Scala Class Fields and Methods

A method is a def that lives inside a class and works with that value's fields. A class value can hold data and do work with it.

A var field can change

The age field is marked var, so a new value can be stored in it. The name field stays as it was.

class Dog(val name: String, var age: Int)

@main def main(): Unit =
  val rex = Dog("Rex", 3)
  rex.age = 4
  println(rex.age)
  println(rex.name)

A method reads the fields

The label method sits inside the class and reads name and age directly. It is defined and called with no parentheses.

class Dog(val name: String, var age: Int):
  def label: String = s"$name is $age"

@main def main(): Unit =
  val rex = Dog("Rex", 3)
  println(rex.label)

Methods can take values and change fields

The ageIn method takes a number of years and adds it to the age. The haveBirthday method changes the stored age itself.

class Dog(val name: String, var age: Int):
  def ageIn(years: Int): Int = age + years
  def haveBirthday(): Unit = age += 1

@main def main(): Unit =
  val rex = Dog("Rex", 3)
  println(rex.ageIn(2))
  rex.haveBirthday()
  println(rex.age)

Example

class Cat(val name: String, var age: Int):
  def label: String = s"$name, age $age"
  def addYears(years: Int): Unit = age += years
  def futureAge(years: Int): Int = age + years

@main def main(): Unit =
  val miso = Cat("Miso", 2)
  println(miso.label)
  miso.addYears(3)
  println(miso.label)
  println(miso.futureAge(10))

Key points

Var field: a field marked var can be given a new value after the value is created.
Val field: a field marked val cannot be given a new value.
Method: a def inside a class, which can read the fields of its own value directly.
Method parameter: a value passed into a method in parentheses, which the method uses in its work.
No parentheses: a method with no parameters and no changes is written and called without parentheses.
Unit method: a method marked Unit does work, such as changing a var field, and gives back no value.

In this example, the Cat class holds a name and a var age, and its label method builds a line from both fields. The addYears method raises the stored age from 2 to 5, so the second label shows 5. The futureAge method adds 10 to the current age and gives back 15 without changing the stored age.

Note

a no-parentheses method is called the same way a field is read, with no parentheses after its name.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Scastie

class Cat(val name: String, var age: Int):
  def label: String = s"$name, age $age"
  def addYears(years: Int): Unit = age += years
  def futureAge(years: Int): Int = age + years

@main def main(): Unit =
  val miso = Cat("Miso", 2)
  println(miso.label)
  miso.addYears(3)
  println(miso.label)
  println(miso.futureAge(10))

Exercise

A Dog value starts with age 3. Its haveBirthday method adds 1 to the age field. What is the age after haveBirthday is called once?

2 3 4 5