fields.md

Fields

Fields are a way to store information about objects of a class. They are declarated using the var and let keywords:

class Person
{
    let id: long = randomID()
    var name: String
    var age: int
}

You can declare fields with or without an initial value. If no initial value is declared, it will be the default value for the type of the field. See the table in Wildcard Literals for more information.

Initialization

Instance fields, i.e. non-static fields are initialized as soon as an object of the class is constructed.

class Test
{
    var s: String = { print "Init"; "." }
}

new Test // prints 'Init'
new Test // prints 'Init'

Static fields belong to the class itself, not individual members. This means they are initialized only once: when the class gets reference for the first time.

class Test
{
    static var s: String = { print "Init"; "." }
}

new Test // prints 'Init'
new Test // does not print 'Init' again

Final Fields and let Declarations

The final modifier or let declarator makes a field immutable. This means it cannot be re-assigned after it has been set to an initial value. Attempting to re-assign a final field will result in a compilation error.

class Test
{
    let i: int = 1
    final var j: int = 2
}

let t = new Test
t.i = 3 // invalid, cannot reassign final field 'i'
t.j = 3 // invalid, s.a.

All final fields have to be initialized with an initial value, or a compilation error will be issued.