Scala中的可更新单例对象

Question

I would like a singleton object in Scala that has an updatable field. Is there a better solution than what I have below that doesn't use a var ?

class Foo {

  def setFoo(newFoo: String): Unit = {
    Foo.foo = newFoo
  }

  def getFoo: String = {
    Foo.foo
  }
}

object Foo {
  var foo = "foo"
}

Answer 1

If you want mutability (ie a variable being constantly updated), var is your only option.

But you can write it in an "immutable way" in which set means creating a new immutable instance:

class Foo(foo: String) {}

object Foo {
    create(foo: String): Foo = new Foo(foo)
}

// ...
Foo foo = Foo.create("foo")
val bar = foo.foo  // "foo"

Answer 2

I would stick with your original implementation, but shield your mutable singleton to the associated class:

class Foo {

  def setFoo(newFoo: String): Unit = {
    Foo.foo = newFoo
  }

  def getFoo: String = {
    Foo.foo
  }
}

private [Foo] object Foo {
  var foo = "foo"
}

Answer 3

Better solution to what you have that does use var could be simply

object Foo {
  var foo = "foo"
}

just this, you don't need the class and those setters, compiler will generate them for you. Now you can just do:

Foo.foo = "bar"

or read value like this

Foo.foo

I don't see a way to have mutable state in object without vars though.