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Writing better functional java code - devnexus

The document discusses functional programming concepts in Java, including lambda expressions, streams, and immutable objects. It provides examples of using lambda expressions to pass functions as arguments. It emphasizes that streams should not mutate objects and explains how to return new copies instead. It also discusses avoiding side effects with forEach and creating higher-order functions to reduce duplicate code.

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Writing better Functional Java code@BrianVerm
@BrianVerm
@BrianVerm
@BrianVerm
@BrianVerm Brian Vermeer Developer Advocate
Don’t do to much in a single lambda @BrianVerm
Lambda Expression In computer programming, a lambda expression is a function definition that is not bound to an identifier. Anonymous functions are often:[1] • arguments being passed to higher-order functions, or • used for constructing the result of a higher-order function that needs to return a function. @BrianVerm
Lambda in Java Anonymous Inner functions Satisfy a Functional Interface They only exist in runtime Single Line <input> -> <output> or Block <input> -> { function body } @BrianVerm
Example @BrianVerm public void execute() { Beer grolsch = new Beer("Grolsch", 4.3); String result = handleBeer(grolsch, beer -> grolsch.getName() + "-" + grolsch.getAlcohol()); System.out.println(result); } private String handleBeer(Beer beer, Function<Beer, String> func) { System.out.println("handling beer " + beer.getName()); return func.apply(beer); } //Output : Grolsch-4.3
Example @BrianVerm public void execute() { Beer grolsch = new Beer("Grolsch", 4.3); String result = handleBeer(grolsch, beer -> grolsch.getName() + "-" + grolsch.getAlcohol()); System.out.println(result); } private String handleBeer(Beer beer, Function<Beer, String> func) { System.out.println("handling beer " + beer.getName()); return func.apply(beer); } //Output : Grolsch-4.3
Don’t do Block Lambda @BrianVerm beer -> { String name; if (beer.getName().contains(" ")) { name = beer.getName().replace(" ", ""); } else { name = beer.getName(); } try { name += Integer.parseInt(beer.getAlcoholPrecentage().toString()); } catch (NumberFormatException nfe) { name += beer.getAlcoholPrecentage(); } return name; }
Transform to a methode @BrianVerm private String createFullName (Beer beer){ String name; if (beer.getName().contains(" ")) { name = beer.getName().replace(" ", ""); } else { name = beer.getName(); } try { name += Integer.parseInt(beer.getAlcoholPrecentage().toString()); } catch (NumberFormatException nfe) { name += beer.getAlcoholPrecentage(); } return name; } String result = handleBeer(grolsch, this::createFullName);
Returning a Stream @BrianVerm
A stream is NOT a data structure @BrianVerm
A stream is NOT a data structure @BrianVerm A stream is NOT a data structure
What is Stream ( in Java) Flow of data derived from a Collection Can create a pipeline of function that can be evaluated Intermediate result Lazy evaluated by nature Can transform data, cannot mutate data @BrianVerm
JAVA Streams @BrianVerm stringLists.stream()
 .map(str -> str.toUpperCase())
 .collect(Collectors.toList()); Intermediate filter 
 distinct
 map 
 flatMap
 Terminal reduce
 collect
 toArray
 count
 max
 min
 limit 
 skip
 sorted
 peek findAny
 findFirst
 forEach
 allMatch
 anyMatch
List<Beer> beers = getBeers();
 Stream<Beer> beerStream = beers.stream();
 
 beerStream.forEach(b ->System.out.println(b.getName())); //1 
 beerStream.forEach(b ->System.out.println(b.getAlcohol())); //2 Only use a Stream once Line 2 will give: 
 
