Work in Progress

We are still in the process of writing the documentation for Scala 3. You can help us to improve the documentation.

Are you searching for the Scala 2 documentation?

Scala 3 — Book

Method Features

Language

This section introduces the various aspects of how to define and call methods in Scala 3.

Defining Methods

Scala methods have many features, including these:

  • Generic (type) parameters
  • Default parameter values
  • Multiple parameter groups
  • Context-provided parameters
  • By-name parameters

Some of these features are demonstrated in this section, but when you’re defining a “simple” method that doesn’t use those features, the syntax looks like this:

def methodName(param1: Type1, param2: Type2): ReturnType =
  // the method body
  // goes here
end methodName   // this is optional

In that syntax:

  • The keyword def is used to define a method
  • The Scala standard is to name methods using the camel case convention
  • Method parameters are always defined with their type
  • Declaring the method return type is optional
  • Methods can consist of many lines, or just one line
  • Providing the end methodName portion after the method body is also optional, and is only recommended for long methods

Here are two examples of a one-line method named add that takes two Int input parameters. The first version explicitly shows the method’s Int return type, and the second does not:

def add(a: Int, b: Int): Int = a + b
def add(a: Int, b: Int) = a + b

It is recommended to annotate publicly visible methods with their return type. Declaring the return type can make it easier to understand it when you look at it months or years later, or when you look at another person’s code.

Calling methods

Invoking a method is straightforward:

val x = add(1, 2)   // 3

The Scala collections classes have dozens of built-in methods. These examples show how to call them:

val x = List(1, 2, 3)

x.size          // 3
x.contains(1)   // true
x.map(_ * 10)   // List(10, 20, 30)

Notice:

  • size takes no arguments, and returns the number of elements in the list
  • The contains method takes one argument, the value to search for
  • map takes one argument, a function; in this case an anonymous function is passed into it

Multiline methods

When a method is longer than one line, start the method body on the second line, indented to the right:

def addThenDouble(a: Int, b: Int): Int =
  // imagine that this body requires multiple lines
  val sum = a + b
  sum * 2

In that method:

  • sum is an immutable local variable; it can’t be accessed outside of the method
  • The last line doubles the value of sum; this value is returned from the method

When you paste that code into the REPL, you’ll see that it works as desired:

scala> addThenDouble(1, 1)
res0: Int = 4

Notice that there’s no need for a return statement at the end of the method. Because almost everything in Scala is an expression—meaning that each line of code returns (or evaluates to) a value—there’s no need to use return.

This becomes more clear when you condense that method and write it on one line:

def addThenDouble(a: Int, b: Int): Int = (a + b) * 2

The body of a method can use all the different features of the language:

  • if/else expressions
  • match expressions
  • while loops
  • for loops and for expressions
  • Variable assignments
  • Calls to other methods
  • Definitions of other methods

As an example of a real-world multiline method, this getStackTraceAsString method converts its Throwable input parameter into a well-formatted String:

def getStackTraceAsString(t: Throwable): String =
  val sw = StringWriter()
  t.printStackTrace(PrintWriter(sw))
  sw.toString

In that method:

  • The first line assigns a new instance of StringWriter to the value binder sw
  • The second line stores the stack trace content into the StringWriter
  • The third line yields the String representation of the stack trace

Default parameter values

Method parameters can have default values. In this example, default values are given for both the timeout and protocol parameters:

def makeConnection(timeout: Int = 5_000, protocol: String = "http") =
  println(f"timeout = ${timeout}%d, protocol = ${protocol}%s")
  // more code here ...

Because the parameters have default values, the method can be called in these ways:

makeConnection()                 // timeout = 5000, protocol = http
makeConnection(2_000)            // timeout = 2000, protocol = http
makeConnection(3_000, "https")   // timeout = 3000, protocol = https

Here are a few key points about those examples:

  • In the first example no arguments are provided, so the method uses the default parameter values of 5_000 and http
  • In the second example, 2_000 is supplied for the timeout value, so it’s used, along with the default value for the protocol
  • In the third example, values are provided for both parameters, so they’re both used

Notice that by using default parameter values, it appears to the consumer that they can use three different overridden methods.

