Expert Java 8

Lambdas, Streams, Optionals
For Afficionados

Nicolai Parlog

codefx.org / @nipafx

Objektforum KA

@objektforum

Public Service Announcement

you need to be proficient with Java 8
some of what follows is merely my opinion
slides at slides.codefx.org

Streams

In APIs

Exception Handling

Finding Elements

Performance

Streams

In APIs

Exception Handling

Finding Elements

Performance

Using streams in your APIs.

Interesting Stream Properties

streams are unmodifiable
streams are easy to transform
streams are lazy

⇝ Good way to enrich data in layers.

Enrichment Example

Map<String, User> userById;
Map<String, Address> addrById;

Stream<User> users() {
	return userById.values().stream();
}

Stream<UserAddress> userAddresses() {
	return users()
		.map(user -> UserAddress.of(
				user, addrById.get(user.id())));
}

Enrichment Example

Map<Address, List<Order>> ordersByAddr;

Stream<Delivery> deliveries() {
	return userAddresses().flatMap(this::deliveriesFor);
}

Stream<Delivery> deliveriesFor(UserAdress userAddr) {
	return ordersFor(userAddr)
		.map(order -> Delivery.of(userAddr, order));
}

Stream<Order> ordersFor(UserAdress userAddr) {
	return ordersByAddr
		.get(userAddr.address()).stream();
}

Guidelines

streams can only be traversed once
⇝ ideally returned streams can be recreated endlessly
during traversal collection must not be mutated
⇝ stick to layers instead of cycles
(can be harder than it sounds)
decide careful where to call parallel()

Accepting Streams

Returning streams is ok.

What about passing them as parameters?

streams can only be traversed once!
caller must assume stream is traversed
works for obvious consumer functions
possibly a transformed stream can be returned

Stream Param Example

// consuming the passed stream
void addUsers(Stream<User> users) {
	users.forEach(this.users::add)
}

// transforming the passed stream
Stream<UserAddress> userAddresses(Stream<User> users) {
	return users
		.map(user -> UserAddress.of(
				user, addrById.get(user.id())));
}

Reflection On APIs

Returning Streams

returning streams is great
(unmodifiable but transformable)
preferably if streams can be recreated
can be used to gradually enrich data

Reflection On APIs

Passing Streams

caller must assume stream is traversed
works for obvious consumers
transformations can be hard to track
never return a traversed stream
(obvious, right?!)

Reflection On APIs

But Look Out

streams can only be traversed once
no mutation during traversal
don’t make chains too long or
debuggability suffers

Streams

In APIs

Exception Handling

Finding Elements

Performance

Handling checked exceptions in Streams.

Setting the Scene

Stream<User> parse(Stream<String> strings) {
	// compile error:
	// "incompatible thrown types ParseException"
	return strings.map(this::parse);
}

User parse(String userString) throws ParseException {
	// ...
}

Which options do we have?

Try in Lambda

Stream<User> parse(Stream<String> strings) {
	return strings
		.map(string -> { try {
				return parse(string);
			} catch (ParseException ex) {
				return null;
			}})
		.filter(Objects::nonNull);
}

super ugly
requires extra clean-up step
handling exception locally can be hard
troublesome elements "disappear"

Try in Method

Stream<User> parse(Stream<String> strings) {
	return strings
			.map(this::tryParse)
			.filter(Objects::nonNull);
}

private User tryParse(String string) {
	try { return parse(string); }
	catch (ParseException ex) { return null; }
}

somewhat ugly
requires extra clean-up step ("far away")
handling exception locally can be hard
troublesome elements "disappear"

Sneaky Throws

How to "trick the compiler":

static Function<T, R> hideException(
		CheckedFunction<T, R, Exception> function) {
	return element -> {
		try {
			return function.apply(element);
		} catch (Exception ex) {
			return sneakyThrow(ex);
		}
	};
}

// sneakyThrow does shenanigans with generics
// and unchecked casts to "confuse the compiler"

Sneaky Throws

Stream<User> parse(Stream<String> strings) {
	return strings
		.map(Util.hideException(this::parse));
}

very surprising (hides a bomb in the stream!)
stream executor has to handle exception
can’t try-catch(ParseException) because
checked exceptions need to be declared
exception aborts stream pipeline

Please never do that!

