Pragmatic Functional Refactoring with Java 8

Pragmatic Functional
Refactoring with Java 8
Raoul-Gabriel Urma (@raoulUK)
Richard Warburton (@RichardWarburto)

First-class Functions
Currying
Immutability
Optional Data Types
Conclusions

Step 1: filtering invoices from Oracle
public List<Invoice> findInvoicesFromOracle(List<Invoice> invoices) {
List<Invoice> result = new ArrayList<>();
for(Invoice invoice: invoices) {
if( invoice.getCustomer() == Customer.ORACLE) {
result.add(invoice);
}
}
return result;
}

Step 2a: abstracting the customer
public List<Invoice> findInvoicesFromCustomer(List<Invoice> invoices,
Customer customer) {
if( invoice.getCustomer() == customer) {
}
}
return result;
}

Step 2b: abstracting the name
public List<Invoice> findInvoicesEndingWith(List<Invoice> invoices,
String suffix) {
if( invoice.getName().endsWith(suffix)) {
}
}
return result;
}

Step 3: messy code-reuse!
private List<Invoice> findInvoices(List<Invoice> invoices,
Customer customer,
String suffix,
boolean flag) {
if(( flag && invoice.getCustomer() == customer)
|| (!flag && invoice.getName().endsWith(suffix))) {
}
}
return result;
}

Step 4a: modeling the filtering criterion
public interface Predicate<T> {
boolean test(T t);
}
Invoice boolean
Predicate<Invoice>

Step 4b: using different criterion with objects
private List<Invoice> findInvoices(List<Invoice> invoices,
Predicate<Invoice> p) {
if( p.test(invoice)) {
}
}
return result;
}

Step 4b: using different criterion with objects
List<Invoice> specificInvoices =
findInvoices(invoices, new FacebookTraining());
class FacebookTraining implements Predicate<Invoice> {
@Override
public boolean test(Invoice invoice) {
return invoice.getCustomer() == Customer.FACEBOOK
&& invoice.getName().endsWith("Training");
}
}

Step 5: method references
public boolean isOracleInvoice(Invoice invoice) {
return invoice.getCustomer() == Customer.ORACLE;
}
public boolean isTrainingInvoice(Invoice invoice) {
return invoice.getName().endsWith("Training");
}
List<Invoice> oracleInvoices =
findInvoices(invoices, this::isOracleInvoice);
List<Invoice> trainingInvoices =
findInvoices(invoices, this::isTrainingInvoice);
method references

Step 6: lambdas
List<Invoice> oracleInvoices =
findInvoices(invoices,
invoice -> invoice.getCustomer() == Customer.ORACLE);
List<Invoice> trainingInvoices =
findInvoices(invoices,
invoice -> invoice.getName().endsWith("Training"));

First-class functions
● Common Object Oriented concept: Function Object
● All it means is the ability to use a function just like a regular value
○ Pass it as argument to a method
○ Store it in a variable
● Helps cope with requirement changes

Composing functions
Functions Composing functions

Composing functions: example
import java.util.function.Predicate;
Predicate<Invoice> isFacebookInvoice = this::isFacebookInvoice;
List<Invoice> facebookAndTraining =
invoices.stream()
.filter( isFacebookInvoice.and(this::isTrainingInvoice))
.collect(toList());
List<Invoice> facebookOrGoogle =
invoices.stream()
creating more complex
functions from building blocks
.filter( isFacebookInvoice.or(this::isGoogleInvoice))
.collect(toList());

Composing functions: why does it work?
public interface Predicate<T> {
boolean test(T t);
default Predicate<T> and(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) && other.test(t);
}
default Predicate<T> or(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) || other.test(t);
}
}
returns a new
function that is
the result of
composing two
functions

Creating function pipelines (1)
public class Email {
private final String message;
public Email(String message) {
this.message = message;
}
public Email addHeader() {
return new Email("Dear Sir/Madam:n" + message);
}
public Email checkSpelling() {
return new Email(message.replaceAll("FTW", "for the win"));
}
public Email addSignature() {
return new Email(message + "nKind regards");
}
}

import java.util.function.Function;
Function<Email, Email> addHeader = Email::addHeader;
Function<Email, Email> processingPipeline =
addHeader.andThen(Email::checkSpelling)
.andThen(Email::addSignature);
andThen andThen
composing
functions
addHeader checkSpelling addSignature
Email
{message="Java
8 FTW!"}
Email{message='Dear
Sir/Madam:
Java 8 for the win!
Kind regards'}

import java.util.function.Function;
Function<Email, Email> addHeader = Email::addHeader;
Function<Email, Email> processingPipeline =
addHeader.andThen(Email::addSignature);
andThen
composing
functions
addHeader addSignature
Email
{message="Java
8 FTW!"}
Email{message='Dear
Sir/Madam:
Java 8 FTW!
Kind regards'}

Example: A Conversion Function
double convert(double amount, double factor, double base)
{
return amount * factor + base;
}
// Usage
double result = convert(10, 1.8, 32);
assertEquals(result, 50, 0.0);

Using the conversion function
// Temperatures
double cInF = convert(10, 1.8, 32);
double higherTemp = convert(30, 1.8, 32);
// Currencies
double dollarsInPounds = convert(10, 0.6, 0);
// Distance
double kilometresInMiles = convert(27, 0.62137, 0);

So what’s the problem?
● Abstraction
○ How do you write something that operates on different conversion
functions?
● Reuse
○ How do you use the conversion function in different places?

