Introduction to Scala

1. Introduction

In this tutorial, we are going to look at Scala – one of the primary languages that run on the Java Virtual Machine.

We’ll begin with the core language features such as values, variables, methods, and control structures. Then, we’ll explore some advanced features such as higher-order functions, currying, classes, objects, and pattern matching.

To get an overview of the JVM languages, check out our quick guide to the JVM Languages

2. Project Setup

In this tutorial, we’ll use the standard Scala installation from https://www.scala-lang.org/download/.

Firstly, let’s add the scala-library dependency to our pom.xml. This artifact provides the standard library for the language:

<dependency>
    <groupId>org.scala-lang</groupId>
    <artifactId>scala-library</artifactId>
    <version>2.12.7</version>
</dependency>

Secondly, let’s add the scala-maven-plugin for compiling, testing, running and documenting the code:

<plugin>
    <groupId>net.alchim31.maven</groupId>
    <artifactId>scala-maven-plugin</artifactId>
    <version>3.3.2</version>
    <executions>
        <execution>
            <goals>
                <goal>compile</goal>
                <goal>testCompile</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Maven has the latest artifacts for scala-lang and scala-maven-plugin.

Finally, we’ll use JUnit for unit testing.

3. Basic Features

In this section, we’ll examine the basic language features through examples. We’ll use the Scala interpreter for this purpose.

3.1. Interpreter

The interpreter is an interactive shell for writing programs and expressions.

Let’s print “hello world” using it:

C:\>scala
Welcome to Scala 2.12.6 (Java HotSpot(TM)
 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. 
Or try :help.

scala> print("Hello World!")
Hello World!
scala>

Above, we start the interpreter by typing ‘scala’ at the command line. The interpreter starts and displays a welcome message followed by a prompt.

Then, we type our expression at this prompt. The interpreter reads the expression, evaluates it and prints the result. Then, it loops and displays the prompt again.

Since it provides immediate feedback, the interpreter is the easiest way to get started with the language. Therefore, let’s use it explore the basic language features: expressions and various definitions.

3.2. Expressions

Any computable statement is an expression.

Let’s write some expressions and see their results:

scala> 123 + 321
res0: Int = 444

scala> 7 * 6
res1: Int = 42

scala> "Hello, " + "World"
res2: String = Hello, World

scala> "zipZAP" * 3
res3: String = zipZAPzipZAPzipZAP

scala> if (11 % 2 == 0) "even" else "odd"
res4: String = odd

As we can see above, every expression has a value and a type.

If an expression does not have anything to return, it returns a value of type Unit. This type has only one value: (). It is similar to the void keyword in Java.

3.3. Value Definition

The keyword val is used to declare values.

We use it to name the result of an expression:

scala> val pi:Double = 3.14
pi: Double = 3.14

scala> print(pi)
3.14

Doing so allows us to reuse the result multiple times.

Values are immutable. Therefore, we cannot reassign them:

scala> pi = 3.1415
<console>:12: error: reassignment to val
       pi = 3.1415
         ^

3.4. Variable Definition

If we need to reassign a value, we declare it as a variable instead.

The keyword var is used to declare variables:

scala> var radius:Int=3
radius: Int = 3

3.5. Method Definition

We define methods using the def keyword. Following the keyword, we specify the method name, parameter declarations, a separator (colon) and return type. After this, we specify a separator (=) followed by the method body.

In contrast to Java, we do not use the return keyword to return the result. A method returns the value of the last expression evaluated.

Let’s write a method avg to compute the average of two numbers:

scala> def avg(x:Double, y:Double):Double = {
  (x + y) / 2
}
avg: (x: Double, y: Double)Double

Then, let’s invoke this method:

scala> avg(10,20)
res0: Double = 12.5

If a method does not take any parameters, we may omit the parentheses during definition and invocation. Additionally, we may omit the braces if the body has only one expression.

Let’s write a parameterless method coinToss which randomly returns “Head” or “Tail”:

scala> def coinToss =  if (Math.random > 0.5) "Head" else "Tail"
coinToss: String

Next, let’s invoke this method:

scala> println(coinToss)
Tail
scala> println(coinToss)
Head

4. Control Structures

Control structures allow us to alter the flow of control in a program. We have the following control structures:

• If-else expression
• While loop and do while loop
• For expression
• Try expression
• Match expression

Unlike Java, we do not have continue or break keywords. We do have the return keyword. However, we should avoid using it.

