Awesome Open Source
Awesome Open Source

Disclaimer: This is not an official Google product.


A small Java 8 utilities library (javadoc), with 0 deps.

  • Stream utilities (BiStream, MoreStreams, Iteration):
    histogram = zip(times, counts).toMap();
  • Optionals provides extra utilities for Optional:
    optional(id.length() > 0, id)
  • Substring finds a substring in a string:
    String user = first('@').toEnd().removeFrom(email);
  • Parallelizer An Executor-friendly, interruptible alternative to parallel streams.
  • Graph utilities (Walker, ShortestPath)


Add the following to pom.xml:



BiStream streams pairs of objects.

This class closely mirrors JDK Stream API (the few extra methods of "its own" are very straight-forward). If you are familiar with Jdk stream, learning curve is minimal.

Example 1: to concatenate Maps:

import static;

Map<AccountId, Account> allAccounts = concat(primaryAccouunts, secondaryAccounts).toMap();

Example 2: to combine two streams:, responses)

Example 3: to build a Map fluently:

Map<DoctorId, Patient> patientsByDoctorId =, patients)
    .filter((doctor, patient) -> patient.likes(doctor))

Example 4: to build Guava ImmutableListMultimap fluently:

ImmutableListMultimap<ZipCode, Address> addressesByZipCode = BiStream.from(addresses)

Example 5: to split a Map into sub-maps:

import static;

Map<Address, PhoneNumber> phonebooks = ...;
Map<State, Map<Address, PhoneNumber>> statePhonebooks = BiStream.from(phonebooks)
    .collect(groupingBy(Address::state, Collectors::toMap))

Example 6: to merge Map entries:

import static;
import static;

Map<Account, Money> totalPayouts =
    .map(Project::payments)  // Stream<Map<Account, Money>>

Example 7: to apply grouping over Map entries:

import static;
import static;
import static;

Map<EmployeeId, Integer> workerHours =
    .map(Project::getTaskAssignments)  // Stream<Map<Employee, Task>>

Example 8: to turn a Collection<Pair<K, V>> to BiStream<K, V>:

BiStream<K, V> stream = RiStream.from(pairs, Pair::getKey, Pair::getValue);

Q: Why not Map<Foo, Bar> or Multimap<Foo, Bar>?

A: Sometimes Foo and Bar are just an arbitrary pair of objects, with no key-value relationship. Or you may not trust Foo#equals() and hashCode(). Instead, drop-in replace your Stream<Pair<Foo, Bar>>/List<Pair<Foo, Bar>> with BiStream<Foo, Bar>/BiCollection<Foo, Bar> to get better readability.

Q: Why not Stream<FooAndBar>?

A: When you already have a proper domain object, sure. But you might find it cumbersome to define a bunch of FooAndBar, PatioChairAndKitchenSink one-off classes especially if the relationship between the two types is only relevant in the local code context.

Q: Why not Stream<Pair<Foo, Bar>>?

A: It's distracting to read code littered with opaque method names like getFirst() and getSecond().


Example 1: to split a stream into smaller-size chunks (batches):

int batchSize = 5;
MoreStreams.dice(requests, batchSize)

Example 2: to iterate over Streams in the presence of checked exceptions or control flow:

The Stream API provides forEach() to iterate over a stream, if you don't have to throw checked exceptions.

When checked exception is in the way, or if you need control flow (continue, return etc.), iterateThrough() and iterateOnce() can help. The following code uses iterateThrough() to write objects into an ObjectOutputStream, with IOException propagated:

Stream<?> stream = ...;
ObjectOutput out = ...;
iterateThrough(stream, out::writeObject);

with control flow:

for (Object obj : iterateOnce(stream)) {
  if (...) continue;
  else if (...) return;

Example 3: to generate a BFS stream:

Stream<V> bfs = MoreStreams.generate(root, node -> node.children().stream())

Example 4: to merge maps:

interface Page {
  Map<Day, Long> getTrafficHistogram();

List<Page> pages = ...;

// Merge traffic histogram across all pages of the web site
Map<Day, Long> siteTrafficHistogram =
    .collect(grouping(BiStream::from, (a, b) -> a + b))


Example 1: turn your recursive algorithm into a lazy Stream:

class DepthFirst<N> extends Iteration<N> {
  private final Set<N> visited = new HashSet<>();
  DepthFirst<N> postOrder(N node) {
    if (visited.add(node)) {
      for (N successor : node.successors()) {
        yield(() -> postOrder(successor));
    return this;

Stream<N> postOrder = new DepthFirst<N>().postOrder(root).iterate();

Example 2: implement Fibonacci sequence as a stream:

class Fibonacci extends Iteration<Long> {
  Fibonacci from(long v0, long v1) {
    yield(() -> from(v1, v0 + v1));
    return this;

Stream<Long> fibonacci = new Fibonacci().from(0, 1).iterate();
    => [0, 1, 2, 3, 5, 8, ...]


