Declare a variable without an initial value

Question

This Scala tutorial has the following to say about declaring variables without an initial value:

If you do not assign any initial value to a variable, then it is valid as follows:
var myVar :Int;  
val myVal :String;

But when I try that code in the Scala REPL, I get these errors:

scala> var myVar :Int;
<console>:10: error: only classes can have declared but undefined members
(Note that variables need to be initialized to be defined)
       var myVar :Int;
           ^

scala> val myVal :String;
<console>:10: error: only classes can have declared but undefined members
       val myVal :String;

Why is this? Is the tutorial for an older version of Scala?
I couldn't find a specific version of Scala that the tutorial is written for, but I am running Scala version 2.11.7 on OpenJDK 64bit, Java 1.8.0_66.

Is the tutorial outdated, or is the problem with my environment?
Is it possible to declare a variable (var or val) without initializing it?

Alvaro Carrasco · Accepted Answer · 2016-05-11 23:13:13Z

41

The error is correct, you can only do that on an abstract class or trait. The tutorial might be assuming that you are writing that code inside of an abstract class.

It is possible to initialize variables to some default value:

var i: Int = _
var s: String = _

But that's essentially the same as:

var i: Int = 0
var s: String = null

answered May 11, 2016 at 23:13

Alvaro Carrasco

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8 Comments

juanmirocks Over a year ago

Where are these "defaults" values coming from? Is it dependent on the type? Does the scala compiler define the defaults for basic types like String ?

juanmirocks Over a year ago

Answering my own question, I guess that all AnyRef are defaulted to null, and that AnyVal must have their default value defined somehow (still don't see where this happens).

KiranM Over a year ago

The var i: Int = _ also gives me error: local variables must be initialized

Alvaro Carrasco Over a year ago

@KiranM What version of scala are you using?

KiranM Over a year ago

@Alvaro - I am using Scala 2.11.0

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