我可以根据构造函数初始化一个值吗？

Question

我有一个课，我要么知道创造的具体价值，要么我需要生成它，这有点贵。只有在实际需要时才可以生成该值？

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

Answer 1

这是可能的，但有一个转折：

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构造的隐私，让二级构造公众。