Objects of a Class

In this lesson, you will create your first object of a class and learn how to set its fields and call its methods.

We'll cover the following

Instantiating a Class

Once a class has been defined, you can create objects from the class blueprint using the new keyword followed by the class identifier.

We usually don’t create objects for the sake of just creating them, rather, we want to work with them in some way. For this reason, we assign the object to a variable. Let’s instantiate our Person class.

For ease, the code for creating the class is also provided below.

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class Person{
  var name: String = "temp"
  var gender: String = "temp"
  var age: Int = 0
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
}
// Creating an object of the Person class
val firstPerson = new Person

Setting the Field Values

Now that we have our object firstPerson, let’s set its field values. To do so, we call the field we wish to set on the object whose field we wish to set.

Let’s set the name, gender, and age of the object firstPerson.

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firstPerson.name = "Sarah"
firstPerson.gender = "female"
firstPerson.age = 25

To print the field values, we pass object.field to a print method.

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class Person{
  var name: String = "temp"
  var gender: String = "temp"
  var age: Int = 0
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
}
// Creating an object of the Person class
val firstPerson = new Person 
firstPerson.name = "Sarah"
firstPerson.gender = "female"
firstPerson.age = 25
println(firstPerson.name)
println(firstPerson.gender)
println(firstPerson.age)

Calling a Class Method

To access the methods of a class, we follow the same syntax we use to access fields. Let’s call the walking and talking methods and see what happens.

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class Person{
  var name: String = "temp"
  var gender: String = "temp"
  var age: Int = 0
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
}
// Creating an object of the Person class
val firstPerson = new Person 
firstPerson.name = "Sarah"
firstPerson.gender = "female"
firstPerson.age = 25
firstPerson.walking
firstPerson.talking

Since our methods don’t have any parameters, we don’t need to pass any arguments when calling them.

firstPerson.walking is called on the object firstPerson and prints a variable name. Since the method is called on an object, it determines if the variable used is one of the properties of that object. As name is defined for the object firstName, the method will print its value.

In the code above, firstPerson.walking takes the value of Sarah and prints Sarah is walking. firstPerson.talking performs the same operation.

Multiple Objects of the Same Class

As classes provide reusable code, it makes sense that we can create multiple objects using the same class blueprint. Along with firstPerson, let’s create more objects of the Person class and set their field name.

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class Person{
  var name: String = "temp"
  var gender: String = "temp"
  var age: Int = 0
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
}
// Creating an object of the Person class
val firstPerson = new Person 
firstPerson.name = "Sarah"
firstPerson.gender = "female"
firstPerson.age = 25
// Creating an object of the Person class
val secondPerson = new Person
secondPerson.name = "Ben"
// Creating an object of the Person class
val thirdPerson = new Person 
thirdPerson.name = "Martin"
// Creating an object of the Person class
val fourthPerson = new Person 
fourthPerson.name = "Hannah"
// Driver Code
println(firstPerson.name)
println(secondPerson.name)
println(thirdPerson.name)
println(fourthPerson.name)

Even though there are multiple name variables, they all are referenced by different objects, hence, modifying one doesn’t modify the others. This is why fields are known as instance variables, because each object has its own set of those variables.

Private and Public Fields

By default, the fields defined in a class are public, i.e., they can be used by the object outside of the class itself. However, if you want to make sure that fields remain valid, it is better to prevent outsiders from accessing those fields. This is done by making the fields private. Private fields can only be accessed by methods defined in the same class. To create a private field, place the private access modifier in front of the field.

Let’s redefine our Person class so that its fields are private.

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class Person{
  private var name: String = "temp"
  private var gender: String = "temp"
  private var age: Int = 0
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
}

Now, let’s create an object and try to set the value for name.

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class Person{
  private var name: String = "temp"
  private var gender: String = "temp"
  private var age: Int = 0
  def walking = println(s"$name is walking")
  def talking = println(s"$name is talking")
}
// Creating an object of the Person class
val firstPerson = new Person 
firstPerson.name = "Sarah"

When you run the above code, you will get an error. This is because firstPerson is an object outside of the class. fistPerson is trying to access name, which is a private property. Hence accessing it outside the class will result in an error. Only members defined within the block of the class have access to private fields. However, in our case name, gender, and age are specifically assigned for each Person object. For such cases, Scala provides constructor parameters.

In the next lesson, we will look at constructor parameters and see how they are used to create an object of a class.