注意：屬性和變量不同，屬性是指一組get和set方法和backing field，變量是指field

var是可變屬性，val是賦值一次，可讀屬性

public class Address { 
    public var name: String = ...
    public var street: String = ...
    public var city: String = ...
    public var state: String? = ...
    public var zip: String = ...
}

//使用屬性
fun copyAddress(address: Address): Address {
    val result = Address() // there's no 'new' keyword in Kotlin
    result.name = address.name // accessors are called
    result.street = address.street
    // ...
    return result
}

定義var屬性的語法：

var <propertyName>: <PropertyType> [= <property_initializer>]
    [<getter>]
    [<setter>]

initializer，getter，setter都是可選的，屬性類型如果從initializer或者父類變量中可以推斷出來，則可以省略，getter，setter沒寫則會自動生成

val必須在定義或者constructor中賦值

重寫getter

val isEmpty: Boolean
    get() = this.size == 0

重寫setter

var stringRepresentation: String
    get() = this.toString()
    set(value) {
        setDataFromString(value) // parses the string and assigns values to other properties
    }

如果只想改變修飾符，而不重新定義getter和setter，可以不用寫body

var setterVisibility: String = "abc"
    private set // the setter is private and has the default implementation

var setterWithAnnotation: Any? = null
    @Inject set // annotate the setter with Inject

Fields

Kotlin中的類沒有field，但是有時寫自定義訪問函數時，需要使用backing field，使用field來訪問backing field，field只能在訪問方法只使用

var counter = 0 // the initializer value is written directly to the backing field
    set(value) {
        if (value >= 0) field = value
    }

除了backing field，還可以使用backing property

private var _table: Map<String, Int>? = null
public val table: Map<String, Int>
    get() {
        if (_table == null) {
            _table = HashMap() // Type parameters are inferred
        }
        return _table ?: throw AssertionError("Set to null by another thread")
    }

編譯時常量

編譯能獲取值的屬性可以用const來修飾

• Top-level或者object的屬性
• 類型為String或者其他primitive type
• 沒有自定義getter

const val SUBSYSTEM_DEPRECATED: String = "This subsystem is deprecated"

延遲初始化的屬性

任何type為非null的屬性都要在構造函數或者定義時初始化，否則需要使用lateinit來修飾

• 只能是var屬性
• 在body里定義，不在primary constructor里定義
• non-null type, not primitive type

public class MyTest {
    lateinit var subject: TestSubject

    @SetUp fun setup() {
        subject = TestSubject()
    }

    @Test fun test() {
        subject.method()  // dereference directly
    }
}