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Deprecation Notice

This project is no longer being maintained, and its usage should be replaced with Vavr (


This project adds some concepts that didn't ship with java 8 lambda project like Either<L,R> or Pair<L,R> etc...

Include from maven


Either cheatsheet

Why Either?

The Either class provides a way to enforce failure handling in the same fashion as Optional is a way to enforce null values handling.

How do I use an Either?

Convention dictates that Left is used for failure path and Right is used for success / happy path.

How do I instantiate an Either?

For a failure:

final Either<String, String> var = Either.left("My failure text");

For a success:

final Either<String, String> var = Either.right("My text");

How can I access the Right value?

There is no way to directly access the value inside an Either in the same way as Optional.get(). This is made on purpose to avoid code in the style of if(either.isRight()) { either.getRight(); } which promotes not handling the failure cases (left side).

final Either<String, String> var = ...;
final String myValue = either.fold(failure -> handleFailure(failure), Function.identity())

How can I transform the value on only one side?

For the Left side:

final Either<String, String> var = ...;
final Either<Integer, String> transformed = var.left().map(Integer::valueOf);

For the Right side:

final Either<String, String> var = ...;
final Either<String, Integer> transformed = var.right().map(Integer::valueOf);

How can I log the failure before transforming it to a default value?

final Logger LOGGER = ...;
final Either<Exception, String> either = ...;
final String myValue = either.left().peek(LOGGER::warn)
                             .fold(failure -> handleFailure(failure), Function.identity());

I have several Either, how can I log the failures of all of them?

final Logger LOGGER = ...;
final Either<Exception, String> either1 = ...;
final Either<Exception, String> either2 = ...;
final Either<Exception, String> either3 = ...;
Arrays.asList(either1, either2, either3).stream().forEach(either -> either.forEach(LOGGER::warn));

How can I conditionally chain functions on the happy path?

private final Either<Exception, String> getUserInput() {
private final Either<Exception, Integer> processUserInput(final String userInput) {
private final Either<Exception, Integer> saveUserInput(final Integer userInput) {
// processUserInput() only gets executed if getUserInput() succeeded, and saveUserInput() only gets executed if processUserInput() succeeded
final Either<Exception, Integer> chained = getUserInput().joinRight(this::processUserInput).joinRight(this::saveUserInput);

How can I combine several Either to use all of their Right values at once?

private final Integer processValues(final String val1, final String val2, final String val3) {
final Either<Exception, String> either1 = ...;
final Either<Exception, String> either2 = ...;
final Either<Exception, String> either3 = ...;

Either<Exception, Integer> result = either1.joinRight(val1 -> either2.joinRight(val2 -> either3.right().map(val3 -> processValues(val1, val2, val3))));

StreamableOptional cheatsheet

Why StreamableOptional

It happens (rather often) to end with a Stream containing Optional-s and to be willing to only retain Present instances.

In Java 8 there are 2 options:

  • the bad: filtering with isPresent() and unwrapping values with get(): stream().filter(Optional::isPresent).map(Optional::get)
  • and the ugly: flat-mapping Optional values to Stream of one or no value: stream().flatMap(opt ->

StreamableOptional is a cleaner shorthand for the second solution above (provided that you use StreamableOptional instead of Optional ): stream.flatMap(StreamableOptional::stream).

How do I retain only present instances

Arrays.asList(StreamableOptional.of(1), StreamableOptional.empty(), StreamableOptional.of(3))
// yields :
// 1
// 2

I already have a stream of traditional Java 8 Optional, how does that help me?

It might not be the ideal setup, however you still have two possibilities:

  • try to replace usages of Optional with StreamableOptional upstream of your code if you can;
  • else : or -> StreamableOptional.ofOptional(opt).stream())

How do I get back a traditional Java 8 Optional?

This might indicate a code smell since:

  • you probably used StreamableOptional to flatMap values into a Stream (and thus have no more use for a StreamableOptional)
  • you can use the same operations on StreamableOptional as on Optional

However, this case can happen when you interact with an API that requires an Optional. Then simply use the toOptional() method.

public apiMethodRequiringAnOptional(Optional<Whatever> maybe);
// call it

Try cheatsheet

Why Try?

The Try class provides convenient ways to handle computations which might fail.

Unlike Either it is right-biased, simplifying the use of familiar higher order functions such as map, flatMap and forEach.

In proper functional languages, Try is a monad where as Either is not, (but this implementation does not satisfy the monadic laws because of how flatMap borrows inspiration from its counterpart in java.util.Optional)

Warning: Do not use Try on Autoclosable resources and expect them to close (try-with-resources) - Try does not close resources!

An example of using Try

private void playWithTry() {
    //a chain of Exception-prone operations
    Try.of(Integer::valueOf, "3f")
        .map(x -> x * 2)
    byte[] mybytes = {0b101, 0b001};
            String myString = Try.of(Float::valueOf, "3F")
                    .map(f -> f * 2)
                    .peek(f -> System.out.println("Float? : " + f))
                    .peekFailure(f -> System.out.println("Exception1? : " + f.getThrowable()))
                    .map(f -> f / 0)
                    .peek(f -> System.out.println("Float? : " + f))
                    .peekFailure(f -> System.out.println("Exception2? : " + f.getThrowable()))
                    .map(f -> mybytes)
                    .peek(b -> System.out.println("bytes? " + b))
                    .peekFailure(f -> System.out.println("Exception3? : " + f.getThrowable()))
                    .map(bytes -> new String(bytes, 0, 10, "UTF-128"))
                    .peek(s -> System.out.println("String? " + s))
                    .peekFailure(f -> System.out.println("Exception4? : " + f.getThrowable()))
                    .recover(s -> s, f -> f.getMessage());
    //Try from a two argument Function
    Try<Integer> myTry = Try.of((a, b) -> a / b, 3, 0);
    //Try to Option
    Optional<Integer> oInt = myTry.toOptional();
    //Try to Either
    Either<Throwable, Integer> integerEither = myTry.toEither();

    //flatMap - a map without nested Try as the result
    Integer divByZero = Try.of(Integer::new, 3)
            .flatMap(i -> div(i, 0))
            .recover(Function.identity(), a -> 0);
    //orElse when Failure
    Integer shouldEqZero = Try.of((a, b) -> a / b, 1, 0).orElse(0);
    //orElseGet when Failure (the fallback Supplier is lazy)
    Integer shouldEqZero = Try.of((a, b) -> a / b, 1, 0).orElseGet(() -> 0);
    //forEach only runs when success
    new Success<>(3)
            .forEach(i -> System.out.println("a number : " + i));
    //escape the Try structure and enter a regular try-catch flow
    Try<Object> myTry = Try.of(() -> {
        throw new IOException("floppy drive too busy");
    try {
        myTry.orElseThrow(e -> new AWTException("hah!"));
    } catch (AWTException e) {
        //handle and/or propagate
private Try<Integer> div(Integer a, Integer b) {
    return Try.of((x,y) -> x / y, a, b);
//this function must end with a value
private String handleFailure(Exception e) {
    return "I'm afraid this didnt work because of " + e.getMessage();