Wrap in Unchecked

Another Util method:

static Function<T, R> uncheckException(
		CheckedFunction<T, R, Exception> function) {
	return element -> {
		try {
			return function.apply(element);
		} catch (Exception ex) {
			throw new IllegalArgumentException(
				element, ex);
		}
	};
}

Wrap in Unchecked

Stream<User> parse(Stream<String> strings) {
	return strings
		.map(Util.uncheckException(this::parse));
}

stream executor has to handle exception
exception aborts stream pipeline

Remove Trouble

Another Util method:

static Function<T, Optional<R>> wrapOptional(
		CheckedFunction<T, R, Exception> function) {
	return element -> {
		try {
			return Optional.of(
				function.apply(element));
		} catch (Exception ex) {
			return Optional.empty();
		}
	};
}

Remove Trouble

Stream<User> parse(Stream<String> strings) {
	return strings
		.map(Util.wrapOptional(this::parse))
		// Java 9: .flatMap(Optional::stream)
		.filter(Optional::isPresent)
		.map(Optional::get);
}

requires extra clean-up step
(at least supported by compiler)
troublesome elements "disappear"

Expose With `Try`

Try<T> is similar to Optional:

has two states: error or success
allows to process them with functions
parameterized in type of success result

Another Util method:

static Function<T, Try<R>> wrapTry(
		CheckedFunction<T, R, Exception> function) {
	return element -> Try.of(
		() -> function.apply(element));
}

Expose With `Try`

Stream<Try<User>> parse(Stream<String> strings) {
	return strings
		.map(Util.wrapTry(this::parse));
}

requires external library (e.g. Javaslang)
encodes possibility of failure in API
makes error available to caller
error is encoded as Exception/Throwable

Expose With `Either`

Either<L, R> is similar to Optional:

has two states: left or right
allows to process them with functions
parameterized in types of left and right
if used for failure/success, exception goes left
(by convention)

Expose With `Either`

Another Util method:

static Function<T, Either<EX, R>> wrapEither(
		CheckedFunction<T, R, EX> function) {
	return element -> {
		try {
			return Either.right(
				function.apply(element));
		} catch (Exception ex) {
			return Either.left((EX) ex);
		}
	};
}

Expose With `Either`

Stream<Either<ParseException, User>> parse(
		Stream<String> strings) {
	return strings
		.map(Util.wrapEither(this::parse));
}

requires external library (e.g. Javaslang)
encodes possibility of failure in API
makes error available to caller
error has correct type

Reflection on Exceptions

don’t be smart and "trick the compiler"
return a clean stream: no nulls!
ideally, use types to express possibility of failure

Streams don’t cooperate well with checked exceptions.

See that as a chance to use functional concepts
for greater good of code base!

Streams

In APIs

Exception Handling

Finding Elements

Performance

Be careful how you find!

Finding First or Any

Stream::findFirst and findAny:

return an arbitrary element from the Stream
if stream has encounter order,
findFirst returns first element

Often used after a filter.

Find Example

Optional<User> findUser(String id) {
	return users.stream()
		.filter(user -> user.id().equals(id))
		.findFirst();
}

Same as the loop:

Optional<User> findUser(String id) {
	for (User user : users)
		if (user.id().equals(id))
			return Optional.of(user);
	return Optional.empty();
}

Small Observation

I sometimes see the following:

code’s correctness depends on only
one element passing the filter
but there are no additional checks

⇝ The easy solution might be the wrong one!

(Applies to the loop as well.)

Finding Only

Make sure there is only one element:

Optional<User> findUser(String id) {
	return users.stream()
		.filter(user -> user.id().equals(id))
		.reduce(toOnlyElement());
}

static BinaryOperator toOnlyElement() {
	return (element, otherElement) -> {
		throw new IllegalArgumentException();
	};
}

Instead of reduce, collect could be used.

Properties of Finding Only

Upsides:

guarantees correctness by failing fast
expresses intent

Downsides:

materializes entire stream

Reflection On Finding

If correctness depends on only one element
surviving an ad-hoc filter:

findFirst, findAny do not suffice
use a reducer or collector to assert uniqueness
comes with a performance penalty

Additional Sources

Beware Of findFirst() And findAny()

Stream

In APIs

Exception Handling

Finding Elements

Performance

Performance is too long to go into.

Great talk by Stuart Marks and Brian Goetz:

Thinking in Parallel (JavaOne 2016)

Optional

Everybody's Favorite Bike Shed!

Usage Patterns

Value-Based Class

Not a Monad

Optional

Usage Patterns

Value-Based Class

Not a Monad

The Java community strongly disagrees
on how to best use Optional.

Some insights into the discussion…

Basic Rules

First some basic rules:

never, ever, ever just call get
without checking isPresent first
prefer functional style
(map, flatMap, ifPresent, orElse, …)
make everyone setting Optional to null
buy a round of drinks or wear a silly hat

Basic Rules

Nobody (?) wants to see …

Optional.ofNullable(mango)
	.ifPresent(System.out::println);

... instead of …

if (mango != null)
	System.out.println(mango);

Different Opinions

don’t use it unless
absolutely necessary
use it as return value
use it everywhere

Don’t Use It!

