How to Use the Auxiliary Constructor in Scala

Before starting with the auxiliary constructor, I recommend that you take a look at my previous blog — Primary Constructor in Scala.

Scala has two types of constructors:

1. primary constructor

2. auxiliary constructor (secondary constructor)

A scala class can contain zero or more auxiliary constructors. The auxiliary constructor in Scala is used for constructor overloading and defined as a method usingthis  name.
The auxiliary constructor must call either previously defined auxiliary constructors or primary constructors in the first line of its body. Hence, every auxiliary constructor invokes directly or indirectly to a primary constructor.

We can call the primary constructor or other auxiliary constructor using this .

Here is an example:

scala> class Employee(empId: Int,name: String,salary :Double){
| def this()
| {
| this(0,"",0.0)
| println("Zero-argument auxiliary constructor")
| }
| println("Primary construtor")
| }
defined class Employee

scala> val emp = new Employee()
Primary construtor
Zero-argument auxiliary constructor
emp: Employee = Employee@7186b202

As shown above, when we create an object using the zero-argument auxiliary constructor, then there is the first statement this(0,””,0.0) which will call to a primary constructor. The first primary constructor body will be executed after the auxiliary constructor body.

We can also call directly to the primary constructor:

scala> val emp = new Employee(101,"Smith",50000)
Primary construtor
emp: Employee = Employee@6f930e0

When you compileEmployee.scala using scalac Employee.scala and convert it into Java code using javap Employee.class, it will generate the following code:

Compiled from "Employee.scala"
public class Employee {
  public Employee(int, java.lang.String, double); //java parameterized constructor.
  public Employee();   // java default constructor.
}

Now, you need to create a Scala class with a primary constructor and multiple auxiliary constructors:

scala> class Employee(empId: Int,name: String,salary :Double){
| def this(empId:Int,name:String)
| {
| this(0,"",0.0) // here it invokes primary constructor.
| println("Two-argument auxiliary constructor")
| }
| def this(empId:Int)
| {
| this(0,"",0.0) // here it invokes primary constructor.
| println("One-argument auxiliary constructor")
| }
| def this()
| {
| this(0) // here it invokes one-argument auxiliary constructor.
| println("Zero-argument auxiliary constructor")
| }
| println("Primary construtor")
| }
defined class Employee

scala> val emp = new Employee()
Primary construtor
One-argument auxiliary constructor
Zero-argument auxiliary constructor
emp: Employee = Employee@291a4791

In the above output, we created an instance with the help of the zero-argument auxiliary constructor. Zero-argument constructors call to one argument auxiliary constructor and one argument constructor to a primary constructor. Therefore, the primary constructor body would be executed first. Then, the one argument constructor body would be executed, followed by the zero-argument. Now, we will have code that looks like this:

scala> val emp = new Employee(101,"John")
Primary construtor
Two-argument auxiliary constructor
emp: Employee = Employee@37e69c43

When you invoke more than one time this in the auxiliary constructor, then it will invoke  apply()  in the class:

scala> class Point(x: Int,y: Int) {
| def this(x: Int) {
| this(x,5)
| println("auxiliary constructor")
| this(x,5);  // it will invokes object's apply method like this.apply(x,5).
| }
| def apply(x:Int,y:Int) = println(s" x= $x and y = $y")
| }
defined class Point

scala> val point = new Point(10)
auxiliary constructor
x= 10 and y = 5
point: Point = Point@310b2b6f

If an apply method does not get define in the class and call this more than one time in the auxiliary constructor, then it will give a compile time error:

scala> class Point(x: Int,y: Int) {
| def this(x: Int) {
| this(x,5)
| println("auxiliary constructor")
| this(x,5)
| }
| }
:15: error: Point does not take parameters
this(x,5)                                                                                                                                1                                scala> class Point(x: Int,y: Int) {                              2                                | def this(x: Int) {                              3                                | this(x,5)                              4                                | println("auxiliary constructor")                              5                                | this(x,5)                              6                                | }                              7                                | }                              8                                :15: error: Point does not take parameters                              9                

In Scala, we can also create a primary constructor with a default value. If you don’t provide value, then it will take the default value, which is provided in the primary constructor.  Otherwise, the value we provide is from an instance created with the help of a parameter name of the class. Here is an example:

scala> class Employee(empId: Int = 0,name: String = "",salary :Double = 0.0){
| println(s"empId = $empId , empName = $name , salary = $salary")
| }
defined class Employee

scala> val emp = new Employee()
empId = 0 , empName = , salary = 0.0
emp: Employee = Employee@713999c2

scala> val emp1 = new Employee(name = "Smith")
empId = 0 , empName = Smith , salary = 0.0
emp1: Employee = Employee@6be766d1

scala> val emp1 = new Employee(name = "Smith", empId = 9)
empId = 9 , empName = Smith , salary = 0.0
emp1: Employee = Employee@5ddb302

If we provide a wrong parameter name of the class at the time of instance creation, then it will give the compile-time error:

scala> val emp1 = new Employee(name = "Smith", empIf = 9)
:13: error: unknown parameter name: empIf
val emp1 = new Employee(name = "Smith", empIf = 9)                                                                                                                                1                                scala> val emp1 = new Employee(name = "Smith", empIf = 9)                              2                                :13: error: unknown parameter name: empIf                              3                                val emp1 = new Employee(name = "Smith", empIf = 9)                                                    

Please leave a comment below, if you have any questions or suggestions. Thanks!