Instead of the switch statement, we have Pattern Matching via match expression. Additionally, we can define our own control abstractions.

4.1. if-else

The if-else expression is similar to Java. The else part is optional. We can nest multiple if-else expressions.

Since it is an expression, it returns a value. Therefore, we use it similar to the ternary operator (?:) in Java. In fact, the language does not have have the ternary operator.

Using if-else, let’s write a method to compute the greatest common divisor:

def gcd(x: Int, y: Int): Int = {
  if (y == 0) x else gcd(y, x % y)
}

Then, let’s write a unit test for this method:

@Test
def whenGcdCalledWith15and27_then3 = {
  assertEquals(3, gcd(15, 27))
}

4.2. While Loop

The while loop has a condition and a body. It repeatedly evaluates the body in a loop while the condition is true – the condition is evaluated at the beginning of each iteration.

Since it has nothing useful to return, it returns Unit.

Let’s use the while loop to write a method to compute the greatest common divisor:

def gcdIter(x: Int, y: Int): Int = {
  var a = x
  var b = y
  while (b > 0) {
    a = a % b
    val t = a
    a = b
    b = t
  }
  a
}

Then, let’s verify the result:

assertEquals(3, gcdIter(15, 27))

4.3. Do While Loop

The do while loop is similar to the while loop except that the loop condition is evaluated at the end of the loop.

Using the do-while loop, let’s write a method to compute factorial:

def factorial(a: Int): Int = {
  var result = 1
  var i = 1
  do {
    result *= i
    i = i + 1
  } while (i <= a)
  result
}

Next, let’s verify the result:

assertEquals(720, factorial(6))

4.4. For Expression

The for expression is much more versatile than the for loop in Java.

It can iterate over single or multiple collections. Moreover, it can filter out elements as well as produce new collections.

Using the for expression, let’s write a method to sum a range of integers:

def rangeSum(a: Int, b: Int) = {
  var sum = 0
  for (i <- a to b) {
    sum += i
  }
  sum
}

Here, a to b is a generator expression. It generates a series of values from a to b.

i <- a to b is a generator. It defines i as val and assigns it the series of values produced by the generator expression.

The body is executed for each value in the series.

Next, let’s verify the result:

assertEquals(55, rangeSum(1, 10))

5. Functions

Scala is a functional language. Functions are first-class values here – we can use them like any other value type.

In this section, we’ll look into some advanced concepts related to functions – local functions, higher-order functions, anonymous functions, and currying.

5.1. Local Functions

We can define functions inside functions. They are referred to as nested functions or local functions. Similar to the local variables, they are visible only within the function they are defined in.

Now, let’s write a method to compute power using a nested function:

def power(x: Int, y:Int): Int = {
  def powNested(i: Int,
                accumulator: Int): Int = {
    if (i <= 0) accumulator
    else powNested(i - 1, x * accumulator)
  }
  powNested(y, 1)
}

Next, let’s verify the result:

assertEquals(8, power(2, 3))

5.2. Higher-Order Functions

Since functions are values, we can pass them as parameters to another function. We can also have a function return another function.

We refer to functions which operate on functions as higher-order functions. They enable us to work at a more abstract level. Using them, we can reduce code duplication by writing generalized algorithms.

Now, let’s write a higher-order function to perform a map and reduce operation over a range of integers:

def mapReduce(r: (Int, Int) => Int,
              i: Int,
              m: Int => Int,
              a: Int, b: Int) = {
  def iter(a: Int, result: Int): Int = {
    if (a > b) {
      result
    } else {
      iter(a + 1, r(m(a), result))
    }
  }
  iter(a, i)
}

Here, r and m are parameters of Function type. By passing different functions, we can solve a range of problems, such as the sum of squares or cubes, and the factorial.

Next, let’s use this function to write another function sumSquares that sums the squares of integers:

@Test
def whenCalledWithSumAndSquare_thenCorrectValue = {
  def square(x: Int) = x * x
  def sum(x: Int, y: Int) = x + y

  def sumSquares(a: Int, b: Int) =
    mapReduce(sum, 0, square, a, b)

  assertEquals(385, sumSquares(1, 10))
}

Above, we can see that higher-order functions tend to create many small single-use functions. We can avoid naming them by using anonymous functions.

