I wrote the following as more of an experiment than anything else, but I thought it would be fun to share and maybe get some feedback!

Motivation: I started looking at some functional languages and noticed how useful the |> pipeline operator is. I started wondering how it might be translated to the Java language.

There are many common functions omitted from the Stream library that could be useful in expressing logic in a more clear fashion. For example, Stream does not have an instance method to concat with another stream-- you have to use a static method. This means that you can't easily chain concat to another operation within a stream; you have to wrap it within a method call. More examples include reversing, delays, zipping, etc. For each of these operations you have to make a helper method, and just like with concat you have to break chaining to wrap the partial solution within a method. This starts getting messy quickly, preventing authors from writing readable one-line expressions.

Consider the following example where static and instance methods are called on a stream:

List<Object> foo = Stream
    .collect(Collectors.toList()) ...

With a pipeline |> operator, you could hide some complexity:

List<Object> foo = list1 |> stream()
    |> map(Some::func)
    |> flatMap(other::stuff))
    |> reverseStream()
    |> concat(list1 |> stream()
        |> map(Some::func)
        |> flatMap(other::stuff))
    |> map(Some::otherFunc)
    |> collect(Collectors.toList());

The following are the methods I wrote which enable piping t |> f:

 * Perform a pipe operation on a parameter and a function.
 * This follows the form: R = T |> F
 * @param t The parameter to be applied
 * @param f The function to be called
 * @param <T> The parameter type
 * @param <R> The result type
 * @return the result of f(t)
static <T, R> R pipe(final T t, final Function<T, R> f) {
    return f.apply(t);

 * Perform a pipe operation on a parameter and a consumer.
 * This follows the form: T |> F
 * @param t The parameter to be accepted
 * @param f The consumer to be called
 * @param <T> The parameter type
static <T> void pipe(final T t, final Consumer<T> f) {

pipe takes in a parameter and a method to accept this parameter. You can either choose to terminate the pipe with a Consumer or chain the pipe with a Function.

For example list.stream().map(Some::func).collect(Collectors.toList()) can be adapted to list |> stream() |> map(Some::func) |> collect(Collectors.toList()) by defining static stream, map and collect functions:

static <T> Function<Collection<T>, Stream<T>> stream() {
    return Collection::stream;

static <T, R> Function<Stream<T>, Stream<R>> map(final Function<T, R> f) {
    return s -> s.map(f);

static <T, A, R> Function<Stream<T>, R> collect(final Collector<? super T, A, R> c) {
    return s -> s.collect(c);

After parsing the pipeline it should be able to generate the code:

pipe(pipe(pipe(list, stream()), map(Some::func)), collect(Collectors.toList()))

Bellow is a code generator I wrote to handle simple cases, such as nested pipelines. It evaluates pipeline expressions recursively from the top-level down, replacing all nested pipelines before the parent:

 * Parse a string representing a pipeline and generate java code 
 * which realizes the pipeline. Pipeline components at the top 
 * level are evaluated first, then each component is parsed to see if
 * it contains nested pipelines.
 * @param pipeline the text representation of the pipeline
 * @return the java code equivalent string
public static String buildPipeline(final String pipeline) {
    return splitPipeline(pipeline).stream().map(p -> {
        for (int index = 0, openBrace = 0, depth = 0; index < p.length(); index++) {
            if (p.charAt(index) == '(' && depth++ == 0) {
                openBrace = index;
            else if (p.charAt(index) == ')' && --depth == 0) {
                final int start = openBrace + 1, stop = index, lastLength = p.length();
                p = p.substring(0, start)
                        + buildPipeline(p.substring(start, stop))
                        + p.substring(stop, lastLength);
                index = p.length() - (lastLength - stop) + 1;
        return p.trim();
    }).reduce((accumulated, next) -> {
        if (accumulated == null) {
            return next;
        return "pipe(" + accumulated + ", " + next + ")";

 * Split a string based on the |> pipeline token. Only pipeline
 * tokens in the top level are evaluated, nested pipelines are 
 * ignored.
 * @param pipeline the text representation of the pipeline
 * @return a list of pipeline components
public static List<String> splitPipeline(final String pipeline) {
    final List<String> splits = new LinkedList<>();
    final StringBuilder builder = new StringBuilder();
    for(int i = 0, depth = 0; i < pipeline.length(); i++) {
        if(pipeline.charAt(i) == '(') {
        } else if(pipeline.charAt(i) == ')') {
        } else if(
            pipeline.charAt(i) == '|'
            && pipeline.charAt(i+1) == '>'
            && depth == 0
        ) {
    return splits;

For fun here's a more complicated output:

static <T> Function<Stream<T>, Stream<T>> reverse() {
    return s -> {
        final Object[] sArray = s.toArray();
        return IntStream.rangeClosed(1, sArray.length)
                .mapToObj(i -> (T) sArray[sArray.length - i]);

static <T> Consumer<Stream<T>> forEach(final Consumer<T> f) {
    return s -> s.forEach(f);

static <T> Function<Stream<T>, Stream<T>> concat(final Stream<T> right) {
    return left -> Stream.concat(left, right);


final String test =
    "ints                                      \n" +
    "    |> stream()                           \n" +
    "    |> concat(ints                        \n" +
    "        |> stream()                       \n" +
    "        |> reverse())                     \n" +
    "    |> forEach(System.out::println)       \n";

Which prints

final List<Integer> ints = Arrays.asList(1, 2, 3, 4, 5);
pipe(pipe(pipe(ints, stream()), concat(pipe(pipe(ints, stream()), reverse()))), forEach(System.out::println))

Which when run will output:

1 2 3 4 5 5 4 3 2 1
  • \$\begingroup\$ I still don't get the WHY? \$\endgroup\$ Commented Dec 16, 2016 at 8:54
  • 1
    \$\begingroup\$ Generally, OP saw a lack of functionality in Stream API, and proposes to parse a custom scripting language to build his own - losing compilator checks, error handling an so on in the process. OP, you took a hammer to squash a fly: Java is perfectly capable of piping as @mnhg 's answer demonstrates. A real problem is the Stream API is insufficient, and (worse!) not extensible. Wrapping around it is apparently in the air. \$\endgroup\$
    – MrBrushy
    Commented Dec 16, 2016 at 14:08

1 Answer 1


This starts getting messy quickly, preventing authors from writing readable one-line expressions.

Based on this statement, I don't get, how you want to use your code. You definitely don't want to start programming in a string (see "evil eval"), nor is it getting less messy with the nested pipe() calls.

What about a more java-approach. Just wrap the stream and add your methods. With some formatting, your are getting pretty close to your layout.

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Pipe<O>
    public static void main(String... args)
        final List<Integer> ints = Arrays.asList(1, 2, 3, 4, 5);

    private static <O> Pipe<O> of(final List<O> objects)
        return new Pipe<>(objects);

    private final Stream<O> stream;

    public Pipe(final List<O> objects)

    private Pipe(final Stream<O> stream)
        this.stream = stream;

    public Pipe<O> concat(Pipe<O> other)
        return new Pipe<>(Stream.concat(stream, other.stream));

    public Pipe<O> reverse()
        final List<O> list = stream.collect(Collectors.toList());
        return new Pipe<>(list);

    private void forEach(Consumer<O> action)

Maybe you could even implement Stream, but I didn't checked this in detail.

  • 2
    \$\begingroup\$ Dammit, I just reinvented jOOL :) \$\endgroup\$ Commented Dec 16, 2016 at 21:38

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