 java.lang.IllegalStateException: stream has already been operated upon or closed @BrianVerm
public Stream<Beer> getMeMyBeers() public void execute() { getMeMyBeers() //don’t know if it is consumed yet!!!! … } Be careful with returning a Stream @BrianVerm
Returning a Stream Private intermediate function When new stream is created every time When result is very large or might be infinite By default return a collection @BrianVerm
Mutable Object @BrianVerm
Functional Programming In computer science, functional programming is a programming paradigma style of building the structure and elements of computer programs that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions or declarations instead of statements. — wikipedia @BrianVerm
@BrianVerm
@BrianVerm – Kevlin Henney “Asking a question should not change the answer, nor should asking it twice”
Immutable objects Less moving parts Easier to reason about code No need to keep a mental map of the state an object is in. @BrianVerm
Stream cannot mutate @BrianVerm private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.setName(“foo”)) //not allowed .collect(Collectors.toList()); }
Stream should not mutate ! @BrianVerm private class Beer { String name; Double alcohol; public Beer setName(String name){ this.name = name; return this; } } private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.setName("foo")) .collect(Collectors.toList()); System.out.println(beers); System.out.println(beersNew); }
Stream should not mutate ! @BrianVerm private class Beer { String name; Double alcohol; public Beer setName(String name){ this.name = name; return this; } } private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.setName("foo")) .collect(Collectors.toList()); System.out.println(beers); System.out.println(beersNew); }
Return a new copy @BrianVerm private class Beer { String name; Double alcohol; public Beer withName(String name){ return new Beer(name, this.alcohol); } } private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.withName("foo")) .collect(Collectors.toList()); System.out.println(beers); System.out.println(beersNew); }
Overusing forEach @BrianVerm
forEach() Terminal functional on a stream. Takes a consumer. Can be used to apply side effects. for-loop without the external iterator @BrianVerm
simple forEach example @BrianVerm List<String> names = List.of("James", "Trisha", "Joshua", "Jessica","Simon", “Heather”, “Roberto”); names.stream() .map(String::toUpperCase) .forEach(System.out::println);
mutation with forEach @BrianVerm List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); beers.stream() .forEach(beer -> beer.setAlcohol(0.0));
overusing forEach @BrianVerm private class Beer { String name; Double alcohol; List<String> reviews; Integer rating; } List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); //enrich with ratings beers.stream().forEach(beer -> beer.setRating(findRating(beer.getName()))); //enrich with reviews beers.stream().forEach(beer -> beer.setReviews(findReviews(beer.getName()))); …
Order of operations @BrianVerm
Assignment Beers -> Brewer -> Country I want the first 3 unique brewers countries from the beer library as comma separated String @BrianVerm beerLib.stream() .map(Beer::getBrewer) .distinct() .limit(3) .map(Brewer::getCountry) .map(String::toUpperCase) .collect(Collectors.joining(“,”));
Assignment Beers -> Brewer -> Country I want the first 3 unique brewers countries from the beer library as comma separated String @BrianVerm beerLib.stream() .map(Beer::getBrewer) .distinct() .limit(3) .map(Brewer::getCountry) .map(String::toUpperCase) .collect(Collectors.joining(“,”)); // wrong
Assignment Beers -> Brewer -> Country I want the first 3 unique brewers countries from the beer library as comma separated String @BrianVerm beerLib.stream() .map(Beer::getBrewer) .map(Brewer::getCountry) .map(String::toUpperCase) .distinct() .limit(3) .collect(Collectors.joining(“,”)); // correct
Infinite stream @BrianVerm IntStream.iterate(0, i -> ( i + 1) % 2)
 .distinct()
 .limit(10)
 .forEach(i -> System.out.println(i));
Infinite stream @BrianVerm IntStream.iterate(0, i -> ( i + 1) % 2)
 .distinct()
 .limit(10)
 .forEach(i -> System.out.println(i));
 // will run forever IntStream.iterate(0, i -> ( i + 1) % 2)
 .limit(10)
 .distinct()
 .forEach(i -> System.out.println(i)); //will terminate
Infinite stream @BrianVerm IntStream.iterate(0, i -> ( i + 1) % 2)
 .parallel() .distinct()
 .limit(10)
 .forEach(i -> System.out.println(i));
 // will run forever on all threads.
Solution 1. Look closely at the order of operations
 - prevent incorrect answers
 
