filterIndexed filterInstanceOf filterOrdered filterOrderedBy foldIndexed group groupBy groupOrdered groupOrderedBy interleaveWith mapIndexed movingProduct movingProductBy movingSum movingSumBy orderByFrequency peekIndexed repeat repeatInfinitely reverse rotate runningProduct runningProductBy runningSum runningSumBy sampleFixedSize samplePercentage scanIndexed shuffle simpleMovingAverage simpleMovingAverageBy simpleRunningAverage simpleRunningAverageBy takeEveryNth takeLast takeUntil throttle uniquelyOccurring window withIndex zipWith zipWithNext exponentialMovingAverageWithAlpha exponentialMovingAverageWithAlphaBy exponentialMovingAverageWithPeriod exponentialMovingAverageWithPeriodBy runningPopulationStandardDeviation runningPopulationStandardDeviationBy runningSampleStandardDeviation runningSampleStandardDeviationBy
127. Summary
128. @todd.ginsberg.com In Summary… Available now, in Java 25
129. @todd.ginsberg.com In Summary… Maintain state ✅ Short Circuit ✅ End of Stream Signal ✅ Finite & Infinite Streams ✅ Parallel Optional ✅ Any input/output ratio ✅
130. @todd.ginsberg.com In Summary… In the future… “We will not add a new intermediate operation to the Stream class for each of the built-in gatherers defined in the Gatherers class, even though for the sake of uniformity it is tempting to do so. ” https://openjdk.org/jeps/485
131. @todd.ginsberg.com In Summary… In the future… “...we might revise the set of built-in gatherers in future releases.” https://openjdk.org/jeps/485
132. Questions? (And Maybe Answers!)
133. @todd.ginsberg.com todd@ginsberg.com https://todd.ginsberg.com Thank You! @todd.ginsberg.com