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I want to create a Scala class where one of its var is read-only from outside the class, but still a var. How can I do it?
If it was a val, there was no need to do anything. By default, the definition implies public access and read-only.
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We call the class like a function, as User() , to create an instance of the class. It is also possible to explicitly use the new keyword, as new User() , although that is usually left out. User has a default constructor which takes no arguments because no constructor was defined.
The difference between val and var is that val makes a variable immutable — like final in Java — and var makes a variable mutable.
Basic ClassClass variables are called, fields of the class and methods are called class methods. The class name works as a class constructor which can take a number of parameters. The above code defines two constructor arguments, xc and yc; they are both visible in the whole body of the class.
Type classes are a powerful and flexible concept that adds ad-hoc polymorphism to Scala. They are not a first-class citizen in the language, but other built-in mechanisms allow to writing them in Scala.
Define a public "getter" to a private var.
scala> class Foo {
| private var _bar = 0
|
| def incBar() {
| _bar += 1
| }
|
| def bar = _bar
| }
defined class Foo
scala> val foo = new Foo
foo: Foo = Foo@1ff83a9
scala> foo.bar
res0: Int = 0
scala> foo.incBar()
scala> foo.bar
res2: Int = 1
scala> foo.bar = 4
<console>:7: error: value bar_= is not a member of Foo
foo.bar = 4
^
Define a trait with the "getter" method:
trait Foo { def bar: T }
Define a class which extends this trait, and which has your variable
private class FooImpl (var bar: T) extends Foo
Restrict the visibility of this class appropriately.
Having a dedicated interface allows you also to use multiple implementation classes at runtime, e.g. to cover special cases more efficiently, lazy loading etc.
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