 2. Only use infinite streams when absolutely necessary. rather use: 
 @BrianVerm IntStream.range(0,10); IntStream.rangeClosed(0,10); IntStream.iterate(0, i -> i < 10, i -> i + 1); //java 9 and up
Create higher-order functions @BrianVerm
Higher-Order function Taking a function as a parameter of a method Insert behaviour instead of just data A new level of abstraction Reduce boilerplate / duplicate code @BrianVerm
Duplicate code @BrianVerm private void logUpper(String str) {
 //super interesting code
 System.out.println(str.toUpperCase());
 //even more interesting code
 }
 
 private void logLower(String str) {
 //super interesting code
 System.out.println(str.toLowerCase());
 //even more interesting code
 }
Insert behaviour @BrianVerm private void logUpper(String string) {
 doSuperCoolStuff(string, s -> s.toUpperCase());
 }
 
 private void logLower(String string) {
 doSuperCoolStuff(string, s -> s.toLowerCase());
 }
 
 private void doFoo(String str, Function<String, String> func) {
 //super interesting code
 System.out.println(func.apply(str));
 //even more interesting code
 }
Real situation (Webshop Basket) MongoDB primary (read/write), multiple secondary (read) Writing only on the primary Mutating a basket: Read from Primary
 Edit
 Run rule engine
 Save to Primary
 @BrianVerm
Real situation (Webshop Basket) MongoDB primary (read/write), multiple secondary (read) Writing only on the primary Mutating a basket: Read from Primary
 Edit
 Run rule engine
 Save to Primary
 @BrianVerm
Mutator @BrianVerm public void mutatorById(String id, Consumer<Basket> action) { Basket basket = repoPrimary.findById(id); action.accept(basket); RuleEngine.run(basket) repoPrimary.save(basket); }
Mutator @BrianVerm public void mutatorById(String id, Consumer<Basket> action) { Basket basket = repoPrimary.findById(id); action.accept(basket); RuleEngine.run(basket) repoPrimary.save(basket); } public void addItem(String uuid, Item item) { mutatorById(uuid, basket -> basket.addItem(item)); } public void removeItem(String uuid, Item item) { mutatorById(uuid, basket -> basket.removeItem(item)); }
Getting an Optional @BrianVerm
Optional Java’s implementation of the Maybe Monad Encapsulation to handle possible null value Consider it a wrapper where a value can be absent Force the user to unpack the Optional before using it. @BrianVerm
Optional @BrianVerm Optional<String> c = null //please avoid this
public void execute() { Optional<String> maybeString = getText(); String unpacked = maybeString.get(); } Unpack Optional @BrianVerm
public void execute() { Optional<String> maybeString = getText(); String unpacked = maybeString.get(); } Unpack Optional @BrianVerm NoSuchElementException
public void execute() { Optional<String> maybeString = getText(); String unpacked = maybeString.get(); } Unpack Optional @BrianVerm public void execute() { Optional<String> maybeString = getText(); if (maybeString.isPresent()) { String unpacked = maybeString.get(); } } NoSuchElementException
Unpack Optional @BrianVerm public void execute() { Optional<String> maybeString = getText(); maybeString.ifPresent( str -> /* doSomething */ ); } public void execute() { Optional<String> maybeString = getText(); maybeString.map(str -> str + “."); }
What else ..?? @BrianVerm
Alternative flow - orElse() - orElseGet() - orElseThrow() @BrianVerm
orElseThrow @BrianVerm public void execute() {
 Optional<String> maybeString = Optional.empty();
 maybeString
 .map(this::runIfExist)
 .orElseThrow(() -> new RuntimeException("Optional was empty"));
 }
orElse @BrianVerm public void execute() {
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElse(runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 }
orElse @BrianVerm public void execute() {
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElse(runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 } only run if optional is filled only run if empty foo
orElseGet @BrianVerm public void execute() {
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElseGet(() -> runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 }