Assumptions

API is verbose and invites misuse
makes stack traces harder to debug
not serializable
unsupported by various frameworks
dereferencing reduces performance
instances increase memory consumption
no benefits over explicit null handling

Don’t Use It!

Conclusions

Optional sucks
only use it if existing API returns it
unpack quickly!

(Mark Struberg, Stephen Connolly, Hugues Johnson)

Limited Return Value

Assumptions

was designed as a return value
not serializable
long-lived instances increase
memory consumption
boxing method arguments is verbose

Limited Return Value

Conclusions

use as return value if
returning null is error-prone
no instance variables
no method parameters
instances should generally be short-lived

(Stuart Marks, Brian Goetz)

⇝ This should be your default choice!

Return Value

Assumptions

returning null is always error-prone
rest as before

Conclusions

use as return value whenever
value can be absent
rest as before

(Stephen Colebourne)

Use Everywhere!

Assumptions

using Optional instead of null
lifts null-handling into the type system
makes any null an implementation error
(great for debugging)
performance arguments can be discarded
unless proven to be relevant

Use Everywhere!

Conclusions

avoid optionality through good design
(good recommendation in general)
use Optional instead of null everywhere
consider providing overloads
for optional method parameters

(Mario Fusco, me)

Use Everywhere!

Overload Example

String bar(Optional<String> drink) {
	return drink.map(this::bar)
			.orElseGet(this::bar);
}

String bar(String drink) { /* ... */ }

String bar() { /* ... */ }

Reflection on Usage

Whatever you decide:

pick my recommendation! :)
make it a team decision
put it into your code style
learn over time

Relaxing rules is easier
than making them stricter!

Additional Sources

The Mother of All Bikesheds (Stuart Marks)

What’s the Point? (huguesjohnson.com)

Design Of Optional (codefx.org)

Pragmatic Approach (joda.org)

Strict Approach (codefx.org)

Optional

Usage Patterns

Value-Based Class

Not a Monad

Optional implements a new "pattern"
that requires us to be careful with what we do.

Value-Based Class?

Did you RTFM?

Optional Javadoc says:

This is a value-based class; use of identity-sensitive operations […] on instances of Optional may have unpredictable results and should be avoided.

What does it mean?

Value Types In Future Java

Future Java (10?) will contain value types:

pass by value
(copied when passed as params)
immutable
no identity

Very similar to today’s primitives.

No Identity?

Class instances have identity:

each new Integer(5) creates a new instance
they are not identical (!=, different locks, …)

Value types will have no identity:

there are no two different int 5
only their value matters

But Isn’t This Java 8?

From value types to value-based classes:

value types require wrappers/boxes
(just like primitives do today)
value-based classes might turn out
wrapping value types
as an optimization the JVM will
create and destroy them at will

⇝ Wrappers have identity but it is unstable

Identity Crisis

Map<User, LocalDateTime> userLogins;
Map<LocalDateTime, String> messages;

String lastLoginMessage(User user) {
	LocalDateTime lastLogin =
		userLogins.get(user); (1)
	String message = "Was " + lastLogin;
	messages.put(lastLogin, message); (2)
	return message;
}

get might return an instance or a value
put might receive an instance or a value

Requirements For VBC

declaration site

final and immutable
equals, hashCode, toString
must only rely on instance state
…

use site

no use of ==, identity hash code,
locking, serialization

(None of this is checked by the JVM.)

VBC in Java 8

java.util: Optional[Double, Long, Int]
java.time: Duration, Instant, Period,
Year, YearMonth, MonthDay,
Local…, Offset…, Zoned…
java.time.chrono: HijrahDate, JapaneseDate, MinguaDate, ThaiBuddhistDate

Reflection on VBC

With Optional and other value-based classes:

never rely on their identity
mainly no ==, locking, serialization

If this works out,
performance hit all but disappears!

Additional Sources

Value-Based Classes (codefx.org)

Optional

Usage Patterns

Value-Based Class

Not a Monad

Optional saves us from null
at the expense of breaking Monad Laws.

(No math, I promise!)

