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Gatherers4j

A library of useful Stream Gatherers (custom intermediate operations) for Java 23+.

Installing

To use this library, add it as a dependency to your build. This library has no additional dependencies.

Maven

Add the following dependency to pom.xml.

<dependency>
    <groupId>com.ginsberg</groupId>
    <artifactId>gatherers4j</artifactId>
    <version>0.6.0</version>
</dependency>

Gradle

Add the following dependency to build.gradle or build.gradle.kts

implementation("com.ginsberg:gatherers4j:0.6.0")

Gatherers In This Library

Streams

Function Purpose
debounce(amount, duration) Limit stream elements to amount elements over duration, dropping any elements over the limit until a new duration starts
dedupeConsecutive() Remove consecutive duplicates from a stream
dedupeConsecutiveBy(fn) Remove consecutive duplicates from a stream as returned by fn
distinctBy(fn) Emit only distinct elements from the stream, as measured by fn
dropLast(n) Keep all but the last n elements of the stream
exactSize(n) Ensure the stream is exactly n elements long, or throw an IllegalStateException
filterWithIndex(predicate) Filter the stream with the given predicate, which takes an element and its index
grouping() Group consecute identical elements into lists
groupingBy(fn) Group consecutive elements that are identical according to fn into lists
interleave(iterable) Creates a stream of alternating objects from the input stream and the argument iterable
interleave(iterator) Creates a stream of alternating objects from the input stream and the argument iterator
interleave(stream) Creates a stream of alternating objects from the input stream and the argument stream
last(n) Constrain the stream to the last n values
maxBy(fn) Return a stream containing a single element, which is the maximum value returned by the mapping function fn
minBy(fn) Return a stream containing a single element, which is the minimum value returned by the mapping function fn
reverse() Reverse the order of the stream
shuffle() Shuffle the stream into a random order using the platform default RandomGenerator
shuffle(rg) Shuffle the stream into a random order using the specified RandomGenerator
throttle(amount, duration) Limit stream elements to amount elements over duration, pausing until a new duration period starts
withIndex() Maps all elements of the stream as-is along with their 0-based index
zipWith(iterable) Creates a stream of Pair objects whose values come from the input stream and argument iterable
zipWith(iterator) Creates a stream of Pair objects whose values come from the input stream and argument iterator
zipWith(stream) Creates a stream of Pair objects whose values come from the input stream and argument stream
zipWithNext() Creates a stream of List objects via a sliding window of width 2 and stepping 1

Mathematics/Statistics

Function Purpose
runningPopulationStandardDeviation() Create a stream of BigDecimal objects representing the running population standard deviation.
runningPopulationStandardDeviationBy(fn) Create a stream of BigDecimal objects as mapped from the input via fn, representing the running population standard deviation.
runningProduct() Create a stream of BigDecimal objects representing the running product.
runningProductBy(fn) Create a stream of BigDecimal objects as mapped from the input via fn, representing the running product.
runningSampleStandardDeviation() Create a stream of BigDecimal objects representing the running sample standard deviation.
runningSampleStandardDeviationBy(fn) Create a stream of BigDecimal objects as mapped from the input via fn, representing the running sample standard deviation.
runningSum() Create a stream of BigDecimal objects representing the running sum.
runningSumBy(fn) Create a stream of BigDecimal objects as mapped from the input via fn, representing the running sum.
simpleMovingAverage(window) Create a moving average of BigDecimal values over the previous window values. See below for options.
simpleMovingAverageBy(fn, window) Create a moving average of BigDecimal values over the previous window values, as mapped via fn.
simpleRunningAverage() Create a running average of BigDecimal values. See below for options.
simpleRunningAverageBy(fn) Create a running average of BigDecimal values as mapped via fn.