5.3. Anonymous Functions

An anonymous function is an expression that evaluates to a function. It is similar to the lambda expression in Java.

Let’s rewrite the previous example using anonymous functions:

@Test
def whenCalledWithAnonymousFunctions_thenCorrectValue = {
  def sumSquares(a: Int, b: Int) =
    mapReduce((x, y) => x + y, 0, x => x * x, a, b)
  assertEquals(385, sumSquares(1, 10))
}

In this example, mapReduce receives two anonymous functions: (x, y) => x + y and x => x * x.

Scala can deduce the parameter types from context. Therefore, we are omitting the type of parameters in these functions.

This results in more concise code compared to the previous example.

5.4. Currying Functions

A curried function takes multiple argument lists, such as def f(x: Int) (y: Int). It is applied by passing multiple argument lists, as in f(5)(6).

It is evaluated as an invocation of a chain of functions. These intermediate functions take a single argument and return a function.

We can also partially specify argument lists, such as f(5).

Now, let’s understand this with an example:

@Test
def whenSumModCalledWith6And10_then10 = {
  // a curried function
  def sum(f : Int => Int)(a : Int, b : Int) : Int =
    if (a > b) 0 else f(a) + sum(f)(a + 1, b)

  // another curried function
  def mod(n : Int)(x : Int) = x % n

  // application of a curried function
  assertEquals(1, mod(5)(6))
    
  // partial application of curried function
  // trailing underscore is required to 
  // make function type explicit
  val sumMod5 = sum(mod(5)) _

  assertEquals(10, sumMod5(6, 10))
}

Above, sum and mod each take two argument lists.
We pass the two arguments lists like mod(5)(6). This is evaluated as two function calls. First, mod(5) is evaluated, which returns a function. This is, in turn, invoked with argument 6. We get 1 as the result.

It is possible to partially apply the parameters as in mod(5). We get a function as a result.

Similarly, in the expression sum(mod(5)) _, we are passing only the first argument to sum function. Therefore, sumMod5 is a function.

The underscore is used as a placeholder for unapplied arguments. Since the compiler cannot infer that a function type is expected, we are using the trailing underscore to make the function return type explicit.

5.5. By-Name Parameters

A function can apply parameters in two different ways – by value and by name – it evaluates by-value arguments only once at the time of invocation. In contrast, it evaluates by-name arguments whenever they are referred. If the by-name argument is not used, it is not evaluated.

Scala uses by-value parameters by default. If the parameter type is preceded by arrow ( =>), it switches to by-name parameter.

Now, let’s use it to implement the while loop:

def whileLoop(condition: => Boolean)(body: => Unit): Unit =
  if (condition) {
    body
    whileLoop(condition)(body)
  }

For the above function to work correctly, both parameters condition and body should be evaluated every time they are referred. Therefore, we are defining them as by-name parameters.

6. Class Definition

We define a class with the class keyword followed by the name of the class.

After the name, we can specify primary constructor parameters. Doing so automatically adds members with the same name to the class.

In the class body, we define the members – values, variables, methods, etc. They are public by default unless modified by the private or protected access modifiers.

We have to use the override keyword to override a method from the superclass.

Let’s define a class Employee:

class Employee(val name : String, var salary : Int, annualIncrement : Int = 20) {
  def incrementSalary() : Unit = {
    salary += annualIncrement
  }

  override def toString = 
    s"Employee(name=$name, salary=$salary)"
}

Here, we are specifying three constructor parameters – name, salary, and annualIncrement.

Since we are declaring name and salary with val and var keywords, the corresponding members are public. On the other hand, we are not using val or var keyword for the annualIncrement parameter. Therefore, the corresponding member is private. As we are specifying a default value for this parameter, we can omit it while calling the constructor.

In addition to the fields, we are defining the method incrementSalary. This method is public.

Next, let’s write a unit test for this class:

@Test
def whenSalaryIncremented_thenCorrectSalary = {
  val employee = new Employee("John Doe", 1000)
  employee.incrementSalary()
  assertEquals(1020, employee.salary)
}

6.1. Abstract Class

We use the keyword abstract to make a class abstract. It is similar to that in Java. It can have all the members that a regular class can have.

Furthermore, it can contain abstract members. These are members with just declaration and no definition, with their definition is provided in the subclass.

Similarly to Java, we cannot create an instance of an abstract class.

Now, let’s illustrate the abstract class with an example.

