426 APPENDIX C: Introduction to Scala
Variables
Scala allows you to decide whether a variable is immutable (read-only) when you declare it. An
immutable variable is declared with the keyword val. This means it is a variable that cannot be
changed. Listing C-2 illustrates creating an immutable variable, and Figure C-2 shows what happens
when you try to change it.
Listing C-2. Immutable Variable
val immutableVar : String = "Hello"
immutableVar = "Hi"
Figure C-3. var changed successfully
Figure C-2. Error when trying to change val
Listing C-3 illustrates creating a mutable variable, and Figure C-3 shows it successfully changed.
Listing C-3. Mutable Variable
var mutableVar = "Hello"
mutableVar = "Hi"
When you assign an initial value to a variable, the Scala compiler can infer the type of the variable
based on the value assigned to it. This is called type inference, as illustrated in Listing C-4.
Listing C-4. Type Inference
var var1= 10
var var2 = "Hello world"
In Listing C-4, Scala will infer var1 to be of the Int type and var2 to be of the String type variable.
Collections
Scala collections distinguish between mutable and immutable collections. A mutable collection
can be updated or extended in place. This means you can change, add, or remove elements
of a collection as a side effect. Immutable collections, by contrast, never change. You still have