如何创建一个内部类，只有外部类可以访问构造函数，而其余的则在任何地方都可见？

Question

我最初想创建一个可以在构造函数中中止实例化的类，但根据此链接，我应该改用 Factory 类。但是现在我想阻止除工厂类之外的任何人创建“Inner”类的对象，同时向所有人提供对内部类方法的访问权限。

我已经尝试过这个答案。

import java.util.Date

object InnerFactory {

    class Inner private constructor(startDate: Date? = null, endDate: Date? = null) {
        fun getTimeDifference(): Long? {
            //calculates time difference but doesn't matter to this example
        }
    }

    fun createInnerObject(startDate: Date? = null, endDate: Date? = null): Inner? {
        if (startDate != null && endDate != null && !endDate.after(startDate)) {
            return null
        }

        return Inner(startDate, endDate)
    }

}

我会像下面这样使用它：

val date1 = Date(1547600000)
val date2 = Date(1547600600)
val inner = InnerFactory.createInnerObject(date1, date2) //should return an instance
val invalidInner = InnerFactory.createInnerObject(date2, date1) //should not return an instance because the "endDate" is before "startDate"
val difference = inner?.getTimeDifference()

将鼠标悬停在“createInnerObject”函数中我对构造函数的使用上时，它说“无法访问 '<init>'：它在 'Inner' 中是私有的”。

Answer 1

你可以做什么：

• 引入一个interface Inner应该公开的所有必要功能
• 制作所有类private并实现该接口

样本：

object InnerFactory {

  interface Inner {
    fun getTimeDifference(): Long?
  }

  private class InnerImpl(startDate: Date? = null, endDate: Date? = null) : Inner {
    override fun getTimeDifference(): Long? = TODO("some implementation")
  }


  fun createInnerObject(startDate: Date? = null, endDate: Date? = null): Inner? {
    if (startDate != null && endDate != null && !endDate.after(startDate)) {
      return null
    }
    return InnerImpl(startDate, endDate) // InnerImpl accessible from here but not from outside of InnerFactory...
  }
}

现在您无法再InnerImpl从外部访问，但仍然可以使用所有必要的功能：

// the following all work as it deals with the interface
val inner = InnerFactory.createInnerObject(date1, date2) //should return an instance
val invalidInner = InnerFactory.createInnerObject(date2, date1) //should not return an instance because the "endDate" is before "startDate"
val difference = inner?.getTimeDifference()

// the following will not compile:
InnerImpl()