Presentation about clean code Scala with counter examples in Java. Live recording is here http://livestre.am/A1SW (Polish)

1. Clean Code Scala Scala == Effective Java ?

5. „Leave camp cleaner than you found it.” Uncle Bob

7. Why do we write bad code ??? Rush (duck tape programmer) Laziness Careless We don’t know how good code looks like Lack of resources We like to do new things (create)

8. „Its harder to read the code than to write it” Joel Spolsky

9. How to improve work quality ? Dont give developers money rewardsbigger reward == worse performance Let them work on what they like and how they like (Google, Atlassian, Facebook) Allow and encourage self improvement

10. Java dev libraries Google Guava Apache Commons Spring Guice Scala !

11. Why Scala ? More concise More powerfull Fun! Fast Becoming popular !

12. Who uses Scala already

13. Class Parameters - Java publicclass Person { private String name; privateintage; publicPerson(String name, int age) { this.name = name; this.age = age; } public String getName() { returnname; } publicvoid setName(String name) { this.name = name; } publicint getAge() { returnage; } publicvoid setAge(int age) { this.age = age; } }

14. Class Parameters - Scala classPerson(var name: String, var age: Int)

15. publicclass Person { private String name; privateintage; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { returnname; } publicvoidsetName(String name) { this.name = name; } publicintgetAge() { returnage; } publicvoidsetAge(int age) { this.age = age; } @Override public String toString() { returnString.format("Person: %s age: %s", name, age); } @Override publicinthashCode() { inthashCode = 0; for(char c: name.toCharArray()) { hashCode += c; } return 11 * hashCode + age; } @Override publicboolean equals(Object other) { if(other == null) returnfalse; if(other instanceof Person) { Person person = (Person)other; returnperson.name.equals(name) && person.age == age; } returnfalse; }

16. Scala case class caseclass Person(name: String, age: Int) Dostajemy za darmo: equals(), hashCode() oraz toString() oraz niezmienną klasę (immutable).

17. To equal or not to equal ? Person p = new Person("Jan Kowalski", 30); Set<Person> set = new HashSet<Person>(); set.add(p); System.out.println(set.contains(p)); // true p.setAge(p.getAge()+1); System.out.println(set.contains(p)); // false ... WTF ???

18. To equal or not to equal ? Iterator<Person> it = set.iterator(); booleancontainedP = false; while (it.hasNext()) { Person nextP = it.next(); if (nextP.equals(p)) { containedP = true; break; } } System.out.println(containedP); // true // ... ???

20. Java – working with Person Object x = new Person("Bill Clinton", 64); if(x instanceof Person) { Person p = (Person)x; System.out.println(„Person name: "+p.getName()); } else { System.out.println("Not a person"); } x = "Lukasz Kuczera"; if(x instanceof Person) { Person p = (Person)x; System.out.println("hello "+p.getName()); } elseif(x instanceof String) { String s = (String)x; if(s.equals("Bill Clinton")) System.out.println("Hello Bill"); else System.out.println("hello: "+s); } else System.out.println("err, ???");

21. Scala – Pattern Matching var x: Any = Person("Lukasz", 28); x match { case Person(name, age) => println("Person name: "+name); case _ => println("Not a person") } x = "Lukasz Kuczera" x match { case Person(name, age) => println("Person name: "+name) case"Bill Clinton" => println("hello Bill") case s: String => println("hello "+s) case _ => "err, ???" } Person name: Lukasz hello Lukasz Kuczera

22. Parameter validation publicclass Person { private String name; privateintage; publicPerson(String name, int age) { if(name == null) { thrownew NullPointerException(); } if(age < 0) { thrownew IllegalArgumentException("Age < 0") } this.name = name; this.age = age; }

23. Parameter validation caseclass Person(name: String, age: Int) { // @elidable(ASSERTION)assert(age > 0, "Age < 0")// by name parameter }

24. Working with arrays Java publicclass Partition { Person[] all; Person[] adults; Person[] minors; { ArrayList<Person> minorsList = new ArrayList<Person>(); ArrayList<Person> adultsList = new ArrayList<Person>(); for(int i=0; i<all.length; i++ ) { (all[i].age<18 ? adultsList: minorsList).add(all[i]); } minors = (Person[]) minorsList.toArray(); adults = (Person[]) adultsList.toArray(); } }

25. Working with arrays Scala val all: Array[Person] val (minors, adults) = all.partition(_.age<18)

26. Null’s – Java Map<String, String> capitals = new HashMap<String, String>(); capitals.put("Poland", "Warsaw"); System.out.println(capitals.get("Polska").trim()); Exception in thread "main" java.lang.NullPointerException

27. Null’s - Scala val capitals = Map("Poland" -> "Warsaw"); val capitalOption: Option[String] = capitals.get("Polska") capitalOption match { case Some(value) => println(value) case None => println("Not found") case _ => } if(capitalOption.isDefined) println(capitalOption.get) println(capitalOption getOrElse "Not found")

28. Scala IO == Java + Commons IO vallines = Source.fromFile("d:scalaMobyDick.txt").getLines; valwithNumbers = lines.foldLeft(List(""))((l,s) => (s.length+" "+s)::l) println(withNumbers.mkString("")) withNumbers.foreach(println) withNumbers.filter(!_.startsWith("0")).map(_.toUpperCase). sort(_.length < _.length).foreach(println)

29. Scala DI - Cake Pattern traitPersonService { deffindByAge(age: Int): Person } traitPersonServiceImplextendsPersonService { deffindByAge(age: Int): Person = new Person("", 12) } traitPersonDAO { defgetAll(): Seq[Person] } traitPersonDAOImplextendsPersonDAO { defgetAll(): Seq[Person] = List(new Person("", 12)) }

30. Scala DI - Cake Pattern traitDirectory { self: PersonServicewithPersonDAO => } classDirectoryComponentextends Directory withPersonServiceImplwithPersonDAOImpl { }

31. Guice Minimize mutability Avoid static state @Nullable