我可以根據構造函數初始化一個值嗎？

Question

我有一個課，我要麼知道創造的具體價值，要麼我需要生成它，這有點貴。只有在實際需要時纔可以生成該值？

val expensiveProperty: A 
constructor(expensiveProperty: A) { 
    this.expensiveProperty = expensiveProperty 
} 
constructor(value: B) { 
    // this doesn't work 
    this.expensiveProperty = lazy { calculateExpensiveProperty(value) } 
} 

Answer 1

這是可能的，但有一個轉折：

class C private constructor(lazy: Lazy<A>) { 
    val expensiveProperty by lazy 

    constructor(value: B) : this(lazy { calculateExpensiveProperty(value) }) 
    constructor(expensiveProperty: A) : this(lazyOf(expensiveProperty)) 
} 

注意我是如何保持主構造的隱私，讓二級構造公衆。