Named parameters

If you prefer, you can also use the names of the method parameters when calling a method. For instance, makeConnection can also be called in these ways:

makeConnection(timeout=10_000)
makeConnection(protocol="https")
makeConnection(timeout=10_000, protocol="https")
makeConnection(protocol="https", timeout=10_000)

In some frameworks named parameters are heavily used. They’re also very useful when multiple method parameters have the same type:

engage(true, true, true, false)

Without help from an IDE that code can be hard to read, but this code is much more clear and obvious:

engage(
  speedIsSet = true,
  directionIsSet = true,
  picardSaidMakeItSo = true,
  turnedOffParkingBrake = false
)

A suggestion about methods that take no parameters

When a method takes no parameters, it’s said to have an arity level of arity-0. Similarly, when a method takes one parameter it’s an arity-1 method. When you create arity-0 methods:

  • If the method performs side effects, such as calling println, declare the method with empty parentheses
  • If the method does not perform side effects—such as getting the size of a collection, which is similar to accessing a field on the collection—leave the parentheses off

For example, this method performs a side effect, so it’s declared with empty parentheses:

def speak() = println("hi")

Doing this requires callers of the method to use open parentheses when calling the method:

speak     // error: "method speak must be called with () argument"
speak()   // prints "hi"

While this is just a convention, following it dramatically improves code readability: It makes it easier to understand at a glance that an arity-0 method performs side effects.

Using if as a method body

Because if/else expressions return a value, they can be used as the body of a method. Here’s a method named isTruthy that implements the Perl definitions of true and false:

def isTruthy(a: Any) =
  if a == 0 || a == "" || a == false then
    false
  else
    true

These examples show how that method works:

isTruthy(0)      // false
isTruthy("")     // false
isTruthy("hi")   // true
isTruthy(1.0)    // true

Using match as a method body

A match expression can also be used as the entire method body, and often is. Here’s another version of isTruthy, written with a match expression :

def isTruthy(a: Matchable) = a match
  case 0 | "" | false => false
  case _ => true

This method works just like the previous method that used an if/else expression. We use Matchable instead of Any as the parameter’s type to accept any value that supports pattern matching.

For more details on the Matchable trait, see the Reference documentation.

Controlling visibility in classes

In classes, objects, traits, and enums, Scala methods are public by default, so the Dog instance created here can access the speak method:

class Dog:
  def speak() = println("Woof")

val d = new Dog
d.speak()   // prints "Woof"

Methods can also be marked as private. This makes them private to the current class, so they can’t be called nor overridden in subclasses:

class Animal:
  private def breathe() = println("I’m breathing")

class Cat extends Animal:
  // this method won’t compile
  override def breathe() = println("Yo, I’m totally breathing")

If you want to make a method private to the current class and also allow subclasses to call it or override it, mark the method as protected, as shown with the speak method in this example:

class Animal:
  private def breathe() = println("I’m breathing")
  def walk() =
    breathe()
    println("I’m walking")
  protected def speak() = println("Hello?")

class Cat extends Animal:
  override def speak() = println("Meow")

val cat = new Cat
cat.walk()
cat.speak()
cat.breathe()   // won’t compile because it’s private

The protected setting means:

  • The method (or field) can be accessed by other instances of the same class
  • It is not visible by other code in the current package
  • It is available to subclasses

Objects can contain methods

Earlier you saw that traits and classes can have methods. The Scala object keyword is used to create a singleton class, and an object can also contain methods. This is a nice way to group a set of “utility” methods. For instance, this object contains a collection of methods that work on strings:

object StringUtils:

  /**
   * Returns a string that is the same as the input string, but
   * truncated to the specified length.
   */
  def truncate(s: String, length: Int): String = s.take(length)

  /**
    * Returns true if the string contains only letters and numbers.
    */
  def lettersAndNumbersOnly_?(s: String): Boolean =
    s.matches("[a-zA-Z0-9]+")

  /**
   * Returns true if the given string contains any whitespace
   * at all. Assumes that `s` is not null.
   */
  def containsWhitespace(s: String): Boolean =
    s.matches(".*\\s.*")

end StringUtils

Extension methods

Extension methods are discussed in the Extension methods section of the Contextual Abstraction chapter. Their main purpose is to let you add new functionality to closed classes. As shown in that section, imagine that you have a Circle class, but you can’t change its source code. For instance, it may be defined like this in a third-party library:

case class Circle(x: Double, y: Double, radius: Double)

When you want to add methods to this class, you can define them as extension methods, like this:

extension (c: Circle)
  def circumference: Double = c.radius * math.Pi * 2
  def diameter: Double = c.radius * 2
  def area: Double = math.Pi * c.radius * c.radius

Now when you have a Circle instance named aCircle, you can call those methods like this:

aCircle.circumference
aCircle.diameter
aCircle.area

See the Extension methods section of this book, and the “Extension methods” Reference page for more details.

Even more

There’s even more to know about methods, including how to:

  • Call methods on superclasses
  • Define and use by-name parameters
  • Write a method that takes a function parameter
  • Create inline methods
  • Handle exceptions
  • Use vararg input parameters
  • Write methods that have multiple parameter groups (partially-applied functions)
  • Create methods that have generic type parameters

See the Reference documentation for more details on these features.

Contributors to this page: