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 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.

Press + to interact
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.

Press + to interact
// 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.

Press + to interact
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.

Press + to interact
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.