A short Scala tutorial for Java Developers

This post is a tutorial guide for Java programmers who want to learn Scala.

I myself have programmed in Java for many years, yet jumping straight into Scala has made me clueless, and so I started to read some baby steps to learn the new language.

The following post is what I have learnt.

Methods Declaration

We use an example of a max function to illustrate the different ways to declare a method.

You do it like this, in a single line.

def max(x: Int, y: Int): Int = if (x < y) y else x

Or, you can skip the return type (Int in this case) and let the compiler infer.

def max(x: Int, y: Int) = if (x < y) y else x

If the method is more than a single line, you can wrap with curly braces.

def max(x: Int, y: Int) = {
  // More lines of code ..
  if (x < y) y else x
}

You might have already notice. The return keyword is actually optional in Scala. If omitted, the last expression is the value that will be returned.

To be verbose, you can specify the return type with the return keyword.

def max(x: Int, y: Int) = {
  return if (x < y) y else x
}

Method with 0 parameter is Special

If you have the following method

def foo() = println("foo!")

You may call it in 2 ways

foo()

Or simply

foo

However, there is a guideline when to use which style. If there is a side effect, you should use the parenthesis. In other words, a getter method can skip the parenthesis.

Method with 1 parameter is Special

Int has a method to that takes 1 parameter of Int to return a sequence.

0.to(10)

You can drop the . and ( ) and simplify to

0 to 10

This alone makes the Scala language beautiful in many ways.

Method names can contain .+*/

Surprisingly, Scala doesn’t have operators, and therefore no operator overloading.

But it can have a method name ‘+’. So an expression 1 + 2 is actually

1.+(2)

But since Method with 1 parameter is Special (read above), 1.+(2) can be simplified to

1 + 2

val and var declarations

To declare a variable,

var foo = 1
foo = 2

To declare a value, which does not allow you to change/reassign,

val foo = 1
// foo = 2 is not possible

Also, a semi-colon at the end of a line is optional.

Class constructor

The constructor is the class declaration itself, and any constructor parameters can be used in other methods

class Color(color: String, index: Int) {
  // foo() uses color
  def foo() = println(color)
}

// Usage
val c = new Color("Blue", 1)
c.foo

If you need to have some code in the constructor, you could write it right in the class body, right after the declaration.

class Color(color: String, index: Int) {

  if (color == null)
      throw new NullPointerException("Color is null")

  if (index <= 0)
      index = 0

  def foo() = println(color)
}

If you have multiple constructors, you can add them with this method.

class Color(color: String, index: Int) {

  def this(color:String) = this(color, 0)

  def foo() = println(color)
}

// Usage
val c = new Color("Blue")

Static/Singleton object

You cannot have static classes or variables in a class.

Instead, if you want to add a static method, you have to use the object declaration, also known as Singleton objects.

object Color {
  def exclaim(s: String) = println(s + " color!")
}

// Usage
Color.exclaim("Pink")
// Prints "Pink color!"

You can also use the static method in the class method.

class Color(color: String, index: Int) {
  def foo() = Color.exclaim(color)
}

object Color {
  def exclaim(s: String) = println(s + " color!")
}

Interface is traits

In Java, you have Interface. In Scala, you use traits. Moreover, you can have non-abstract methods in traits.

trait Friendly {
  def greet() = "Hi"
}

To have a class implement the traits, you use the extends keyword. And if you need to override the method, you need to explicitly use override def.

class Dog extends Friendly {
  override def greet() = "Woof"
}

Similar to Java, a class can extend 1 class and multiple traits.

Another difference is that Scala can mix in traits at instantiation time. In the following, we create another trait and use the with keyword to instantiate a Dog with that trait.

trait ExclamatoryGreeter extends Friendly {
  override def greet() = super.greet() + "!"
}

// Usage
val pup = new Dog with ExclamatoryGreeter
println(pup.greet())
// Prints "Woof!"

Array does not use subscript [ ]

To access an array, you use ( ) instead of [ ]. It is not a matter of symbol choice. Scala uses ( ) because an array is an object with methods.

To access the 4th element of an array, you write

myArray(3)

Behind the scene, it is in fact calling a method apply.

myArray.apply(3)

Similarly, for setting an array element, you write

myArray(3) = "foo"

Which is interpreted as

myArray.update(3, "foo")

Functions are first class constructs

Java is imperative style. Scala is imperative, but excels in functional style too.

Being a functional language, functions are first class constructs. We use an example of printing args.

args.foreach(arg => println(arg))

The foreach is being passed a function

arg => println(arg)

The above function has a few characteristics:

It is an anonymous function (has no name)
It get passed a single parameter named arg and the type is being inferred by compiler
The main code is simply the println
Yeah, => is used, also call a right arrow

A more complete example of an anonymous method with explicit parameter type

(x: Int, y: Int) => {
  // More code
  x + y
}

As we said, functions are first class constructs, so you basically could assign it to a variable

var add = (x: Int, y: Int) => {
  // More code
  x + y
}

Then use it

add(2, 3)
// returns 5

for arg in args

This is how you use for (arg in args)

for (arg <- args)
          println(arg)

Some characteristics:

For each element in args, it is assigned to arg using val (notvar`, so you can re-assign)
Yeah, it is using a <-, which you can interpret as ‘in’
It is not a <= because that would mean less-than-or-equal

Array, List, and Tuple

Much about immutability from Java is different in Scala.

Array is mutable
List is immutable
Tuple is immutable, and can contain different types

You can read more about Array and List.

However, I would want to point out about Tuple, as that is never heard of in Java. In Java, when you want to return multiple objects, you will probably create a POJO (plain old java object) to contain the multiple objects. Using tuple, you can avoid POJO like classes.

val color = ("blue", 258, 'b')

You can then access the tuple using a dot, underscore, and the one-based index of the element.

println(color._1)
// Prints blue

Set and Map

The way immutability works for Set and Map is different.

To use a mutable Set, you import the mutable HashSet.

import scala.collection.mutable.HashSet

val colorSet = new HashSet[String]
colorSet += "blue"
colorSet += ("red", "green")

To use an immutable Set, you import the immutable version.

import scala.collection.immutable.HashSet

val colorSet = HashSet[String]("blue", "red", "green")

Similarly for map, there is a mutable and immutable version. Let’s take a look at just the mutable HashMap.

import scala.collection.mutable.HashMap

val colorMap = new HashMap[Int, String]
colorMap += 1 -> "Blue"
colorMap += 2 -> "Red"

The expression 1 -> "Blue" means 1.->("Blue"). The method -> is available for any object in Scala, and it returns a 2-element tuple. So basically a 2-element tuple of (Int, String) is added to colorMap.

You might think the equivalent is

colorMap += (1, "Blue")

However, that would not work as += method will interpret as you want to add 2 elements – a Int and a String – where in fact you want to add a tuple (Int, String). Hence you need to add ( ) for the tuple

colorMap += ((1, "Blue"))

You can also create Map with a shorthand

var a = Map(1 -> "Blue", 2 -> "Red", 3 -> "Green")

Once again, that’s same as

var a = Map( (1, "Blue"), (2, "Red"), (3, "Green") )

Junda Ong

A Short Scala Tutorial for Java Developers