Scala Variable Declarations

Scala has two types of variables, immutable and mutable. You declare immutable variables using val and mutable variables using var. Irrespective of the type, you must initialize the variable as you declare.

In this article, we will learn how to declare variables.

My set up details:

1. I am using Scala Eclipse IDE, 4.0.0.
2. JavaSE-1.8

To run the examples. Select the statements and run in scala interpreter or right click on the file and then Run-as->Scala Application.

Declare Immutable Variables

Let’s define an object and introduce an immutable int variable a. Since it is immutable we declare it using val keyword.

ValExample.scala:

object ImmutableVariableExample {
  //declare an immutable variable
  val a = 2
  print(a)
  
  def main(args: Array[String]): Unit = {    
  }
}

Output:

2

Let’s re-assign variable a to 3. This should fail.

object ImmutableVariableExample {
  //declare an immutable variable
  val a = 2
  print(a)
  //uncomment below code and it fail as val is immutable
  a = 3
  def main(args: Array[String]): Unit = {    
  }
}
}

Try running it and the scala interpreter will throw error as val is immutable and can’t be re-assigned.

 :12: error: reassignment to val
         a = 3
           ^

Declare Mutable Variables

Declare mutable variable with var. The above example can be re-written using var and you will be able to re-assign the variable.

VarExample.scala:

object MutableVariableExample {
  //declare a mutable variable
  var a = 2
  println(a)
  println("Now re-assign it to 3")
  a = 3
  println(a)
  def main(args: Array[String]): Unit = {    
  }
}

Output:

2
Now re-assign it to 3
3

Type Inference

In our above examples, we didn’t specify the variable types in the declarations. The examples still worked because of Scala’s ability to infer the types based on the initialized data.

You can however specify a type explicitly if you want.

For example,

VarExample:

object MutableVariableExample {
  //declare a mutable variable
  var a = 2
  println(a)
  println("Now re-assign it to 3")
  a = 3
  println(a)
  
  var b:Int = 3
  var c:String = "Hi";
  
  println(b + c)
  
  def main(args: Array[String]): Unit = {    
  }
}

Output:

2
Now re-assign it to 3
3
3Hi

Declaring nested variables with same name

In this example, I will declare the same variable again in a nested block. In java, it will throw an error but it is OK to do in Scala.

NestedVarExample.scala:

object NestedVarExample {
  //declare a mutable variable
  var a = "a"
  println(a)
  if (a == "a") {
    println("declare var a again")
    var a = "aa"
    println(a)
  }

  var b = "b"
  println(b)
  if (b == "b") {
    println("declare val b again")
    var b = "bb"
    println(b)
  }

  def main(args: Array[String]): Unit = {
  }
}

One more thing, if you notice, I used ‘==’ to compare strings. In Scala, its perfectly fine to compare two Strings with the == operator. Of course in Java, we compare two Strings with the equals method.

Output:

a
declare var a again
aa
b
declare val b again
bb

Download source code

This as an example of declaring variables in Scala. You can download the source code here: variablesExample.zip