Left Identity

For a Monad, this should always be true:

Objects.equals(
	ofNullable(x).flatMap(f),
	f.apply(x));

But:

Function f = s -> of("mango")
Optional ofMap =
	ofNullable(null).flatMap(f);
Optional apply = f.apply(null);
// Optional[] != Optional["mango"]

Associativity

For a Monad, this should always be true:

Objects.equals(
	ofNullable(x).map(f).map(g),
	ofNullable(x).map(f.andThen(g)));

But:

Function f = s -> null;
Function g = s -> "mango";
Optional map = of("kiwi").map(f).map(g);
Optional then = of("kiwi").map(f.andThen(g));
// Optional[] != Optional["mango"]

Root Cause Analysis

Optional maps null to empty()
flatMap and map are not executed
on empty optionals
the first occurrence of null/empty
stops the chain of executions

So What?

refactoring can change
which code gets executed
functions that can "recover" from null
might not get executed
particularly error-prone when
functions have side effects
(they generally should not but it happens)

Reflection on Monads

be aware that Optional is no well-behaved monad
see it as a way to avoid handling null
be aware that refactoring can cause problems
if null was special cased

Additional Sources

More on Optional Being Broken (atlassian.com)

… and Why It Matters (sitepoint.com)

Default Methods

Interface Evolution

General Approach

New Version

interface is transitional (old and new outline)
default methods ensure existing code works

Transition

client moves from old to new outline
default methods ensure code keeps working

New Version

removes old outline

Adding Methods

Reasonable default impl exists:

New Version

add the method with default impl
internal impls can override
internal callers use new method

Transition

external callers use the method

That’s it.

Adding Methods

No reasonable default impl exists:

New Version

add method with default impl throwing UOE
override method in all internal impls

Transition

external impls override the method
external callers use the method

New Version

make method abstract
internal callers use new method

Removing Methods

No external impls exist:

New Version

deprecate method
internal callers stop calling method

Transition

external callers stop using the method

New Version

remove the method

(No default methods required.)

Removing Methods

External impls exist:

New Version

deprecate method
provide default impl throwing UOE
internal callers stop calling method

Transition

external callers stop using the method
external impls of the method are removed

New Version

remove the method

Replacing Methods

Applies with new signature (name, parameters, …),
where methods are "functionally equivalent".

Otherwise it’s a matter of adding new
and removing old method.

Replacing Methods

New Version

add new with default impl calling old
provide default impl of old calling new
deprecate old
internal impls override new instead of old
internal callers use new instead of old

Wtf, circular call?

ensures it does not matter which version is impl’d
must be thoroughly documented; tests help

Replacing Methods

Transition

external impls override new instead of old
external callers use new instead of old

New Version

make new abstract
remove old

Reflection On Evolution

If clients can be expected to update their code
default methods allow interface evolution
without breaking client code.

Mode is always the same:

release version with transitional outline
give clients time to update
release version with new outline

Additional Source

Interface Evolution (codefx.org)

New Javadoc Tags (codefx.org)

About Me

Half price with code mlparlog until 6th Apr.!

Nicolai Parlog
codefx.org
sitepoint.com/java
@nipafx / +NicolaiParlog

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Image Credits

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question-mark: Milos Milosevic, CC-BY 2.0

Expert Java 8

Lambdas, Streams, OptionalsFor Afficionados

Public Service Announcement

Streams

Streams

Interesting Stream Properties

Enrichment Example

Enrichment Example

Guidelines

Accepting Streams

Stream Param Example

Reflection On APIs

Returning Streams

Reflection On APIs

Passing Streams

Reflection On APIs

But Look Out

Streams

Setting the Scene

Try in Lambda

Try in Method

Sneaky Throws

Sneaky Throws

Wrap in Unchecked

Wrap in Unchecked

Remove Trouble

Remove Trouble

Expose With Try

Expose With Try

Expose With Either

Expose With Either

Expose With Either

Reflection on Exceptions

Streams

Finding First or Any

Find Example

Small Observation

Finding Only

Properties of Finding Only

Reflection On Finding

Additional Sources

Stream

Optional

Everybody's Favorite Bike Shed!

Optional

Basic Rules

Basic Rules

Different Opinions

Don’t Use It!

Assumptions

Don’t Use It!

Conclusions

Limited Return Value

Assumptions

Limited Return Value

Conclusions

Return Value

Assumptions

Conclusions

Use Everywhere!

Assumptions

Use Everywhere!

Conclusions

Use Everywhere!

Overload Example

Reflection on Usage

Additional Sources

Optional

Value-Based Class?

Value Types In Future Java

No Identity?

But Isn’t This Java 8?

Identity Crisis

Requirements For VBC

VBC in Java 8

Reflection on VBC

Additional Sources

Optional

Left Identity

Associativity

Lambdas, Streams, Optionals
For Afficionados

Expose With `Try`

Expose With `Try`

Expose With `Either`

Expose With `Either`

Expose With `Either`