Kotlin笔记 类属性和类变量

注意：属性和变量不同，属性是指一组get和set方法和backing field，变量是指field

var是可变属性，val是赋值一次，可读属性

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

//使用属性
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
}

定义var属性的语法：

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

initializer，getter，setter都是可选的，属性类型如果从initializer或者父类变量中可以推断出来，则可以省略，getter，setter没写则会自动生成

val必须在定义或者constructor中赋值

重写getter

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

重写setter

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

如果只想改变修饰符，而不重新定义getter和setter，可以不用写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中的类没有field，但是有时写自定义访问函数时，需要使用backing field，使用field来访问backing field，field只能在访问方法只使用

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

除了backing field，还可以使用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")
    }

编译时常量

编译能获取值的属性可以用const来修饰

• Top-level或者object的属性
• 类型为String或者其他primitive type
• 没有自定义getter

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

延迟初始化的属性

任何type为非null的属性都要在构造函数或者定义时初始化，否则需要使用lateinit来修饰

• 只能是var属性
• 在body里定义，不在primary constructor里定义
• 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
    }
}