Kotlin 有没有办法在数据类构建时转换属性的值

发布于03月30日

When creating a data class I frequently find that I want to transform one of the properties, usually to normalize it or to make a defensive copy. For example, here I want productCode to always be lowercase:

data class Product(val productCode: String)

我try 添加一个init%的挡路，希望Kotlin 足够聪明，让我手动处理构造函数参数对属性的赋值:

data class Product(val productCode: String) {
    init {
        this.productCode = productCode.toLowerCase()
    }
}

but it treats this as a reassignment.

我宁愿不必手工编写equals/hashCode/toString/copy，IDE生成的方法也不会更好.

Is there any way to transform constructor parameters in a data class?

推荐答案

No. For equality and toString to work, the properties need to be in the primary constructor.

但是，您可以创建一个工厂方法:

data class Product private constructor(val productCode: String) {

  companion object Factory {
     fun create(productCode: String) : Product {
        return Product(productCode.toLowerCase())
     }
  }
}

By making the constructor private you force usage of this create method.

If you want to get 'hacky', you can pretend you're still calling the constructor, by renaming create to invoke and making it an operator function:

data class Product private constructor(val productCode: String) {

    companion object {

        operator fun invoke(productCode: String): Product {
            return Product(productCode.toLowerCase())
        }
    }
}

调用Product("foo")将调用invoke方法.

^{注意:构造函数仍然通过copy方法公开，请参见https://youtrack.jetbrains.com/issue/KT-11914}