This chapter is from the book
This chapter is from the book
This chapter is from the book
5.6 Auxiliary Constructors
As in Java or C++, a Scala class can have as many constructors as you like. However, a Scala class has one constructor that is more important than all the others, called the primary constructor. In addition, a class may have any number of auxiliary constructors.
We discuss auxiliary constructors first because they are easier to understand. They are similar to constructors in Java or C++, with just two differences.
The auxiliary constructors are called this. (In Java or C++, constructors have the same name as the class—which is not so convenient if you rename the class.)
Each auxiliary constructor must start with a call to a previously defined auxiliary constructor or the primary constructor.
Here is a class with two auxiliary constructors:
class Person {
private var name = ""
private var age = 0
def this(name: String) { // An auxiliary constructor
this() // Calls primary constructor
this.name = name
}
def this(name: String, age: Int) { // Another auxiliary constructor
this(name) // Calls previous auxiliary constructor
this.age = age
}
}
We will look at the primary constructor in the next section. For now, it is sufficient to know that a class for which you don’t define a primary constructor has a primary constructor with no arguments.
You can construct objects of this class in three ways:
val p1 = new Person // Primary constructor
val p2 = new Person("Fred") // First auxiliary constructor
val p3 = new Person("Fred", 42) // Second auxiliary constructor