Streams à la carte: Extensible stream pipelines with object algebras.

View the Project on GitHub biboudis/streamalg


Build Status

The StreamAlg repository contains the source code artifact that accompanies the Streams à la carte: Extensible Pipelines with Object Algebras paper, to appear at the 29th European Conference on Object-Oriented Programming (ECOOP'15).


We address extensibility shortcomings in libraries for lazy-streaming queries with a new design. The architecture underlying this design borrows heavily from Oliveira and Cook's object algebra solution to the expression problem, extended with a design that exposes the push/pull character of the iteration, and an encoding of higher-kinded polymorphism.

In this library we apply our design to Java and show that the addition of full extensibility is accompanied by high performance, matching or exceeding that of the original, highly-optimized Java streams library.

In this repository we present a fundamental set of sequential operators map, filter, reduce, count, take/limit and iterate.

Additionally we present the behaviors that are discussed in the paper: push, pull, fused pull, logging, id (for blocking terminal operators), future (for non-blocking terminal operators).

Getting Started


The project runs with Java 8.


Clone the project:

git clone

The project is built with maven and its dependencies are automatically resolved: Guava, JMH and JUnit. To run the test suite simply run:

mvn test

The tests cover all examples included in the paper (operators, behaviors) and cases used as motivation as well. The streams package is covered at: 87% classes, 90% methods, 91% lines.

Running Benchmarks

Benchmarks are reproduced by executing:


Project Structure

The basic packages of this artifact are the following:

Object Algebras


The following factories implement different combinations of behaviors:

Higher Kinded types

The types that participate in higher-kinded polymorphism scenarios are: Future, Id, Pull and Push.

GADT encoding

The encoding appears in the gadt.evaluator package.

Fluent APIs

In streamalg/fluent/Stream.cs and streamalg/fluent/Stream.scala we describe a possible solution of obtaining fluent APIs of Streams a la carte in C# and Scala.


The script simply builds the JMH benchmarks über-jar and then uses the command line interface of JMH to pass the arguments of the experiments. The script will run all benchmarks in streamalg/src/main/java/benchmarks/* and their description is included in the paper. The user can run the benchmark script as is or by passing a regular-expression for filter to select only some of them. The micro-benchmark suite can be passed the number of elements N for map, count, operations with large number of elements, N_small that is used for cart and limit/take examples. N_limit is used as the parameter for limit and F is used for benchmarks with fused pipelines.

For more information on JMH the user can run it directly, e.g., to get the help dialog java -jar target/microbenchmarks.jar -h.

We omitted baseline tests from the paper (although we included them in the repo) as the focus of the paper is not on comparing hand-optimized tight loops with streaming pipelines. We have investigated this in previous work ( and it is something that we would like to investigate about the OpenJDK specifically in the immediate future.

We include three basic categories of benchmarks: basic pipelines with various combinations about both Pull and Push algebras, fused pipelines to exercise map and filter fusion and help with the comparison between the non-fused pipelines, iterator pipelines to demonstrate differences of the Pull algebra and the obtaining of an iterator from Java 8 Streams and take pipelines (the take operator is the same as the limit operator in Java 8 Streams).


Aggelos Biboudis (@biboudis), Nick Palladinos (@NickPalladinos), George Fourtounis (@gf0ur) and Yannis Smaragdakis (@YSmaragdakis).