orElseGet @BrianVerm public void execute() {
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElseGet(() -> runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 } only run if optional is filled foo
what else? - When using orElse(x), make sure x doesn’t contain any side effects - Only use orElse() to assign a default value - Use orElseGet() to run an alternative flow @BrianVerm
List of Optionals @BrianVerm beerLib.stream() .map(Beer::getDescription) //returns optional
List of Optionals @BrianVerm //java 8 style beerLib.stream() .map(Beer::getDescription) //returns optional .filter(Optional::isPresent) .map(Optional::get) .forEach(System.out::println);
List of Optionals @BrianVerm //java 8 flatMap beerLib.stream() .map(Beer::getDescription) //returns optional .flatMap(o -> o.map(Stream::of).orElse(Stream.empty())) .forEach(System.out::println);
List of Optionals @BrianVerm //java 9 flatMap beerLib.stream() .map(Beer::getDescription) //returns optional .flatMap(Optional::stream) .forEach(System.out::println);
Exceptions @BrianVerm
Checked Exceptions & Lambda @BrianVerm 
 public Beer doSomething(Beer beer) throws IsEmptyException { …} Function <Beer,Beer> fBeer = beer -> doSomething(beer)
Checked Exceptions & Lambda @BrianVerm public Beer doSomething(Beer beer) throws IsEmptyException { …} Function <Beer,Beer> fBeer = beer -> doSomething(beer)
Checked Exceptions & Lambda @BrianVerm public Beer doSomething(Beer beer) throws IsEmptyException { …} beerLib.stream() .map(beer -> {
 try{
 return doSomething(beer);
 } catch (IsEmptyException e) {
 throw new RuntimeException(e);
 }
 };) .collect(Collectors.toList()); //not very pretty
Checked Exceptions & Lambda @BrianVerm public Beer doSomething(Beer beer) throws IsEmptyException { …} private Beer wrappedDoSomeThing(Beer beer) {
 try{
 return doSomething(beer);
 } catch (IsEmptyException e) {
 throw new RuntimeException(e);
 }
 } beerLib.stream() .map(this::wrappedDoSomeThing) .collect(Collectors.toList());
Exception Utility @BrianVerm @FunctionalInterface
 public interface CheckedFunction<T, R> {
 public R apply(T t) throws Exception;
 } public static <T, R> Function<T, R> wrap(CheckedFunction<T, R> function) {
 return t -> {
 try {
 return function.apply(t);
 } catch (Exception ex) {
 throw new RuntimeException(ex);
 }
 };
 }; beerLib.stream() .map(wrap(beer -> doSomething(beer))) .collect(Collectors.toList());
Either type @BrianVerm public class Either<L, R> { private final L left; private final R right; private Either(L left, R right) { this.left = left; this.right = right; } public static <L,R> Either<L,R> Left( L value) { return new Either(value, null); } public static <L,R> Either<L,R> Right( R value) { return new Either(null, value); } … }
Either type @BrianVerm private Either<Exception, String> canGoWrong(Integer input) { if (input > 10) { return Either.Left(new RuntimeException("larger then 10")); } return Either.Right("["+input+"]"); } List<Either<Exception, String>> canGoWrongs = IntStream.range(0,12) .mapToObj(i -> canGoWrong(i)) .collect(Collectors.toList());
Either type @BrianVerm private Either<Exception, String> canGoWrong(Integer input) { if (input > 10) { return Either.Left(new RuntimeException("larger then 10")); } return Either.Right("["+input+"]"); } List<Either<Exception, String>> canGoWrongs = IntStream.range(0,12) .mapToObj(i -> canGoWrong(i)) .collect(Collectors.toList()); canGoWrongs.stream() .map(e -> e.mapRight(s -> s.toUpperCase())) .flatMap(o -> o.map(Stream::of).orElseGet(Stream::empty)) .forEach(System.out::println);
Try - Failure (Exception) - Success (Type) - VAVR @BrianVerm
Try - Failure (Exception) - Success (Type) - VAVR @BrianVerm List<Try<String>> output = teams.stream() .map(CheckedFunction1.liftTry(this::mayThrowException)) .collect(Collectors.toList());
@BrianVerm
@BrianVerm Brian Vermeer @BrianVerm brianvermeer@snyk.io

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Writing better functional java code - devnexus

