JDK 8 Functional Programming …For Groovy Developers

John Engelman • @johnrengelman • github.com/johnrengelman • ratpack.io • objectpartners.com/category/blog

Caveats • Scala • Clojure • Erlang • Haskell

…Big pile of Salt

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public static TransformablePublisher stream(Publisher publisher) { return Streams.transformable(subscriber -> require().streamSubscribe((handle) -> publisher.subscribe(new Subscriber() { @Override public void onSubscribe(final Subscription subscription) { handle.event(() -> subscriber.onSubscribe(subscription) ); } @Override public void onNext(final T element) { handle.event(() -> subscriber.onNext(element)); } @Override public void onComplete() { handle.complete(subscriber::onComplete); } @Override public void onError(final Throwable cause) { handle.complete(() -> subscriber.onError(cause)); } }) )); }

java.util.function https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html

JDK8 Functional Interfaces Conceptually, a functional interface has exactly one abstract method. - https://docs.oracle.com/javase/8/docs/api/java/ lang/FunctionalInterface.html

public interface Supplier { T get(); }

Supplier supply() { return (() -> { return new Foo(); }); } lambda expression parameters list body

Supplier supply() { return (() -> new Foo()); } implicit return where have we seen that before?

Supplier supply() { return Foo::new } method handle

Supplier supply() { return { new Foo() } } Supplier supply() { //http://mrhaki.blogspot.com/2015/03/groovy-goodness- use-constructor-as.htm Foo.metaClass.&invokeConstructor }

public interface Consumer { void accept(T t); }

Consumer consume() { return (val -> System.out.println(val)); } Consumer consumeMethod() { return System.out::println; }

Consumer consume() { return { val -> println val.name } } Consumer consumeMethod() { this.&println }

public interface Function { R appy(T t); }

Function convert() { return (foo -> { return new Bar(foo.name); }); } Function convertMethod() { return Bar::new; }

Function convert() { return { foo -> new Bar(foo.name) } } Function convertMethod() { Bar.metaClass.&invokeConstructor }

public interface BiConsumer { void accept(T t, U u); } public interface BiFunction { R apply(T t, U u); }

Where is TriFunction?

• BiConsumer • BiFunction • BinaryOperator • BiPredicate • BooleanSupplier • Consumer • DoubleBinaryOperator • DoubleConsumer • DoubleFunction • DoublePredicate • DoubleSupplier • DoubleToIntFunction • DoubleToLongFunction • DoubleUnaryOperator • Function • IntBinaryOperator • IntConsumer • IntFunction • IntPredicate • IntSupplier • IntToDoubleFunction • IntToLongFunction • IntUnaryOperator • LongBinaryOperator • LongConsumer • LongFunction • LongPredicate • LongSupplier • LongToDoubleFunction • LongToIntFunction • LongUnaryOperator • ObjDoubleConsumer • ObjIntConsumer • ObjLongConsumer • Predicate • Supplier • ToDoubleBiFunction • ToDoubleFunction • ToIntBitFunction • ToIntFunction • ToLongBiFunction • ToLongFunction • UnaryOperator Java • Closure Groovy

java.util.stream https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html

List filter(List foos) { return foos.stream() .filter(foo -> foo.name.equals(“bar") ).collect(Collectors.toList()); } List map(List foos) { return foos.stream() .map(Bar::new).collect(Collectors.toList()); } List distinct(List foos) { return foos.stream().distinct().collect(Collectors.toList()); }

List filter(List foos) { foos.findAll { foo -> foo.name == 'bar' } } List map(List foos) { foos.collect { new Bar(foo) } } List distinct(List foos) { foos.unique() }

public static TransformablePublisher stream(Publisher publisher) { return Streams.transformable(subscriber -> require().streamSubscribe((handle) -> publisher.subscribe(new Subscriber() { @Override public void onSubscribe(final Subscription subscription) { handle.event(() -> subscriber.onSubscribe(subscription) ); } @Override public void onNext(final T element) { handle.event(() -> subscriber.onNext(element)); } @Override public void onComplete() { handle.complete(subscriber::onComplete); } @Override public void onError(final Throwable cause) { handle.complete(() -> subscriber.onError(cause)); } }) )); }

public static TransformablePublisher stream(Publisher publisher) { return Streams.transformable { subscriber -> require().streamSubscribe { handle -> publisher.subscribe(new Subscriber() { @Override public void onSubscribe(final Subscription subscription) { handle.event { subscriber.onSubscribe(subscription) } } @Override public void onNext(final T element) { handle.event { subscriber.onNext(element) } } @Override public void onComplete() { handle.complete(subscriber.&onComplete) } @Override public void onError(final Throwable cause) { handle.complete { subscriber.onError(cause) } } }) } } }

Default Interface Methods • Game changer public interface Baz { String getName(); default Bar bar() { return new Bar(this); } } Bar interfaceMethod(Baz baz) { return baz.bar(); } Bar useInterface() { return interfaceMethod(() -> "John"); } becomes a Supplier lambda implicitly coerced to interface

Default Interface Methods • Not directly* supported in Groovy * Groovy 2.3 introduced Traits which are similar

References/More Info • https://github.com/danveloper/uberconf2014- from-groovy-to-java8/blob/master/from-groovy-to- java8.pdf • Anything w/ Venkat • https://www.youtube.com/watch? v=Ee5t_EGjv0A