Example 1: to combine two Optional instances into a single one:

Optional<Couple> couple = Optionals.mapBoth(optionalHusband, optionalWife, Couple::new);

Example 2: to run code when two Optional instances are both present:

Optionals.ifPresent(findTeacher(), findStudent(), Teacher::teach);

Example 3: or else run a fallback code block:

static import;

Optional<Teacher> teacher = findTeacher(...);
Optional<Student> student = findStudent(...);
ifPresent(teacher, student, Teacher::teach)             // teach if both present
    .or(() -> ifPresent(teacher, Teacher::workOut))     // teacher work out if present
    .or(() -> ifPresent(student, Student::doHomework))  // student do homework if present
    .orElse(() -> log("no teacher. no student"));       // or else log

Example 4: wrap a value in Optional if it exists:

static import;

Optional<String> id = optional(request.hasId(), request.getId());

All Optionals utilites propagate checked exception from the the lambda/method references.


Example 1: strip off a prefix if existent:

String httpStripped = Substring.prefix("http://").removeFrom(uri);

Example 2: strip off any scheme prefix from a uri:

String schemeStripped = Substring.upToIncluding(first("://")).removeFrom(uri);

Example 3: split a string in the format of "name=value" into name and value:

Substring.first('=').split("name=value").map((name, value) -> ...);

Example 4: replace trailing "//" with "/" :

Substring.suffix("//").replaceFrom(path, "/");

Example 5: strip off the suffix starting with a dash (-) character :


Example 6: extract a substring using regex :

String quoted = Substring.first(Pattern.compile("'(.*?)'"), 1)

Example 7: find the substring between the first and last curly braces ({) :

String body = Substring.between(first('{'), last('}'))


  • Retry blockingly or async
  • Configurable and extensible backoff strategies
  • Retry on exception or by return value
  • Everything is @Immutable and @ThreadSafe

To retry blockingly

Blocking the thread for retry isn't always a good idea at server side. It is however simple and being able to propagate exceptions directly up the call stack is convenient:

Account fetchAccountWithRetry() throws IOException {
  return new Retryer()
      .upon(IOException.class, Delay.ofMillis(1).exponentialBackoff(1.5, 4))

To retry asynchronously

CompletionStage<Account> fetchAccountWithRetry(ScheduledExecutorService executor) {
  return new Retryer()
      .upon(IOException.class, Delay.ofMillis(1).exponentialBackoff(1.5, 4))
      .retry(this::getAccount, executor);

To retry an already asynchronous operation

If getAccount() itself already runs asynchronously and returns CompletionStage<Account>, it can be retried using the retryAsync() method.

And for demo purpose, let's use Fibonacci backoff strategy, with a bit of randomization in the backoff to avoid bursty traffic, why not?

CompletionStage<Account> fetchAccountWithRetry(ScheduledExecutorService executor) {
  Random rnd = new Random();
  return new Retryer()
                .map(d -> d.randomized(rnd, 0.3)))
      .retryAsync(this::getAccount, executor);

A side note: using Stream to transform will eagerly evaluate all list elements before retryAsync() is called. If that isn't desirable (like, you have nCopies(10000000, delay)), it's best to use some kind of lazy List transformation library. For example, if you use Guava, then:

Lists.transform(nCopies(1000000, Delay.ofMillis(30)), d -> d.randomized(rnd, 0.3))

To retry based on return value

Sometimes the API you work with may return error codes instead of throwing exceptions. Retries can be based on return values too:

new Retryer()
    .uponReturn(ErrorCode::BAD, Delay.ofMillis(10).exponentialBackoff(1.5, 4))

Or, use a predicate:

new Retryer()
    .ifReturns(r -> r == null, Delay.ofMillis(10).exponentialBackoff(1.5, 4))

Backoffs are just List<Delay>

exponentialBackoff(), fibonacci(), timed() and randomized() are provided out of the box for convenience purpose only. But at the end of the day, backoffs are just old-school boring Lists. This makes the backoff strategies extensible. You can create the List in any way you are used to, using any Java library. For example, there isn't a uniformDelay() in this library, because there is already Collections.nCopies(n, delay).