Currying the conversion function
DoubleUnaryOperator convert(double factor, double base) {
return amount -> amount * factor + base;
}
DoubleUnaryOperator convertCtoF = convert(1.8, 32);
double result = convertCtoF.applyAsDouble(10);
assertEquals(result, 50, 0.0);

Partial Application Examples
// Temperatures
DoubleUnaryOperator convertCtoF = convert(1.8, 32);
// Currencies
DoubleUnaryOperator convert$to£ = convert(0.6, 0);
// Distance
DoubleUnaryOperator convertKmToMi = convert(0.62137, 0);

Summary
● Currying is about splitting up the arguments of a function.
● Partial Application is a function “eating” some of its arguments and
returning a new function

Mutable objects
public class TrainJourney {
private int price;
private TrainJourney onward;
public TrainJourney(int price, TrainJourney onward) {
this.price = price;
this.onward = onward;
}
public int getPrice() {
return price;
}
public TrainJourney getOnward() {
return onward;
}
public void setPrice(int price) {
this.price = price;
}
public void setOnward(TrainJourney onward) {
}
}

Immutable objects
public class TrainJourneyImmutable {
private final int price;
private final TrainJourneyImmutable onward;
public TrainJourneyImmutable(int price, TrainJourney onward) {
this.price = price;
}
public int getPrice() {
return price;
}
public TrainJourneyImmutable getOnward() {
return onward;
}
public TrainJourneyImmutable withPrice(int price) {
return new TrainJourneyImmutable(price, getOnward());
}
public TrainJourneyImmutable withOnward(TrainJourneyImmutable onward) {
return new TrainJourneyImmutable(getPrice(), onward);
}
}

Scenario: be able to link together train
journeys to form longer journeys.

Mutable approach
public TrainJourney link(TrainJourney start, TrainJourney continuation) {
if (start == null) {
return continuation;
}
TrainJourney t = start;
while (t.getOnward() != null) {
t = t.getOnward();
}
t.setOnward(continuation);
return start;
}
return the continuation journey if
there is no start journey
find the last stop in the first journey
modify it to link to the continuation
journey

Mutable approach: problem
TrainJourney firstJourney = link(start, continuation);
TrainJourney secondJourney = link(start, continuation);
visit(secondJourney,
tj -> { System.out.print(tj.price + " - "); });
static void visit(TrainJourney journey, Consumer<TrainJourney> c)
{
if (journey != null) {
c.accept(journey);
visit(journey.onward, c);
}
}

Immutable approach
public TrainJourneyImmutable link(TrainJourneyImmutable start,
TrainJourneyImmutable continuation) {
return start == null ? continuation
: new TrainJourneyImmutable(start.getPrice(),
link(start.getOnward(), continuation));
}

Related topics
● Domain Driven Design
○ Value Classes are Immutable
● Core Java Improvements
○ New date & time library in Java 8 has many Immutable Objects
○ Current Value Types proposal is immutable
● Tooling
○ final keyword only bans reassignment
○ JSR 308 - improved annotation opportunities
○ Mutability Detector
○ Findbugs

Immutable objects reduce the scope for bugs.

Don’t we all love it?
Exception in thread "main" java.lang.NullPointerException

public String getCarInsuranceName(Person person) {
return person.getCar().getInsurance().getName();
}
Where’s the NPE?

Defensive checking
if (person != null) {
Car car = person.getCar();
if (car != null) {
Insurance insurance = car.getInsurance();
if (insurance != null) {
return insurance.getName();
}
}
}
return "No Insurance";
}

Optional
● Java 8 introduces a new class java.util.Optional<T>
○ a single-value container
● Explicit modelling
○ Immediately clear that its an optional value
○ better maintainability
● You need to actively unwrap an Optional
○ force users to think about the absence case
○ fewer errors

Updating model
public class Person {
private Optional<Car> car;
public Optional<Car> getCar() { return car; }
}
public class Car {
private Optional<Insurance> insurance;
public Optional<Insurance> getInsurance() { return insurance; }
}
public class Insurance {
private String name;
public String getName() { return name; }
}

Why it doesn’t work?
Optional<People> optPerson = Optional.ofNullable(person);
Optional<String> name =
optPeople.map(Person::getCar)
.map(Car::getInsurance)
.map(Insurance::getName);
returns Optional<Optional<Car>>
Invalid, the inner Optional object
doesn’t support the method
getInsurance!
public class Person {
private Optional<Car> car;
public Optional<Car> getCar() { return car; }
}

Chaining methods with flatMap
return Optional.ofNullable(person)
.flatMap(Person::getCar)
.flatMap(Car::getInsurance)
.map(Insurance::getName)
.orElse("Unknown");
}
default value if the resulting
Optional is empty

Consistent use of Optional replaces the use of null

Summary of benefits
● First-class functions let you cope for requirement changes
● Currying lets you split the parameters of a function to re-use code logic
● Immutability reduces the scope for bugs
● Optional data types lets you reduce null checking boilerplate and
prevent bugs

Training Website: http://java8training.com
http://manning.com/urma http://tinyurl.com/java8lambdas

Any Questions?
Richard Warburton
@richardwarburto
Raoul-Gabriel Urma
@raoulUK

Example currying use cases
● Large factory methods
○ Partially apply some of the arguments and then pass this factory
object around.
● Parser Combinators
○ many(‘a’) - function which parses a, aa, aaa, etc.

A curry function
// int -> (int -> int)
IntFunction<IntUnaryOperator>
curry(IntBinaryOperator biFunction) {
return f -> s -> biFunction.applyAsInt(f, s);
}
// Usage:
IntFunction<IntUnaryOperator> add = curry((f, s) -> f +
s);
int result = add.apply(1)
.applyAsInt(2);
assertEquals(3, result);

Generalised curry function
// F -> (S -> R)
<F, S, R> Function<F, Function<S, R>>
curry(BiFunction<F, S, R> biFunction) {
return f -> s -> biFunction.apply(f, s);
}

Pragmatic Functional Refactoring with Java 8

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