Poly-paradigm Java

Speaker
@pavletko
pav@p12v.comPavel Tcholakov

Programming styles
• Imperative
• Object-oriented
• Aspect-oriented
• Functional
• Concurrent
• Actor
• Message-passing
• Logic, or rule-based

Lambda expressions
(Pojo p) -> {
return p.getProperty();
}

Streams
IntStream.range(0, 100)
.filter(i -> i % 2 == 0)
.map(i -> i * i)
.sum();

Longest string’s length
public int longestStringLength(List<String> strings) {
return strings.stream()
.mapToInt(String::length)
.max()
.getAsInt();
}

public String longestString(List<String> strings) {
.mapToInt(String::length)
.max()
// Ehh, now what???
}

final String[] maxString = {null};
strings.stream()
.forEach(s -> {
if (s.length() > maxString[0].length()) {
maxString[0] = s;
}
});
return maxString[0];
}

Longest string
.reduce((s1, s2) ->
s1.length() > s2.length() ? s1 : s2)
.get();
}

Longest string
.max(Comparator.comparingInt(s -> s.length()))
.get();
}

Streams and functions
public List<String> findAllPalindromes(List<String> strings) {
.filter(s -> Objects.equals(s, StringUtils.reverse(s)))
.collect(Collectors.toList());
}

Functions, continued
public List<String> findAllPalindromes(List<String> strings) {
.filter(s -> Objects.equals(s, StringUtils.reverse(s)))
}

Higher order function
public List<String> findUnchangedStrings(
List<String> strings,
Function<String, String> transformation) {
.filter(s -> Objects.equals(s, transformation.apply(s)))
}

Higher order, continued
public List<String> palindromes(List<String> strings) {
return findUnchangedThings(strings, StringUtils::reverse);
}
public List<String> findShouting(List<String> strings) {
return findUnchangedThings(strings, StringUtils::capitalize);
}

Generic enough?
public List<String> findUnchangedStrings(
List<String> strings,
Function<String, String> transformation) {
.filter(s -> Objects.equals(s, transformation.apply(s)))
}

Type parameters
public <T> List<T> findUnchangedThings(
List<? extends T> things,
Function<? super T, ? extends T> transformation) {
return things.stream()
.filter(t -> Objects.equals(t, transformation.apply(t)))
}

public List<String> palindromes(List<String> strings) {
return findUnchangedThings(strings, StringUtils::reverse);
}
public List<String> findShouting(List<String> strings) {
return findUnchangedThings(strings, StringUtils::capitalize);
}
public List<Integer> nonNegative(List<Integer> ints) {
return findUnchangedThings(ints, Math::abs);
}
public <T> List<T> findUnchangedThings(
List<? extends T> things,
Function<? super T, ? extends T> transformation) {
return things.stream()
.filter(t -> Objects.equals(t, transformation.apply(t)))
}

Internal DSLs
Vagrant.configure("2") do |config|
config.vm.box = "hashicorp/precise64"
config.vm.provision :shell, path: "bootstrap.sh"
config.vm.network :forwarded_port, guest: 80, host: 4567
end

JavaSlang pattern matching
Stream.of(0, 1, 2, 3, 13, 14, null, -1)
.peek(n -> out.print(format("%d -> ", n)))
.map(Match.as(Object.class)
.when(Objects::isNull).then("!")
.whenIs(0).then("zero")
.whenIsIn(1, 13, 14).then(i -> "first digit 1: " + i)
.whenType(Double.class).then(d -> "Found a double: " + d)
.whenApplicable((Number num) -> "number: " + num).thenApply()
.otherwise(() -> "no match"))
.map(Object::toString)

Concurrency-oriented
programming
• We identify all the truly concurrent activities in our
real world activity.
• We identify all message channels between the
concurrent activities.
• We write down all the messages which can flow on
the different message channels.
— Joe Armstrong

Akka actor in Java
public class Greeter extends AbstractActor {
String greeting = "";
public Greeter() {
receive(ReceiveBuilder.
match(WhoToGreet.class,
message -> greeting = "hello, " + message.who).
match(Greet.class,
message -> sender()
.tell(new Greeting(greeting), self())).
build());
}
}

Prolog
fact.
another_fact.
jug(jozi_jug).
jug(virtual_jug).
java_meetup(java_day).
attendee(pavel, java_day).
attendee(nitsan, java_day).
attendee(richard, linux_meetup).
interested(java, Who) :-
attendee(Who, Meetup),
java_meetup(Meetup).

N-queens in Prolog
solution([]).
solution([X/Y|Others]) :-
solution(Others),
member(Y, [1,2,3,4,5,6,7,8]),
noattack(X/Y, Others).
noattack(_, []).
noattack(X/Y, [X1/Y1|Others]) :-
Y == Y1,
Y1 - Y == X1 - X,
Y1 - Y == X - X1,
noattack(X/Y,Others).

