Scala: Statements in the class body gets executed while initializing an instance

 If a class body has any statements, then those gets executed while initializing an object.

class Point(x : Int, y : Int){
     println("Initializing point instance")
     println(s"x = $x, y = $y")
     
     def getX() = {
          x
     }
     
     def getY() = {
           y
     }
     
     override def toString() = {
          s"x = $x, y = $y"
     }
}

When you define an instance of Point, two println statements which are part of Point class gets executed. These are similar to initializer blocks in Java.

 

scala> class Point(x : Int, y : Int){
     | 
     |   println("Initializing point instance")
     |   println(s"x = $x, y = $y")
     | 
     |   def getX() = {
     |    x
     |   }
     | 
     |   def getY() = {
     |    y
     |   }
     | 
     |  override def toString() = {
     |    s"x = $x, y = $y"
     |  }
     | }
class Point

scala> val p1 = new Point(10, 20)
Initializing point instance
x = 10, y = 20
val p1: Point = x = 10, y = 20

scala> 

scala> val p1 = new Point(1, 2)
Initializing point instance
x = 1, y = 2
val p1: Point = x = 1, y = 2

 

Statements in the class body is used to validate instance properties.

 

For example, I want to restrict Point to take only non-negative values.

class Point(x : Int, y : Int){
     println("Initializing point instance")
     println(s"x = $x, y = $y")

     if ( x < 0 || y < 0 ) { 
          throw new IllegalArgumentException("x and y must be >= 0")
     }

     def getX() = {
          x
     }
     
     def getY() = {
          y
     }

     override def toString() = {
          s"x = $x, y = $y"
     }
}

When you try to initialize Point instance with negative values, you will get java.lang.IllegalArgumentException.

scala> class Point(x : Int, y : Int){
     |      println("Initializing point instance")
     |      println(s"x = $x, y = $y")
     | 
     |      if ( x < 0 || y < 0 ) { 
     |           throw new IllegalArgumentException("x and y must be >= 0")
     |      }
     | 
     |      def getX() = {
     |           x
     |      }
     |      
     |      def getY() = {
     |           y
     |      }
     | 
     |      override def toString() = {
     |           s"x = $x, y = $y"
     |      }
     | }
class Point

scala> 

scala> val pi = new Point(10, -20)
Initializing point instance
x = 10, y = -20
java.lang.IllegalArgumentException: x and y must be >= 0
  ... 33 elided

scala> val pi = new Point(-10, 20)
Initializing point instance
x = -10, y = 20
java.lang.IllegalArgumentException: x and y must be >= 0
  ... 33 elided

scala> val pi = new Point(-10, -20)
Initializing point instance
x = -10, y = -20
java.lang.IllegalArgumentException: x and y must be >= 0
  ... 33 elided

scala> val pi = new Point(10, 20)
Initializing point instance
x = 10, y = 20
val pi: Point = x = 10, y = 20

 

 

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