why val work but var doesn't in constructor in this case (scala)

Question

And one of the answer is as below:

trait T1 { def a: String; def x = a.length }
class Test(val a: String) extends T1

But if only works:

define a in T1 as "def a", if I use "val a" or "var a", then it doesn't work
define a in Test as "val a", if I just use "a" which means default to "var a", it also doesn't work.

Why does it happen?

"It doesn't work" doesn't describe the error you're seeing. Always describe the actual error. — Jon Skeet
Works for me: scala> trait T1 { val a: String; def x = a.length } defined trait T1 scala> class Test(val a: String) extends T1 defined class Test scala> new Test("foo").x res0: Int = 3 — Dima
It doesn't "default to var a" - if you write class Test(a: String) then that is not the same as class Test(var a: String). For case classes it's the same as val a but not var a. — Jesper

Dima Dima · Accepted Answer · 2014-12-11T13:46:06

"#1" should work, as I mentioned in the comment. Regarding #2, there two things:

class Test(a: String) is not the same as class Test(var a: String) (if anything, it is actually closer to val declaration, but isn't the same either, except for case classes). The latter declares a as a mutable class member, while the former just makes it an argument to the constructor, not a member of the class at all. For this reason, it fails in your case: you have to override the abstract member a of T1 in order to be able to extend it.
class Test(var a: String) won't work either. This is because a is declared in T1 as def, which makes it immutable. You can override a def with a def or a val, but you cannot override it with a mutable value. If you need it to be mutable in Test, you have to declare it as a var in T1 as well.