Sudoku in Clojure core.logic
(defn sudokufd [hints]
(let [vars (repeatedly 81 lvar)
rows (->> vars (partition 9) (map vec) (into []))
cols (apply map vector rows)
sqs (for [x (range 0 9 3)
y (range 0 9 3)]
(get-square rows x y))]
(run 1 [q]
(== q vars)
(everyg #(fd/in % (fd/domain 1 2 3 4 5 6 7 8 9)) vars)
(init vars hints)
(everyg fd/distinct rows)
(everyg fd/distinct cols)
(everyg fd/distinct sqs))))

public interface Task {
boolean isInGoalState();
void handleEvents(Set<Event> events);
void applyRules(Consumer<Event> bus);
}

public boolean isInGoalState() {
return state == MemberState.FOLLOWER &&
lastHeartbeat.plus(ELECTION_TIMEOUT).isAfter(clock.now());
}
public void handleEvents(Set<Event> events) {
for (Event evt : events) {
if (evt instanceof AppendEntriesRpcReceived) {
lastHeartbeat = clock.now();
}
}
}
public void applyRules(Consumer<Event> eventBus) {
if (state == MemberState.FOLLOWER) {
if (lastHeartbeat.plus(ELECTION_TIMEOUT).isAfter(clock.now())) {
state = MemberState.CANDIDATE;
eventBus.accept(new MemberStateChange(state));
eventBus.accept(new LeaderVote(serverId));
} else {
// keep calm and carry on
}
} else {
// nothing to do in other states
}
}

Causes of complexity
• State
• Control
• Code volume
• Others: complexity breeds complexity

FRP architecture
• Essential State
➡ A Relational definition of the stateful components of the
system
• Essential Logic
➡ Derived-relation definitions, integrity constraints and (pure)
functions
• Accidental State and Control
➡ A declarative specification of a set of performance
optimisations for the system

for (int i = 1; i <= 100; i++) {
if (i % 15 == 0) {
out.println("FizzBuzz");
} else if (i % 3 == 0) {
out.println("Fizz");
} else if (i % 5 == 0) {
out.println("Buzz");
} else {
out.println(i);
}
}

static Predicate<Integer> divisibleBy(Integer div) {
return (i) -> i % div == 0;
}

class Replacement {
final Predicate<Integer> when; final String output;
public Replacement(Predicate<Integer> when, String output) {
this.when = when; this.output = output;
}
}
List<Replacement> fizzAndOrBuzz =
Collections.unmodifiableList(Arrays.asList(
new Replacement(divisibleBy(3), "Fizz"),
new Replacement(divisibleBy(5), "Buzz")));

static String replace(Integer i, List<Replacement> rules) {
return rules.stream()
.filter(replacement -> replacement.when.test(i))
.map(replacement -> replacement.output)
.reduce(String::concat)
.orElse(i.toString());
}

Stream<String> applicableReplacements = rules.stream()
.map(replacement -> replacement.output);
return applicableReplacements
}

static String fizzBuzz(Integer i) {
return replace(i, fizzAndOrBuzz);
}
// …
IntStream.rangeClosed(1, 100)
.mapToObj(FizzBuzz::fizzBuzz)
.forEach(out::println);

static Stream<String> fizzBuzz(IntStream intStream) {
return intStream.mapToObj(FizzBuzz::fizzBuzz);
}
// …
IntStream infiniteInts = IntStream.iterate(1, i -> i + 1);
fizzBuzz(infiniteInts)
.limit(100)

public interface FizzBuzz {
class Replacement { final Predicate<Integer> when; final String output; … }
static Predicate<Integer> divisibleBy(Integer d) { return i -> i % d == 0; }
List<Replacement> fizzBuzzRules = unmodifiableList(asList(
new Replacement(divisibleBy(3), "Fizz"),
new Replacement(divisibleBy(5), "Buzz")));
return rules.stream()
.map(replacement -> replacement.output)
}
static void main(String... args) {
IntStream.rangeClosed(1, 100)
.mapToObj(i -> replace(i, fizzBuzzRules))
}
}

Performance
Benchmark Mode Cnt Score Error Units
FizzBenchmark.functionalSolution avgt 5 13.122 ± 4.030 us/op
FizzBenchmark.functionalSolution2 avgt 5 16.922 ± 2.558 us/op
FizzBenchmark.imperativeSolution avgt 5 1.579 ± 0.142 us/op
2015 MacBook Pro 15”, i7 2.2GHz

Feedback
@pavletko
pav@p12v.comPavel Tcholakov

“Simplicity can only be attained if it is
recognised, sought and prized.”
— Excerpt From: Ben Moseley, “Out of the Tar Pit.”

Poly-paradigm Java

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