Or, to concatenate two different backoff strategies together (first uniform and then exponential), the Java 8 Stream API has a good tool for the job:

new Retryer()
          Stream.concat(nCopies(3, Delay.ofMillis(1)).stream(),
                        Delay.ofMillis(2).exponentialBackoff(1.5, 4).stream()))

What about to retry infinitely? Collections.nCopies(Integer.MAX_VALUE, delay) isn't infinite but close. JDK only uses O(1) time and space for creating it; same goes for Delay#exponentialBackoff() and Delay#fibonacci().

To handle retry events

Sometimes the program may need custom handling of retry events, like, for example, to increment a stats counter based on the error code in the exception. Requirements like this can be done with a custom Delay implementation:

class RpcDelay extends Delay<RpcException> {

  @Override public Duration duration() {

  @Override public void beforeDelay(RpcException e) {
    updateStatsCounter(e.getErrorCode(), "before delay", duration());

  @Override public void afterDelay(RpcException e) {
    updateStatsCounter(e.getErrorCode(), "after delay", duration());

return new Retryer()
    .retry(this::sendRpcRequest, executor);

Or, to get access to the retry attempt number, which is also the list's index, here's an example:

class RpcDelay extends Delay<RpcException> {
  RpcDelay(int attempt, Duration duration) {...}

  @Override public void beforeDelay(RpcException e) {
    updateStatsCounter(e.getErrorCode(), "before delay " + attempt, duration());

  @Override public void afterDelay(RpcException e) {...}

List<Delay<?>> delays = Delay.ofMillis(10).fibonacci(...);
return new Retryer()
          IntStream.range(0, delays.size())
              .mapToObj(i -> new RpcDelay(i, delays.get(i).duration())))

To keep track of exceptions

If the method succeeds after retry, the exceptions are by default logged. As shown above, one can override beforeDelay() and afterDelay() to change or suppress the logging.

If the method fails after retry, the exceptions can also be accessed programmatically through exception.getSuppressed().


Represents a value that may have failed with an exception. Tunnels checked exceptions through streams or futures.


For a stream operation that would have looked like this if checked exception weren't in the way:

   .filter(b -> b.length > 0)

Maybe can be used to wrap the checked exception through the stream operations:

import static;
import static;

Stream<Maybe<byte[], IOException>> stream =
    .filter(byValue(b -> b.length > 0));
List<byte[]> contents = new ArrayList<>();
Iterate.through(stream, m -> contents.add(m.orElseThrow()));
return contents;


In asynchronous programming, checked exceptions are wrapped inside ExecutionException or CompletionException. By the time the caller catches it, the static type of the causal exception is already lost. The caller code usually resorts to instanceof MyException. For example, the following code recovers from AuthenticationException:

CompletionStage<User> assumeAnonymousIfNotAuthenticated(CompletionStage<User> stage) {
  return stage.exceptionally((Throwable e) -> {
    Throwable actual = e;
    if (e instanceof ExecutionException || e instanceof CompletionException) {
      actual = e.getCause();
    if (actual instanceof AuthenticationException) {
      return new AnonymousUser();
    // The following re-throws the exception and possibly wraps it.
    if (e instanceof RuntimeException) {
      throw (RuntimeException) e;
    if (e instanceof Error) {
      throw (Error) e;
    throw new CompletionException(e);

Alternatively, if the asynchronous code returns Maybe<Foo, AuthenticationException> instead, then upon getting a Future<Maybe<Foo, AuthenticationException>>, the exception can be handled type safely using maybe.catchException() or maybe.orElse() etc.

CompletionStage<User> assumeAnonymousIfNotAuthenticated(CompletionStage<User> stage) {
  CompletionStage<Maybe<User, AuthenticationException>> authenticated =
      Maybe.catchException(AuthenticationException.class, stage);
  return authenticated.thenApply(maybe -> maybe.orElse(e -> new AnonymousUser()));

Conceptually, what is Maybe?