  • 1.
  • 2.
  • 3.
  • 4.
  • 5.
  • 6.
    Don’t do tomuch in a single lambda @BrianVerm
  • 7.
    Lambda Expression In computer programming,a lambda expression is a function definition that is not bound to an identifier. Anonymous functions are often:[1] • arguments being passed to higher-order functions, or • used for constructing the result of a higher-order function that needs to return a function. @BrianVerm
  • 8.
    Lambda in Java AnonymousInner functions Satisfy a Functional Interface They only exist in runtime Single Line <input> -> <output> or Block <input> -> { function body } @BrianVerm
  • 9.
    Example @BrianVerm public void execute(){ Beer grolsch = new Beer("Grolsch", 4.3); String result = handleBeer(grolsch, beer -> grolsch.getName() + "-" + grolsch.getAlcohol()); System.out.println(result); } private String handleBeer(Beer beer, Function<Beer, String> func) { System.out.println("handling beer " + beer.getName()); return func.apply(beer); } //Output : Grolsch-4.3
  • 10.
    Example @BrianVerm public void execute(){ Beer grolsch = new Beer("Grolsch", 4.3); String result = handleBeer(grolsch, beer -> grolsch.getName() + "-" + grolsch.getAlcohol()); System.out.println(result); } private String handleBeer(Beer beer, Function<Beer, String> func) { System.out.println("handling beer " + beer.getName()); return func.apply(beer); } //Output : Grolsch-4.3
  • 11.
    Don’t do BlockLambda @BrianVerm beer -> { String name; if (beer.getName().contains(" ")) { name = beer.getName().replace(" ", ""); } else { name = beer.getName(); } try { name += Integer.parseInt(beer.getAlcoholPrecentage().toString()); } catch (NumberFormatException nfe) { name += beer.getAlcoholPrecentage(); } return name; }
  • 12.
    Transform to amethode @BrianVerm private String createFullName (Beer beer){ String name; if (beer.getName().contains(" ")) { name = beer.getName().replace(" ", ""); } else { name = beer.getName(); } try { name += Integer.parseInt(beer.getAlcoholPrecentage().toString()); } catch (NumberFormatException nfe) { name += beer.getAlcoholPrecentage(); } return name; } String result = handleBeer(grolsch, this::createFullName);
  • 13.
  • 14.
    A stream isNOT a data structure @BrianVerm
  • 15.
    A stream isNOT a data structure @BrianVerm A stream is NOT a data structure
  • 16.
    What is Stream( in Java) Flow of data derived from a Collection Can create a pipeline of function that can be evaluated Intermediate result Lazy evaluated by nature Can transform data, cannot mutate data @BrianVerm
  • 17.
    JAVA Streams @BrianVerm stringLists.stream()
 .map(str ->str.toUpperCase())
 .collect(Collectors.toList()); Intermediate filter 
 distinct
 map 
 flatMap
 Terminal reduce
 collect
 toArray
 count
 max
 min
 limit 
 skip
 sorted
 peek findAny
 findFirst
 forEach
 allMatch
 anyMatch
  • 18.
    List<Beer> beers =getBeers();
 Stream<Beer> beerStream = beers.stream();
 
 beerStream.forEach(b ->System.out.println(b.getName())); //1 
 beerStream.forEach(b ->System.out.println(b.getAlcohol())); //2 Only use a Stream once Line 2 will give: 
 
