Hi, everbody. My question is Can field be overriden by subclasse's method? If the answer is YES, then how can I do it?

scala> class A { val a = 5 }
defined class A

scala> class B extends A { override def a = (Math.random * 10).toInt }
<console>:5: error: error overriding value a in class A of type Int;
 method a needs to be an immutable value
       class B extends A { override def a = (Math.random * 10).toInt }

According to the Liskov Substitutability Principle, if B is a subclass of A, then instances of B should always be valid where instances of A are.

Let's imagine Scala let you compile that code.  Here's a little test for instances of A, written in a Specs stylee.

"A.a" should {
  "not vary between accesses" in {
    anA.a mustEqual anA.a
  }
}

My class B would make that test fail sometimes.  In other words, overriding and removing guarantees is a bad thing to do.  It will break code that relies on those guarantees.

It is a little surprising that a lazy val can't override a val though.

2009/1/20 Roclae <haixu.huang@live.cn>

For example:
I know that,
class A {
 def greeting = {"Hello!"}
}

class B {
 override val greeting = "Hello again!"
}

But can I xchange the overriding order?
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OK, I got it. nice reply. Thank U very much. :)

Ricky Clarkson wrote:
>
> scala> class A { val a = 5 }
> defined class A
>
> scala> class B extends A { override def a = (Math.random * 10).toInt }
> :5: error: error overriding value a in class A of type Int;
> method a needs to be an immutable value
> class B extends A { override def a = (Math.random * 10).toInt }
>
> According to the Liskov Substitutability Principle, if B is a subclass of
> A,
> then instances of B should always be valid where instances of A are.
>
> Let's imagine Scala let you compile that code. Here's a little test for
> instances of A, written in a Specs stylee.
>
> "A.a" should {
> "not vary between accesses" in {
> anA.a mustEqual anA.a
> }
> }
>
> My class B would make that test fail sometimes. In other words,
> overriding
> and removing guarantees is a bad thing to do. It will break code that
> relies on those guarantees.
>
> It is a little surprising that a lazy val can't override a val though.
>
> 2009/1/20 Roclae
>
>>
>> For example:
>> I know that,
>> class A {
>> def greeting = {"Hello!"}
>> }
>>
>> class B {
>> override val greeting = "Hello again!"
>> }
>>
>> But can I xchange the overriding order?
>> --
>> View this message in context:
>> http://www.nabble.com/Hi%2C-everbody.-My-question-is-Can-field-be-overri...
>> Sent from the Scala mailing list archive at Nabble.com.
>>
>>
>
>