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val similar to Java final?

3 replies on 1 page. Most recent reply: Nov 23, 2008 8:04 PM by Vladimir Kelman

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Kean Heng Lim

Posts: 5
Nickname: 53770
Registered: Jan, 2008

val similar to Java final?

Posted: May 23, 2008 8:51 AM

Hi

Some questions from beginner of Scala -

If val similar to Java final but not the same, then what is the difference, in short?

Why both val and var are needed? While a val variable can't be re-assigned but it's always can be re-defined, doesn't that has the same effect as using var?

scala> val msg="Hello"
msg: java.lang.String = Hello

scala> msg="World"
<console>:5: error: reassignment to val
msg="World"
^

scala> val msg="Hello, World"
msg: java.lang.String = Hello, World

Regards
KH

Callum Rhodes

Posts: 3
Nickname: 54709
Registered: Mar, 2008

Re: val similar to Java final?

Posted: May 24, 2008 9:28 AM

Hi Kean

Although a val can be re-defined the origonal still exists, consider the two cases:


scala> val msg = "hello"

scala> def printMsg(): Unit = print(msg)

scala> printMsg() // Prints "hello"

scala> val msg = "hello world"

scala> printMsg() // Prints "hello" still

scala> print(msg) // This one prints "hello world"

On the other hand:


scala> var msg = "hello"

scala> def printMsg(): Unit = print(msg)

scala> printMsg() // Prints "hello"

scala> msg = "hello world"

scala> printMsg() // Now it prints "hello world"

There are a couple of other things to note, 1) you can re-define vars too, and 2) in a '.scala' file (script or otherwise) when re-defining vals/vars you must be in a different scope ie:


val msg = "hello"
{
   val msg = "hello world"
}

works but


val msg = "hello"
val msg = "hello world"

doesn't.

Callum

Jeff Heon

Posts: 40
Nickname: jfheon
Registered: Feb, 2005

Re: val similar to Java final?

Posted: May 26, 2008 7:27 AM

If my memory is correct, each statement in the interpreter is inside a nested scope, which explains that the val is not redefined at all, it is just defined in an inner scope.

Vladimir Kelman

Posts: 46
Nickname: vkelman
Registered: Feb, 2008

Re: val similar to Java final?

Posted: Nov 23, 2008 8:04 PM

Interestingly enough, F# language uses a special #light directive and, after you applied this directive, it begins to function like Scala interpreter, making subsequent statements to define nested scopes.
In C#, on the other hand, even the following is prohibited:

int i = 5;
{
int i = 3;
}

i.e. in C# in a nested scope you still cannot define a variable with the same name as on in an outer scope.

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