 java.lang.IllegalStateException: stream has already been operated upon or closed @BrianVerm
  • 19.
    public Stream<Beer> getMeMyBeers() publicvoid execute() { getMeMyBeers() //don’t know if it is consumed yet!!!! … } Be careful with returning a Stream @BrianVerm
  • 20.
    Returning a Stream Privateintermediate function When new stream is created every time When result is very large or might be infinite By default return a collection @BrianVerm
  • 21.
  • 22.
    Functional Programming In computerscience, functional programming is a programming paradigma style of building the structure and elements of computer programs that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions or declarations instead of statements. — wikipedia @BrianVerm
  • 23.
  • 24.
    @BrianVerm – Kevlin Henney “Askinga question should not change the answer, nor should asking it twice”
  • 25.
    Immutable objects Less movingparts Easier to reason about code No need to keep a mental map of the state an object is in. @BrianVerm
  • 26.
    Stream cannot mutate @BrianVerm privatefinal List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.setName(“foo”)) //not allowed .collect(Collectors.toList()); }
  • 27.
    Stream should notmutate ! @BrianVerm private class Beer { String name; Double alcohol; public Beer setName(String name){ this.name = name; return this; } } private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.setName("foo")) .collect(Collectors.toList()); System.out.println(beers); System.out.println(beersNew); }
  • 28.
    Stream should notmutate ! @BrianVerm private class Beer { String name; Double alcohol; public Beer setName(String name){ this.name = name; return this; } } private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.setName("foo")) .collect(Collectors.toList()); System.out.println(beers); System.out.println(beersNew); }
  • 29.
    Return a newcopy @BrianVerm private class Beer { String name; Double alcohol; public Beer withName(String name){ return new Beer(name, this.alcohol); } } private final List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); public void execute() { List<Beer> beersNew = beers.stream() .map(beer -> beer.withName("foo")) .collect(Collectors.toList()); System.out.println(beers); System.out.println(beersNew); }
  • 30.
  • 31.
    forEach() Terminal functional ona stream. Takes a consumer. Can be used to apply side effects. for-loop without the external iterator @BrianVerm
  • 32.
    simple forEach example @BrianVerm List<String>names = List.of("James", "Trisha", "Joshua", "Jessica","Simon", “Heather”, “Roberto”); names.stream() .map(String::toUpperCase) .forEach(System.out::println);
  • 33.
    mutation with forEach @BrianVerm List<Beer>beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); beers.stream() .forEach(beer -> beer.setAlcohol(0.0));
  • 34.
    overusing forEach @BrianVerm private classBeer { String name; Double alcohol; List<String> reviews; Integer rating; } List<Beer> beers = List.of(new Beer("Heineken", 5.2), new Beer("Amstel", 5.1)); //enrich with ratings beers.stream().forEach(beer -> beer.setRating(findRating(beer.getName()))); //enrich with reviews beers.stream().forEach(beer -> beer.setReviews(findReviews(beer.getName()))); …
  • 35.
  • 36.
    Assignment Beers -> Brewer-> Country I want the first 3 unique brewers countries from the beer library as comma separated String @BrianVerm beerLib.stream() .map(Beer::getBrewer) .distinct() .limit(3) .map(Brewer::getCountry) .map(String::toUpperCase) .collect(Collectors.joining(“,”));
  • 37.
    Assignment Beers -> Brewer-> Country I want the first 3 unique brewers countries from the beer library as comma separated String @BrianVerm beerLib.stream() .map(Beer::getBrewer) .distinct() .limit(3) .map(Brewer::getCountry) .map(String::toUpperCase) .collect(Collectors.joining(“,”)); // wrong
  • 38.
    Assignment Beers -> Brewer-> Country I want the first 3 unique brewers countries from the beer library as comma separated String @BrianVerm beerLib.stream() .map(Beer::getBrewer) .map(Brewer::getCountry) .map(String::toUpperCase) .distinct() .limit(3) .collect(Collectors.joining(“,”)); // correct
  • 39.
    Infinite stream @BrianVerm IntStream.iterate(0, i-> ( i + 1) % 2)
 .distinct()
 .limit(10)
 .forEach(i -> System.out.println(i));
  • 40.
    Infinite stream @BrianVerm IntStream.iterate(0, i-> ( i + 1) % 2)
 .distinct()
 .limit(10)
 .forEach(i -> System.out.println(i));
 // will run forever IntStream.iterate(0, i -> ( i + 1) % 2)
 .limit(10)
 .distinct()
 .forEach(i -> System.out.println(i)); //will terminate
  • 41.
    Infinite stream @BrianVerm IntStream.iterate(0, i-> ( i + 1) % 2)
 .parallel() .distinct()
 .limit(10)
 .forEach(i -> System.out.println(i));
 // will run forever on all threads.
  • 42.
    Solution 1. Look closelyat the order of operations
 - prevent incorrect answers
 
