Scala Book

Two Types of Variables


In Java you declare new variables like this:

String s = "hello";
int i = 42;
Person p = new Person("Joel Fleischman");

Each variable declaration is preceded by its type.

By contrast, Scala has two types of variables:

  • val creates an immutable variable (like final in Java)
  • var creates a mutable variable

This is what variable declaration looks like in Scala:

val s = "hello"   // immutable
var i = 42        // mutable

val p = new Person("Joel Fleischman")

Those examples show that the Scala compiler is usually smart enough to infer the variable’s data type from the code on the right side of the = sign. We say that the variable’s type is inferred by the compiler. You can also explicitly declare the variable type if you prefer:

val s: String = "hello"
var i: Int = 42

In most cases the compiler doesn’t need to see those explicit types, but you can add them if you think it makes your code easier to read.

As a practical matter it can help to explicitly show the type when you’re working with methods in third-party libraries, especially if you don’t use the library often, or if their method names don’t make the type clear.

The difference between val and var

The difference between val and var is that val makes a variable immutable — like final in Java — and var makes a variable mutable. Because val fields can’t vary, some people refer to them as values rather than variables.

The REPL shows what happens when you try to reassign a val field:

scala> val a = 'a'
a: Char = a

scala> a = 'b'
<console>:12: error: reassignment to val
       a = 'b'

That fails with a “reassignment to val” error, as expected. Conversely, you can reassign a var:

scala> var a = 'a'
a: Char = a

scala> a = 'b'
a: Char = b

In Scala the general rule is that you should always use a val field unless there’s a good reason not to. This simple rule (a) makes your code more like algebra and (b) helps get you started down the path to functional programming, where all fields are immutable.

“Hello, world” with a val field

Here’s what a “Hello, world” app looks like with a val field:

object Hello3 extends App {
    val hello = "Hello, world"

As before:

  • Save that code in a file named Hello3.scala
  • Compile it with scalac Hello3.scala
  • Run it with scala Hello3

A note about val fields in the REPL

The REPL isn’t 100% the same as working with source code in an IDE, so there are a few things you can do in the REPL that you can’t do when working on real-world code in a project. One example of this is that you can redefine a val field in the REPL, like this:

scala> val age = 18
age: Int = 18

scala> val age = 19
age: Int = 19

val fields can’t be redefined like that in the real world, but they can be redefined in the REPL playground.

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