Scala中的同伴对象

Question

can any one explain the following output. I have a simple Scala code like this..

object compOrNotTest {
  def main(args: Array[String]) {
    var emp = new Employee("tom", 20)
    println(emp)
    println(Employee.adult(emp))
    Employee.printName("Roland", 38)

    var emp2 = new Employee("Harry", 37)
    Employee.printName(emp2)
  }
}

class Employee(name: String, age: Int) {
  val ageOfEmplyedd: Int = age
  val nameEmp: String = name

  override def toString() = this.name + " age : " + this.age

  def printName() {
    println("name is in Class " + nameEmp)
  }
}

object Employee {
  def adult(emp: Employee) = {
    if (emp.ageOfEmplyedd > 18)
      true
    else
      false
  }

  def printName(name: String, age: Int) = {
    val emp1 = new Employee(name, age)
    println("Name is : " + emp1.printName())
  }

  def printName(emp1: Employee) = {
    //val emp1 = new Employee(name, age)
    println("Name is :  "+ emp1.printName())
  }
}

And the output I am getting is

tom age : 20
true
name is in Class Roland
Name is : ()
name is in Class Harry
Name is : ()

My question is that why , when I am calling from Companion object I am getting only Name is : () . I am expecting something like Name is : name is in Class Roland . Please help me out to understand the how scala works in this case. Thanks a lot

Answer 1

The return type of Employee.printName (in class Employee ) is Unit . This is because this function was declared using procedure syntax (a function declaration with no = sign in it, which has been deprecated and which will no longer be supported in a future version of Scala ) that has an associated return type of Unit . The Unit value that is returned is represented in Scala as () .

The following function declarations are equivalent:

// Using (soon-to-be-deprecated) procedure syntax.
def printName() {
  println("name is in Class " + nameEmp)
}

// Using an explicit return type.
def printName(): Unit = {
  println("name is in Class " + nameExp)
}

// Using an inferred return type (note "=" in declaration). The last statement is the call
// to println, which returns Unit, so a return type of Unit is inferred.
def printName() = {
  println("name is in Class " + nameExp)
}

If you wanted to return the string that was printed, you would need something like this:

def printName() = {
  val s = "name is in Class " + nameEmp
  println(s)
  s
}

Or, using an explicit return type, instead of inferring it from the last statement:

def printName(): String = {
  val s = "name is in Class " + nameEmp
  println(s)
  s
}