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Creating an Object Using Constructor Parameters

In this lesson, we will look at another way of creating an object class and learn about constructors.

We'll cover the following...

We can pass arguments to a class the same way we can pass arguments to functions. They are known as constructor parameters as they are assigned a value when the object is constructed using a class.

Let’s look at the syntax below:

As you can see, where we initially only had class classIdentifier, we now also have a parameter list. The rest of the code for creating a class is exactly the same.

Let’s now redefine our Person class using constructor parameters.

Scala
class Person(var name: String, var gender: String, var age: Int) {
  private var years = 15
    
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
  def yearsFromNow = {
    var newAge = years + age
    println(s"In $years years from $name will be $newAge")
  }
}

name, gender, and age are now constructor parameters. We also have a new field, years, which is assigned a value 15. years is private, so it can only be accessed by the members of the class. yearsFromNow is a new method which calculates the age of a Person object years from their current age. As yearsFromNow is a member of the Person class, it can access years.

Constructor parameters are also fields of that class; hence, they can be used just as fields would be used.

We will now create an instance of the Person class using the constructor parameters.

Scala
// Creating an object of the Person class
val firstPerson = new Person("Sarah", "Female", 25)

The expression on Line 2 is known as a constructor as it is constructing an instance of a class.

Just as before, we can access name, gender, and age.

Scala
class Person(var name: String, var gender: String, var age: Int) {
  private var years = 15
    
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
  def yearsFromNow = {
    var newAge = years + age
    println(s"In $years years from $name will be $newAge")
  }
}
// Creating an object of the Person class
val firstPerson = new Person("Sarah", "Female", 25) 
// Accessing name, gender, and age
println(firstPerson.name)
println(firstPerson.gender)
println(firstPerson.age)

Let’s see what happens when we call the yearsFromNow method.

Scala
class Person(var name: String, var gender: String, var age: Int) {
  private var years = 15
    
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
  def yearsFromNow = {
    var newAge = years + age
    println(s"In $years years from $name will be $newAge")
  }
}
// Creating an object of the Person class
val firstPerson = new Person("Sarah", "Female", 25) 
// Calling yearsFromNow method on the object firstPerson
firstPerson.yearsFromNow

Our methods are simply printing an output, not returning anything. If your methods have a return value, you can store that value in a variable and use it whenever required.

In the next lesson, we will look at a unique Scala feature: singleton objects.