Java8 and Functional Programming

Java8 and Functional Programming

• 1.
  Java8 Yiguang Hu
• 2.
  What has beenhappening in Computer ❖ Moor’s Law no longer applicable ❖ Computer speed is not increasing unlimited ❖ MultiCore Computer ❖ Pre-Java8 does not take advantage of multicore computer Power ❖ Threading is hard, multicore make it harder
• 3.
  Computer Languages ❖ Groovy,Scala, Clojure, Kotlin, JavaScript/CoffeeScript, Go, Swift, Lua, ErLang, Haskell, Ruby,C#,F#,… ❖ Closure ❖ Lambda Expression ❖ Pre-Java8 Did not have this, so what?
• 4.
  What is Closure “Incomputer science, a closure is a first-class function with free variables that are bound in the lexical environment.”
• 5.
  Groovy Closure def myConst= 5 def incByConst = { num -> num + myConst } println incByConst(10) //==>15 myConst = 20 println incByConst(10) //==>30 def result=[] (1..10).collect(result,{a->a*a}) result==[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
• 6.
  What is LambdaExpression ❖ A concise representation of an anonymous function that can be passed around: ❖ doesn't have a name, but has ❖ a list of parameters ❖ a body ❖ a return type ❖ possibly a list of exceptions
• 7.
  Example Lambda Expression //Anonymousversion Runnable r1 = new Runnable(){ @Override public void run(){ System.out.println("Hello world one!"); } }; // Lambda Runnable Runnable r2 = () -> System.out.println("Hello world two!"); // Run em! r1.run(); r2.run(); Compare the one-liner lambda with the 4+ lines legacy code!
• 8.
  What’s new inJava8 ❖ Lambda expression ❖ Default methods ❖ Stream ❖ optionals ❖ Date/Time API
• 9.
  What Lambda Expression brings? ❖Pass code concisely-functions are passed as values ❖ Collection: ❖ List<Person> pl ;//Given a list of Persons ❖ looping: pl.forEach( p -> p.printName() ); ❖ filtering and chaining: ❖ pl.stream().filter(p->p.getAge()>30).sort(comparing((a)- >a.getAge())).forEach(Person::printName); ❖ This one line replaces 20 lines of traditional code and does more (later on stream)
• 10.
  functional interfaces ❖ functionalinterface specifies exactly one abstract method ❖ Examples: ❖ Predicate<T>:boolean test(T t) ❖ Comparator<T>:compare(T o1, T o2)
• 11.
  Default Methods ❖ pl.stream().filter(p- >p.getAge()>30).sort(comparing((a)- >a.getAge()).reversed().thenComparing(Person:: getHeight())).forEach(Person::printName);
• 12.
  Default Methods ❖ Interfacecan contain method signatures for which an implementation class doesn’t have to provide implementation ❖ interface can now provide default method implementation
• 13.
  Streams ❖ Manipulate collectionof data in a declarative way. ❖ Streams can be processed in parallel transparently without having to write any multithreaded code!
• 14.
  Stream example ❖ pl.stream().filter(p->p.getAge()>30).sort(comparing((a)- >a.getAge()).reversed().thenComparing(Person::getHeigh t())).map(Person::getName()).collect(toList)) ❖To Exploit multicore architecture and execute the code in parallel ❖ pl.parallelStream().filter(p- >p.getAge()>30).sort(comparing((a)- >a.getAge()).reversed().thenComparing(Person::getHeigh t())).map(Person::getName()).collect(toList))
• 15.
  Hi Map Collect/Reduce ❖pl.parallelStream().filter(p- >p.getAge()>30).sort(comparing((a)- >a.getAge()).reversed().thenComparing(Person::getHei ght())).map(Person::getName()).collect(toList)) ❖ pl.parallelStream().filter(p- >p.getAge()>30).sort(comparing((a)- >a.getAge()).reversed().thenComparing(Person::getHei ght())).map(Person::getSallary()).reduce(Integer::sum)).get ()
• 16.
  Optionals ❖ String version= computer.getSoundcard().getUSB().getVersion(); ❖ Problem: Multiple NPE possible. Traditional way: String version = "UNKNOWN"; if(computer != null){ Soundcard soundcard = computer.getSoundcard(); if(soundcard != null){ USB usb = soundcard.getUSB(); if(usb != null){ version = usb.getVersion(); } } }
• 17.
  Optionals ❖ The problemwith Null ❖ source of error: NPE ❖ bloats code ❖ meaningless ❖ break java philosophy ❖ a hole in type system
• 18.
  –Tony Hoare Introducing Nullreference in ALGOL W in 1965 was “my billion-dollar mistake”
• 19.
  Optionals ❖ Groovy safenavigation operator+Elvis operator ❖ String version = computer?.soundcard?.uSB.version ?: "UNKNOWN";
• 20.
  Optionals ❖ how tomodel “absence of a value” ❖ Scala introduced Option[T] ❖ Java 8 call it Optional[T] ❖ Optional<String> version = optcomputer.map(Computer::getSoundcard).map(Sound Card::getUSB).map(USB::getVersion)
• 21.
  Beyond Java 8 ❖Functional programming: Focus on What ❖ Scala example ❖ flatten(List(List(1, 1), 2, List(3, List(5, 8)))) ❖ result: List(1, 1, 2, 3, 5, 8) ❖ Imperative programming: Focus on How ❖ Many lines needed to do the above 1-line job ❖ Functional programming is itself a great topic!