Functional

(0 to 9).toList.partition ((x:Int) => x%3 == 0)

Object-oriented

class Counter:
  var n = 0
  def get () : Int = { val tmp = n; n = n + 1; tmp }
val c = new Counter()
c.get()
c.get()

class C (f1:Int, val f2:Int, var f3:Int):
  //...

• f1 private and immutable
• f2 public immutable
• f3 public mutable

scala> val c = new C (2, 3, 5)
c: C = C@8bd1b6a
scala> c.f1
<console>:10: error: value f1 is not a member of C
scala> c.f2
res1: Int = 3
scala> c.f3
res2: Int = 5
scala> c.f2 = 10
<console>:9: error: reassignment to val
scala> c.f3 = 10
c.f3: Int = 10

Scala

class C1(x:Int): 
  def double() = x+x

Expressed in Java

public class C1 {
  private final int x;
  
  public C1(int x) { 
    this.x = x; 
  }
  public int double() {
    return x+x;
  }
}

Scala

class C2(val x:Int): 
  def double() = x+x

Expressed in Java

public class C2 {
  private final int x;
  public C2(int x) {
    this.x = x;
  }
  public int x() {
    return x;
  }
  public int double() {
    return x+x;
  }
}

Scala

class C3(var x:Int): 
  def double() = x+x

Expressed in Java

public class C3 {
  private int x;
  public C3(int x) { this.x = x; }
  public int x() { return x; }
    
  public void setX(int x) { this.x = x; }
    
  public int double() { return x+x; }
}          

Java

public class C {
  public static void main (String[] args) {
    //...
  }
}

Scala

object C:
  def main (args:Array[String]) : Unit = 
    //...

Java

• static components belong to a class
• All other belong to an instance of the class

class C {
  int f1;
  int m1 () { return f1; }
  static int f2;
  static int m2 () { return f2; }
}

Scala: Companion object replaces static

• All data and methods belong to objects
• Companion object is related to other instances of the same class

class C:
  var f1:Int = 0
  def m1 () : Int = f1
object C:
  var f2:Int = 0
  def m2 () : Int = f2

class Point private (private var x:Int, private var y:Int):
  def translate (xDisp:Int, yDisp:Int) : Unit = 
    x = x + xDisp
    y = y + yDisp
object Point:
  def apply (x:Int, y:Int) : Point = 
    if 0 <= x && x <= 10 && 0 <= y && y <= 10 then 
      new Point (x, y)
    else
      throw IllegalArgumentException (s"Both x and y must be in range [0,10], but x=$x and y=$y.")

// java.lang.IllegalArgumentException: 
// both x and y must be in range [0,10], but x=1 and y=100.
val p = Point.apply (1, 100)
// succeeds
val q = Point.apply (1, 10)
// name of "apply" method can be omitted
val r = Point (1, 10) 
// compile error: constructor Point in class Point 
// cannot be accessed
val s = new Point (1, 100)