• 屬性宣告

fun copyAddress(address: Address): Address {
    val result = Address()
    result.name = address.name
    result.street = address.street
    // ...
    return result
}

• Getters以及setters
  語法如下：

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

• Backing fields
  需要注意的是，在Kotlin內field不能直接被宣告
  在屬性需要Backing fields時，Kotlin會自動提供

var counter = 0
    set(value) {
        if (value >= 0)
            field = value
    }

• Backing properties
  隱含的backing field若在使用上不符合的話，可以透過Backing properties達到

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