Rewriting Java In Scala

Rewriting Java in Scala ...and Making Your Code Lovely

Relevant About Me New to Scala – Summer 2009 http://www.colinhowe.co.uk http://twitter.com/colinhowe

Format Try to keep this as interactive as possible. Introduce a concept Use the concept on some Java Blue is used to highlight the essence

Case Classes (1) Provide pattern matching Default toString, equals, hashCode

Case Classes Example - Java @Override public boolean equals(Object obj) { if (!(obj instanceof Point)) { return false; } Point other = (Point)obj; return other.x == x && other.y == y; } @Override public int hashCode() { return x * 17 + y; } } class Point { private int x; private int y; public Point(int x, int y) { this.x = x; this.y = y; } public String toString() { return "Point(x: " + x + ", y: " + y + ")"; }

Case Classes Example - Scala case class Point(x : Int, y : Int)

Filter Take a list of X and find only certain X E.g. find all even numbers in a list

Filter Example - Java Predicate evenNumbers = new Predicate() { @Override public boolean evaluate(Object o) { Integer n = (Integer)o; // Assume list contains only integers return n % 2 == 0 ; } }; CollectionUtils.filter( list , evenNumbers);

Filter Example - Java List<Integer> evens = new LinkedList<Integer>(); for (Integer i : list ) { if ( i % 2 == 0 ) evens.add(i); }

Filter Example - Scala val evens = list .filter( _ % 2 == 0 )

Pattern Matching Think switch on steroids result match { case "A" | "B" => println("A or B!") case "C" => println("Just C.") case e : Exception => println("Broken: " + e) case _ => println("Anything else!") }

Matching Example - Java @Override public boolean equals(Object obj) { if ( !(obj instanceof Point) ) { return false; } Point other = (Point)obj; return other.x == x && other.y == y ; }

Matching Example - Scala override def equals(o : Any) : Boolean = o match { case p : Point => p.x == this.x && p.y == this.y case _ => false }

Map Take a list of X and turn it into a list of Y Lists are the same size

Map Example (1) - Java for (int i : list ) { result.add( i * 2 ); }

Map Example (1) - Scala val result = list .map( _ * 2 )

Map Example (2) - Java for (int n : list) { List<Integer> factors = new LinkedList<Integer>(); int p = 2; while (p <= n) { if (n % p == 0) { factors.add(p); n /= p; } else { p++; } } result.add(factors); }

Map Example (2) - Scala def factorise(n : Int) : List[Int] = { val factors = scala.collection.mutable.ListBuffer[Int]() var currentValue = n var p = 2 while (p <= currentValue) { if (currentValue % p == 0) { factors += p currentValue = currentValue / p } else { p = p + 1 } } factors.toList }

Map Example (2) - Scala def factorise(n : Int, p : Int) : List[Int] = { if (n == 1) Nil else if (n % p == 0) p :: factorise(n / p, p) else factorise(n, p + 1) } def factorise(n : Int) : List[Int] = factorise(n, 2)

Map Example (2) - Scala def factorise(n : Int, p : Int, factors : List[Int]) : List[Int] = { if (n == 1) factors else if (n % p == 0) factorise(n / p, p, p :: factors) else factorise(n, p + 1, factors) } def factorise(n : Int) : List[Int] = factorise(n, 2, Nil)

Map Example (2) - Scala val result = list.map(factorise)

Fold Take a list of X and turn it into a single Y by combining each element in the list left = do the operation on the left-most element first

Fold Example (1) - Java int total = 0 ; for (int i : someList ) { total += i ; }

Fold Example (1) - Scala val sum = someList .foldLeft( 0 )( _ + _ )

Fold Example (2) - Java int successCount = 0 ; for (Message m : messages ) { successCount += process(m) }

Fold Example (2) - Scala val successCount = messages.foldLeft( 0 )( _ + process(_) )

Spot Common Constructs Spot re-used constructs Replace with a more functional construct Try to extract out algorithms and data

Common Constructs - Java for (Car car : cars) { if (!groups.containsKey(car.colour)) { groups.put(car.colour, new HashSet<Car>()); } groups.get(car.colour).add(car); }

Common Constructs - Scala def group[T, K](list : List[T], key : (T => K)) : Map[K, Set[T]] = { val groups = scala.collection.mutable.Map[K, Set[T]]() for (t <- list) { if (!groups.contains(key(t))) { groups(key(t)) = Set[T]() } groups(key(t)) = Set[T](t) ++ groups(key(t)) } groups.toMap } val carGroups = group(cars, ((_:car).colour))

Generators For loops with multiple iterators for (x <- 1 to 10; y <- 1 to 20)

Generators Example (1) - Java for (int x = 1; x <= 10; x++) { for (int y = x + 1; y <= 10; y++) { list.add(new Pair(x, y)); } }

Generators Example (1) - Scala val xys = for (x <- 1 to 10; y <- x + 1 to 10) yield (x, y)

Generators Example (2) - Java for (Message message : messages ) { if ( message.isHighPriority() ) { processImmediately(message); } }

Generators Example (2) - Scala for (message <- messages ; if message.isHighPriority ) { processImmediately(message) }

Exception Handling Not as forced – no checked exceptions

Exceptions Example - Java final String line; try { line = reader.readLine(); } catch (IOException e) { throw new RuntimeException(e); } // Do stuff with the line

Exceptions Example - Scala val line = reader.readLine() // Do stuff with the line

Exceptions Example - Scala try { val line = reader.readLine() // Do stuff with the line } catch { case e : Exception => println(e) }

Reduce Fold but without a 0 th value Does not work on the empty list

Reduce Example (1) - Java int currentMax = someList.get(0) ; for (int i : someList ) { if (i > currentMax) { currentMax = i ; } }

Reduce Example (1) - Scala val currentMax = someList .reduceLeft(0)( max )

Random Tricks Slight break from the format. Covering a few random things that have no proper equivalent in Java.

Releasing Resources withBufferedReader(new File("readme")) { reader => reader.readLine } def withBufferedReader(file : File)(op : BufferedReader => Unit) = { val reader = new BufferedReader(new FileReader(file)) try { op(reader) } finally { reader.close() } }

Testing Way more fun in Scala. Make your own DSLs :)

Testing DSL Example def testSimpleAddition { "1 + 1" evaluates 2 "2 + 3 + 4" evaluates 9 } implicit def evaluates(source : String) = new { def evaluates(value : Int) { // Invoke the expression evaluator and check the value } }

Some Guidelines Write small functions... … do pass around functions

Some Guidelines Abstract out algorithms

Some Guidelines Write small classes / traits

Some Guidelines If you see unwanted state... … you can probably remove it

Some Guidelines Immutability is King

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Rewriting Java In Scala

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