Use Cases

Running average of Stream<BigDecimal>

Stream
    .of("1.0", "2.0", "10.0")
    .map(BigDecimal::new)        
    .gather(Gatherers4j.simpleRunningAverage())
    .toList();

// [1, 1.5, 4.3333333333333333]

Moving average of Stream<BigDecimal>

Stream
    .of("1.0", "2.0", "10.0", "20.0", "30.0")
    .map(BigDecimal::new)
    .gather(Gatherers4j.simpleMovingAverage(2))
    .toList();

// [1.5, 6, 15, 25]

Remove consecutive duplicate elements

Stream
    .of("A", "A", "A", "B", "B", "C", "C", "D", "A", "B", "C")
    .gather(Gatherers4j.dedupeConsecutive())
    .toList();

// ["A", "B", "C", "D", "A", "B", "C"]

Remove consecutive duplicate elements, where duplicate is measured by a function

record Person(String firstName, String lastName) {}

Stream
    .of(
        new Person("Todd", "Ginsberg"),
        new Person("Emma", "Ginsberg"),
        new Person("Todd", "Smith")
    )
    .gather(Gatherers4j.dedupeConsecutiveBy(Person::lastName))
    .toList();

// [Person("Todd", "Ginsberg"), Person("Todd", "Smith")]

Remove duplicate elements, where duplicate is measured by a function

record Person(String firstName, String lastName) {}

Stream
    .of(
        new Person("Todd", "Ginsberg"),
        new Person("Emma", "Ginsberg"),
        new Person("Todd", "Smith")
    )
    .gather(Gatherers4j.distinctBy(Person::firstName))
    .toList();

// [Person("Todd", "Ginsberg"), Person("Emma", "Ginsberg")]

Keep all but the last n elements

Stream.of("A", "B", "C", "D", "E")
    .gather(Gatherers4j.dropLast(2))
    .toList();

// ["A", "B", "C"]

Ensure the stream is exactly n elements long

// Good

Stream.of("A", "B", "C").gather(Gatherers4j.exactSize(3)).toList();
// ["A", "B", "C"]

// Bad
Stream.of("A").gather(Gatherers4j.exactSize(3)).toList();
// IllegalStateException

Filter a stream, knowing the index of each element

Stream.of("A", "B", "C", "D")
      .gather(Gatherers4j.filterWithIndex((index, element) -> index % 2 == 0 || element.equals("D")))
      .toList();

// ["A", "C", "D"]

Group identical elements

Stream.of("A", "A", "B", "B", "B", "C")
    .gather(Gatherers4j.grouping())
    .toList();

// [["A", "A"], ["B", "B", "B"], ["C"]]

Group identical elements as measured by a function

Stream.of("A", "B", "AA", "BB", "CC", "DDD")
    .gather(Gatherers4j.groupingBy(String::length))
    .toList();

// [["A", "B"], ["AA", "BB", "CC"], ["DDD"]]

Interleave streams of the same type into one stream

final Stream<String> left = Stream.of("A", "B", "C");
final Stream<String> right = Stream.of("D", "E", "F");

left.gather(Gatherers4j.interleave(right)).toList();

// ["A", "D", "B", "E", "C", "F"]

Limit the stream to the last n elements

Stream
    .of("A", "B", "C", "D", "E", "F", "G")
    .gather(Gatherers4j.last(3))
    .toList();

// ["E", "F", "G"]

Find the object with the maximum mapped value

record Employee(String name, int salary) {}

streamOfEmployees
    .gather(Gatherers4j.maxBy(Employee:salary))
    .toList();

// Employee("Big Shot", 1_000_000)

Find the object with the minimum mapped value

record Person(String name, int age) {}

streamOfPeople
    .gather(Gatherers4j.minBy(Person:age))
    .toList();

// Person("Baby", 1)

Reverse the order of the stream

Stream
    .of("A", "B", "C")
    .gather(Gatherers4j.reverse())
    .toList();

// ["C", "B", "A"]

Include index with original stream values

Stream
    .of("A", "B", "C")
    .gather(Gatherers4j.withIndex())
    .toList();

// [IndexedValue(0, "A"), IndexedValue(1, "B"), IndexedValue(2, "C")]

Shuffle the stream into a random order

Stream
     .of("A", "B", "C", "D" ,"E")
     .gather(Gatherers4j.shuffle())
     .toList();