 2. Only use infinite streams when absolutely necessary. rather use: 
 @BrianVerm IntStream.range(0,10); IntStream.rangeClosed(0,10); IntStream.iterate(0, i -> i < 10, i -> i + 1); //java 9 and up
  • 43.
  • 44.
    Higher-Order function Taking afunction as a parameter of a method Insert behaviour instead of just data A new level of abstraction Reduce boilerplate / duplicate code @BrianVerm
  • 45.
    Duplicate code @BrianVerm private voidlogUpper(String str) {
 //super interesting code
 System.out.println(str.toUpperCase());
 //even more interesting code
 }
 
 private void logLower(String str) {
 //super interesting code
 System.out.println(str.toLowerCase());
 //even more interesting code
 }
  • 46.
    Insert behaviour @BrianVerm private voidlogUpper(String string) {
 doSuperCoolStuff(string, s -> s.toUpperCase());
 }
 
 private void logLower(String string) {
 doSuperCoolStuff(string, s -> s.toLowerCase());
 }
 
 private void doFoo(String str, Function<String, String> func) {
 //super interesting code
 System.out.println(func.apply(str));
 //even more interesting code
 }
  • 47.
    Real situation (WebshopBasket) MongoDB primary (read/write), multiple secondary (read) Writing only on the primary Mutating a basket: Read from Primary
 Edit
 Run rule engine
 Save to Primary
 @BrianVerm
  • 48.
    Real situation (WebshopBasket) MongoDB primary (read/write), multiple secondary (read) Writing only on the primary Mutating a basket: Read from Primary
 Edit
 Run rule engine
 Save to Primary
 @BrianVerm
  • 49.
    Mutator @BrianVerm public void mutatorById(Stringid, Consumer<Basket> action) { Basket basket = repoPrimary.findById(id); action.accept(basket); RuleEngine.run(basket) repoPrimary.save(basket); }
  • 50.
    Mutator @BrianVerm public void mutatorById(Stringid, Consumer<Basket> action) { Basket basket = repoPrimary.findById(id); action.accept(basket); RuleEngine.run(basket) repoPrimary.save(basket); } public void addItem(String uuid, Item item) { mutatorById(uuid, basket -> basket.addItem(item)); } public void removeItem(String uuid, Item item) { mutatorById(uuid, basket -> basket.removeItem(item)); }
  • 51.
  • 52.
    Optional Java’s implementation ofthe Maybe Monad Encapsulation to handle possible null value Consider it a wrapper where a value can be absent Force the user to unpack the Optional before using it. @BrianVerm
  • 53.
  • 54.
    public void execute(){ Optional<String> maybeString = getText(); String unpacked = maybeString.get(); } Unpack Optional @BrianVerm
  • 55.
    public void execute(){ Optional<String> maybeString = getText(); String unpacked = maybeString.get(); } Unpack Optional @BrianVerm NoSuchElementException
  • 56.
    public void execute(){ Optional<String> maybeString = getText(); String unpacked = maybeString.get(); } Unpack Optional @BrianVerm public void execute() { Optional<String> maybeString = getText(); if (maybeString.isPresent()) { String unpacked = maybeString.get(); } } NoSuchElementException
  • 57.
    Unpack Optional @BrianVerm public voidexecute() { Optional<String> maybeString = getText(); maybeString.ifPresent( str -> /* doSomething */ ); } public void execute() { Optional<String> maybeString = getText(); maybeString.map(str -> str + “."); }
  • 58.
  • 59.
    Alternative flow - orElse() -orElseGet() - orElseThrow() @BrianVerm
  • 60.
    orElseThrow @BrianVerm public void execute(){
 Optional<String> maybeString = Optional.empty();
 maybeString
 .map(this::runIfExist)
 .orElseThrow(() -> new RuntimeException("Optional was empty"));
 }
  • 61.
    orElse @BrianVerm public void execute(){
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElse(runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 }
  • 62.
    orElse @BrianVerm public void execute(){
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElse(runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 } only run if optional is filled only run if empty foo
  • 63.
    orElseGet @BrianVerm public void execute(){
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElseGet(() -> runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 }
  • 64.
