scala 中 trait 中使用的抽象值有什么问题吗? [英] Is there something wrong with an abstract value used in trait in scala?

问题描述

我有

trait Invoker {
  val method: Method
}

Intellij IDEA 代码检查警告我特征中使用了抽象值".一切都编译得很好.在特征中具有抽象值有什么问题吗?如果是这样，我应该如何指定 trait 的所有扩展器都必须定义一个方法属性?

Intellij IDEA code inspection is warning me that "Abstract value used in trait". Everything compiles fine. Is there something wrong with having an abstract value in a trait? If so, how should I specify that all extenders of the trait must define a method property?

推荐答案

这意味着以下奇怪之处:<​​/p>

What is meant by this is the following weirdness:

trait A {
  val i: String
  def j: String
}

class C extends A {
  println ("val i = " + i)
  println ("def j = " + j)

  val i = "i"
  def j = "j"
}

val c = new C
// prints
// val i = null
// def j = j

所以，正如你所看到的，i 在它最终被构造函数覆盖之前被初始化为它的默认值(null for AnyRef)在 C 中.(def 声明会立即重新引用.)

So, as you can see i is initialised to it default value (null for AnyRef) before it is finally overridden by the constructor in C. (def declarations are re-referenced immediately.)

为了避免这种情况，如果可能的话，必须将 val 初始化放在构造函数的开头.

To avoid this one would have to put the val initialisations to the beginning of the constructor, if possible.

在以下情况下的其他奇怪之处(以及如何解决它)

考虑

trait A {
  val i: String
  def j: String
}

abstract class D extends A {
  println ("val i = " + i)
  println ("def j = " + j)
}

class C extends D {
  val i = "i"
  def j = "j"
}
val c = new C
// prints
// val i = null
// def j = null

现在我们似乎不走运；看起来好像我们没有机会在我们的超类 D 尝试之前初始化 val i and def j打印它们.为了解决这个问题，我们必须使用早期定义(§5.1.6 Scala 参考):

Now we seem to be out of luck; it looks as if there is no chance for us to initialise val i and def j before our superclass D tries to print them. In order to solve this problem, we must use Early definitions (§5.1.6 Scala reference):

class C extends {
  val i = "i"
  def j = "j"
} with D

val c = new C
// prints
// val i = i
// def j = j

它有效！

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