// ex: ["B", "E", "A", "C", "D"] -- or some other randomly arranged stream

Shuffle the stream into a random order, with a specific RandomGenerator

Stream
     .of("A", "B", "C", "D" ,"E")
     .gather(Gatherers4j.shuffle(RandomGenerator.of("someGenerator")))
     .toList();

// ex: ["B", "E", "A", "C", "D"] -- or some other randomly arranged stream

Throttle the number of elements consumed in a period

Stream
    .of("A", "B", "C")
    .gather(Gatherers4j.throttle(2, Duration.ofSeconds(1))) // Two per second
    .toList();

// ["A", "B",   "C"]
              ^
              |
              +----------- Pause

Zip two streams of together into a Stream<Pair>

The left and right streams can be of different types.

final Stream<String> left = Stream.of("A", "B", "C");
final Stream<Integer> right = Stream.of(1, 2, 3);

left.gather(Gatherers4j.zip(right)).toList();

// [Pair("A", 1), Pair("B", 2), Pair("C", 3)]

Zip elements of a stream together

This converts a Stream<T> to a Stream<List<T>>

Stream
     .of("A", "B", "C", "D", "E")
     .gather(Gatherers4j.zipWitNext())
     .toList();

// [["A", "B"], ["B", "C"], ["C", "D"], ["D", "E"]]

Streams of BigDecimal

Functions which modify output and are available on all BigDecimal gatherers (simple average, moving average, and standard deviation).

Function Purpose
treatNullAsZero() When an element in the Stream<BigDecimal> is null treat it as BigDecimal.ZERO instead of skipping it in the calculation.
treatNullAs(BigDecimal) When an element in the Stream<BigDecimal> is null treat it as the BigDecimal value given instead of skipping it in the calculation.
withMathContext(MathContext) Switch the MathContext for all calculations to the non-null MathContext given. The default is MathContext.DECIMAL64.
withOriginal() Include the original stream value in addition to the calculated value.

Note that rounding mode, precision, and scale are derived from the MathContext.

Example of simpleRunningAverage()

This example creates a stream of double, converts each value to a BigDecmial, and takes a simpleMovingAverage over 10 trailing values. It will includePartialValues and sets the MathContext to the values given. Additionally, nulls are treated as zeros, and the calculated average is returned along with the original value.

someStreamOfBigDecimal()
    .gather(Gatherers4j
        .simpleMovingAverage(10)
        .includePartialValues()
        .withMathContext(MathContext.DECIMAL32)
        .treatNullAsZero()
        .withOriginal()
    )
    .toList();

// Example output:
[
  WithOriginal[original=0.8462487, calculated=0.8462487], 
  WithOriginal[original=0.8923297, calculated=0.8692890], 
  WithOriginal[original=0.2556937, calculated=0.6647573], 
  WithOriginal[original=0.2901778, calculated=0.5711125], 
  WithOriginal[original=0.4945578, calculated=0.5558016], 
  WithOriginal[original=0.3173066, calculated=0.5160525], 
  WithOriginal[original=0.6377766, calculated=0.5334417], 
  WithOriginal[original=0.1729199, calculated=0.4883765], 
  WithOriginal[original=0.7408201, calculated=0.5164258], 
  WithOriginal[original=0.7169926, calculated=0.5364825], 
  WithOriginal[original=0.5174489, calculated=0.5036025], 
  WithOriginal[original=0.5895662, calculated=0.4733262], 
  WithOriginal[original=0.4458275, calculated=0.4923396], 
  // etc...
]

Project Philosophy

  1. Consider adding a gatherer if it cannot be implemented with map, filter, or a collector without enclosing outside state.
  2. Resist the temptation to add functions that only exist to provide an alias. They seem fun/handy but add surface area to the API and must be maintained forever.
  3. All features should be documented and tested.

Contributing

Please feel free to file issues for change requests or bugs. If you would like to contribute new functionality, please contact me before starting work!

Copyright © 2024 by Todd Ginsberg