    orElseGet @BrianVerm public void execute(){
 Optional<String> maybeString = Optional.of("foo");
 String newString = maybeString
 .map(this::runIfExist)
 .orElseGet(() -> runIfEmpty());
 System.out.println(newString);
 }
 private String runIfExist(String str) {
 System.out.println("only run if optional is filled ");
 return str;
 }
 private String runIfEmpty() {
 System.out.println("only run if empty");
 return "empty";
 } only run if optional is filled foo
  • 65.
    what else? - Whenusing orElse(x), make sure x doesn’t contain any side effects - Only use orElse() to assign a default value - Use orElseGet() to run an alternative flow @BrianVerm
  • 66.
  • 67.
    List of Optionals @BrianVerm //java8 style beerLib.stream() .map(Beer::getDescription) //returns optional .filter(Optional::isPresent) .map(Optional::get) .forEach(System.out::println);
  • 68.
    List of Optionals @BrianVerm //java8 flatMap beerLib.stream() .map(Beer::getDescription) //returns optional .flatMap(o -> o.map(Stream::of).orElse(Stream.empty())) .forEach(System.out::println);
  • 69.
    List of Optionals @BrianVerm //java9 flatMap beerLib.stream() .map(Beer::getDescription) //returns optional .flatMap(Optional::stream) .forEach(System.out::println);
  • 70.
  • 71.
    Checked Exceptions &Lambda @BrianVerm 
 public Beer doSomething(Beer beer) throws IsEmptyException { …} Function <Beer,Beer> fBeer = beer -> doSomething(beer)
  • 72.
    Checked Exceptions &Lambda @BrianVerm public Beer doSomething(Beer beer) throws IsEmptyException { …} Function <Beer,Beer> fBeer = beer -> doSomething(beer)
  • 73.
    Checked Exceptions &Lambda @BrianVerm public Beer doSomething(Beer beer) throws IsEmptyException { …} beerLib.stream() .map(beer -> {
 try{
 return doSomething(beer);
 } catch (IsEmptyException e) {
 throw new RuntimeException(e);
 }
 };) .collect(Collectors.toList()); //not very pretty
  • 74.
    Checked Exceptions &Lambda @BrianVerm public Beer doSomething(Beer beer) throws IsEmptyException { …} private Beer wrappedDoSomeThing(Beer beer) {
 try{
 return doSomething(beer);
 } catch (IsEmptyException e) {
 throw new RuntimeException(e);
 }
 } beerLib.stream() .map(this::wrappedDoSomeThing) .collect(Collectors.toList());
  • 75.
    Exception Utility @BrianVerm @FunctionalInterface
 public interfaceCheckedFunction<T, R> {
 public R apply(T t) throws Exception;
 } public static <T, R> Function<T, R> wrap(CheckedFunction<T, R> function) {
 return t -> {
 try {
 return function.apply(t);
 } catch (Exception ex) {
 throw new RuntimeException(ex);
 }
 };
 }; beerLib.stream() .map(wrap(beer -> doSomething(beer))) .collect(Collectors.toList());
  • 76.
    Either type @BrianVerm public classEither<L, R> { private final L left; private final R right; private Either(L left, R right) { this.left = left; this.right = right; } public static <L,R> Either<L,R> Left( L value) { return new Either(value, null); } public static <L,R> Either<L,R> Right( R value) { return new Either(null, value); } … }
  • 77.
    Either type @BrianVerm private Either<Exception,String> canGoWrong(Integer input) { if (input > 10) { return Either.Left(new RuntimeException("larger then 10")); } return Either.Right("["+input+"]"); } List<Either<Exception, String>> canGoWrongs = IntStream.range(0,12) .mapToObj(i -> canGoWrong(i)) .collect(Collectors.toList());
  • 78.
    Either type @BrianVerm private Either<Exception,String> canGoWrong(Integer input) { if (input > 10) { return Either.Left(new RuntimeException("larger then 10")); } return Either.Right("["+input+"]"); } List<Either<Exception, String>> canGoWrongs = IntStream.range(0,12) .mapToObj(i -> canGoWrong(i)) .collect(Collectors.toList()); canGoWrongs.stream() .map(e -> e.mapRight(s -> s.toUpperCase())) .flatMap(o -> o.map(Stream::of).orElseGet(Stream::empty)) .forEach(System.out::println);
  • 79.
    Try - Failure (Exception) -Success (Type) - VAVR @BrianVerm
  • 80.
    Try - Failure (Exception) -Success (Type) - VAVR @BrianVerm List<Try<String>> output = teams.stream() .map(CheckedFunction1.liftTry(this::mayThrowException)) .collect(Collectors.toList());
  • 81.
  • 82.