First, let’s create an abstract class IntSet to represent the set of integers:

abstract class IntSet {
  // add an element to the set
  def incl(x: Int): IntSet

  // whether an element belongs to the set
  def contains(x: Int): Boolean
}

Next, let’s create a concrete subclass EmptyIntSet to represent the empty set:

class EmptyIntSet extends IntSet {	 	 
  def contains(x : Int) = false	 	 
  def incl(x : Int) =	 	 
  new NonEmptyIntSet(x, this)	 	 
}

Then, another subclass NonEmptyIntSet represent the non-empty sets:

class NonEmptyIntSet(val head : Int, val tail : IntSet)
  extends IntSet {

  def contains(x : Int) =
    head == x || (tail contains x)

  def incl(x : Int) =
    if (this contains x) {
      this
    } else {
      new NonEmptyIntSet(x, this)
    }
}

Finally, let’s write a unit test for NonEmptySet:

@Test
def givenSetOf1To10_whenContains11Called_thenFalse = {
  // Set up a set containing integers 1 to 10.
  val set1To10 = Range(1, 10)
    .foldLeft(new EmptyIntSet() : IntSet) {
        (x, y) => x incl y
    }

  assertFalse(set1To10 contains 11)
}

6.2. Traits

Traits correspond to Java interfaces with the following differences:

• able to extend from a class
• can access superclass members
• can have initializer statements

We define them as we define classes but using the trait keyword. Besides, they can have the same members as abstract classes except for constructor parameters. Furthermore, they are meant to be added to some other class as a mixin.

Now, let’s illustrate traits using an example.

First, let’s define a trait UpperCasePrinter to ensure the toString method returns a value in the upper case:

trait UpperCasePrinter {
  override def toString =
    super.toString toUpperCase
}

Then, let’s test this trait by adding it to an Employee class:

@Test
def givenEmployeeWithTrait_whenToStringCalled_thenUpper = {
  val employee = new Employee("John Doe", 10) with UpperCasePrinter
  assertEquals("EMPLOYEE(NAME=JOHN DOE, SALARY=10)", employee.toString)
}

Classes, objects, and traits can inherit at most one class but any number of traits.

7. Object Definition

Objects are instances of a class. As we have seen in previous examples, we create objects from a class using the new keyword.

However, if a class can have only one instance, we need to prevent the creation of multiple instances. In Java, we use the Singleton pattern to achieve this.

For such cases, we have a concise syntax called object definition – similar to the class definition with one difference. Instead of using the class keyword, we use the object keyword. Doing so defines a class and lazily creates its sole instance.

We use object definitions to implement utility methods and singletons.

Let’s define a Utils object:

object Utils {
  def average(x: Double, y: Double) =
    (x + y) / 2
}

Here, we are defining the class Utils and also creating its only instance.

We refer to this sole instance using its name Utils. This instance is created the first time it is accessed.

We cannot create another instance of Utils using the new keyword.

Now, let’s write a unit test for the Utils object:

assertEquals(15.0, Utils.average(10, 20), 1e-5)

7.1. Companion Object and Companion Class

If a class and an object definition have the same name, we call them as companion class and companion object respectively. We need to define both in the same file. Companion objects can access private members from their companion class and vice versa.

Unlike Java, we do not have static members. Instead, we use companion objects to implement static members.

8. Pattern Matching

Pattern matching matches an expression to a sequence of alternatives. Each of these begins with the keyword case. This is followed by a pattern, separator arrow (=>) and a number of expressions. The expression is evaluated if the pattern matches.

We can build patterns from:

• case class constructors
• variable pattern
• the wildcard pattern _
• literals
• constant identifiers

Case classes make it easy to do pattern matching on objects. We add case keyword while defining a class to make it a case class.

Thus, Pattern matching is much more powerful than the switch statement in Java. For this reason, it is a widely used language feature.

Now, let’s write the Fibonacci method using pattern matching:

def fibonacci(n:Int) : Int = n match {
  case 0 | 1 => 1
  case x if x > 1 =>
    fibonacci (x-1) + fibonacci(x-2) 
}

Next, let’s write a unit test for this method:

assertEquals(13, fibonacci(6))

9. Conclusion

In this tutorial, we introduced the Scala language and some of its key features. As we have seen, it provides excellent support for imperative, functional and object-oriented programming.

As usual, the full source code can be found over on GitHub.