Scala Methods – Syntax, Declaration, Use case, Examples

Original Source: scala-methods on Leobenkel.com.

Introduction to Scala Methods

In order to understand this article, one should understand Scala Variables. After learning about Scala Variables, you can learn about Scala Methods.

What is a method?

A method, sometimes called a function, in computer programming, is a black box which takes input(s), executes one or more operations, and finally yields an output value.

For instance, there might be a black box called add which would combine two values, a and b, and sum them together.

For the below example the values are 1 and 2.

1 and 2 -> add -> 3 (output)

In this case, the black box (add) contains a + b.

In computer science:

• the inputs are called arguments
• the black box is called a method or a function
• the output of the black box is called the return value

How to declare a Scala Method ?

The keyword to declare a Scala method is def.

Below, find an example of a method to add two numbers.

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

Detailed review of the syntax:

• def declares the start of a method
• add is the name of the method
• In between ( and ), find the arguments of the method, separated by coma , :
  • a: Int is the first argument
  • b: Int is the second argument
• After the arguments, : precedes the type of the return value of the method, in this example, we have : Int
• = declares the start of the content of the method
• { opens the context
• a + b is what the method will do with the arguments ( the content of the black box )
• } closes the context

For those with proficiency in other computer languages: Other computer languages commonly use the keyword return to return the output of a method. In Scala, the last statement of a context is its return value.

Bonus: What is a context in Scala ?

This section is aimed at those with proficiency in other computer languages. Feel free to jump to the next section, How to shorten method syntax, if you are a beginner.

A context is a block of code that has a return value. Values and variables declared within a context are destroyed when exiting the context. Scala is built on top of the Java Virtual Machine (JVM) so the garbage collector is going to remove unused variables.

In the example below, a context is being opened to set the value name. This context allows the execution of more code before determining the value of name. For instance, methods can be called.

val name = {
  // do more things, like calling methods:
  val temporaryValue = generateRandomName()
  temporaryValue + "_123"
}

How to shorten the syntax to declare a method ?

The context declaration can be omitted when there is only one line of code in the method:

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

It is also possible to omit the return type of the method:

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

But adding the return type will generally speed up the compilation process, read more about it on this Reddit thread.

How to use a Scala Method ?

The example below shows how to use a declared method, following the syntax explained above.

val output = add(1, 2)

Call the name of the method, add, and give it the values that a and b should be: 1 and 2. In this example, output will take the value 3.

Practical example of Scala Methods

Below you can find a sandbox environment to practice declaring and using Scala Methods.

def multiply(a: Double, b: Double): Double = {
  a * b
}

val weeksPerMonth = 4
val monthsPerYear = 12

val weeksPerYear = multiply(weeksPerMonth, monthsPerYear)

println(weeksPerYear)

Feel free to play with it.

If this does not work, directly go to Scastie.

Bonus: Declare a method as a value

This section is aimed at those with proficiency in other computer languages. Feel free to jump to the next section, What to remember, if you are a beginner.

In Scala, you can declare lambda method. There is a Stackoverflow post about Lambda method you can read to learn more about the theory behind it.

Below find the first part of declaring a method as a value, starting by the type:

val myMethod: InputType => OutputType

val declares an immutable variable, otherwise called a value.

Then, the : precedes the type.

Finally, there is the type InputType => OutputType, the => operator are telling Scala that the Type of this value is going to transform InputType into OutputType when used.

See a working example below:

val triple: Double => Double = _ * 3

_ is a new piece of syntax: it is a wildcard to replace variables. _ is used often within Scala code.

Here, it works in combination with the Type. Since the type is Double to Double ( => can be read as “to” or “transforming into” ) , then _ can only be of type Double.

To use this value triple, see the example below:

val output = triple(5)

Use a method as a value in the same way as if it was a normal method.

Practical example of Scala Lambda Method

Below you can find a sandbox environment to practice declaring and using Scala Lambda Methods.

val triple: Double => Double = _ * 3

triple(4)

Feel free to play with it.

If this does not work, go directly to Scastie.

What to remember?

A few things to remember about Scala methods:

• def is to start the declaration of the method
• ( and ) encapsulate the argument(s) of the method
• : declares the return type of the method. Same as for Scala Variable
• = announces the start of the content of the method
• { and } encapsulate the context of the method
• => declares a lambda method type
• _ replaces a variable implicitly

Conclusion

You now have the tools to use methods in Scala code.

Feel free to leave comments if you have any questions.