  • An (otherwise) Java Optional parameterized by the exception type.
  • A computation result that could have failed with expected exception.
  • Helps with awkward situations in Java where checked exception isn't the sweet spot.

What's not Maybe?

  • It's not Haskell Maybe (Optional is her cousin).
  • It's not Haskell Either either. In Java we think of return values and exceptions, not mathematical "Left" and "Right".
  • It's not to replace throwing and catching exceptions. Java code should do the Java way. When in Rome.
  • It's not designed to write code more "functional" just because you can. Use it where it helps.


The problem

The following code converts a list of objects:

List<Result> convert(List<Input> inputs) {
  List<Result> list = new ArrayList<>();
  for (Input input : inputs) {
  return list;

Intuitively, the contract is that the order of results are in the same order as the inputs.

Now assume the input can be of two different kinds, with one kind to be converted through a remote service. Like this:

List<Result> convert(List<Input> inputs) {
  List<Result> list = new ArrayList<>();
  for (Input input : inputs) {
    if (input.needsRemoteConversion()) {
    } else {
  return list;

In reality, most remote services are expensive and could use batching as an optimization. How do you batch the ones needing remote conversion and convert them in one remote call?

Perhaps this?

List<Result> convert(List<Input> inputs) {
  List<Result> local = new ArrayList<>();
  List<Input> needRemote = new ArrayList<>();
  for (Input input : inputs) {
    if (input.needsRemoteConversion()) {
    } else {
  List<Result> remote = remoteService.batchConvert(needRemote);
  return concat(local, remote);

Close. Except it breaks the ordering of results. The caller no longer knows which result is for which input.

Tl;Dr: maintaining the encounter order while dispatching objects to batches requires careful juggling of the indices and messes up the code rather quickly.

The tool

Funnel is a simple class designed for this use case:

List<Result> convert(List<Input> inputs) {
  Funnel<Result> funnel = new Funnel<>();
  Funnel.Batch<Input, Result> remoteBatch = funnel.through(remoteService::batchConvert);
  for (Input input : inputs) {
    if (input.needsRemoteConversion()) {
    } else {

That is, define the batches with funnel.through() and then inputs can flow through arbitrary number of batch conversions. Conversion results flow out of the funnel in the same order as inputs entered the funnel.


An Executor-friendly, interruptible alternative to parallel streams.

Designed for running a (large) pipeline of IO-bound (as opposed to CPU-bound) sub tasks in parallel, while limiting max concurrency.

For example, the following snippet uploads a large number of pictures in parallel:

ExecutorService threadPool = Executors.newFixedThreadPool(numThreads);
try {
  new Parallelizer(threadPool, numThreads)
      .parallelize(pictures, this::upload);
} finally {

Note that this code will terminate if any picture fails to upload. If upload() throws IOException and an IOException should not terminate the batch upload, the exception needs to be caught and handled:

  new Parallelizer(threadPool, numThreads)
      .parallelize(pictures, pic -> {
        try {
        } catch (IOException e) {

Why not parallel stream?



Some major shopping-list differences:

  • Parallelizer works with any existing ExecutorService.
  • Parallelizer supports an in-flight tasks limit.
  • Thread unsafe input streams or Iterators are okay.
  • Upon failure, all pending tasks are canceled.
  • Exceptions from worker threads are wrapped so that stack trace isn't misleading.

But fundamentally:

  • Parallel streams are for CPU-bound tasks. JDK has built-in magic to optimally use the available cores.
  • Parallelizer is for IO-bound tasks.

Why not just submitting to a fixed thread pool?


ExecutorService threadPool = Executors.newFixedThreadPool(numThreads);
try {
  pictures.forEach(pic -> threadPool.submit(() -> upload(pic)));
  threadPool.awaitTermination(100, SECONDS);
} finally {

Again, use case is different:

  1. The thread pool queues all pending tasks. If the input stream is too large to fit in memory, you'll get an OutOfMemoryError.
  2. Exceptions (including NullPointerException, OutOfMemoryError) are silently swallowed (but may print stack trace). To propagate the exceptions, the Future objects need to be stored in a list and then Future#get() needs to be called on every future object after all tasks have been submitted to the executor.
  3. Tasks submitted to an executor are independent. One task failing doesn't automatically terminate the pipeline.

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