为什么用成员变量声明为元组的这种擦除警告？ [英] Why this erasure warning with member variables declared as a tuple?

问题描述

看看这个Scala类：

  class示例{
 val（x，y）：（ Int，Int）=（1，2）
} 
  

编译此结果会产生警告：

  Example.scala：2：警告：类型模式
中的非变量类型参数Int Int，Int ）未被选中，因为通过擦除
 val（x，y）消除了：（Int，Int）=（1，2）
 ^ 
  

删除显式类型注释可避免此警告：

 类示例{
 val（x，y）=（1，2）
} 
  

为什么我会收到警告，为什么删除显式类型注释会删除它呢？据我所知，没有什么真正的变化， x 和 y 仍为 Int类型不带类型批注。

解决方案

您可以将示例重写为：

 类示例{
 val Tuple2（x，y）：Tuple2 [Int，Int] = Tuple2（1，2）
 } 
  

此模式匹配实际上由2个匹配组成-现在它说：取右边的对象键入 Tuple2 [Int，Int] 并在 unapply [Int，Int] > Tuple2 随播对象。然后， unapply [Int，Int] 会验证该对象确实具有 Tuple2 类型，其结果值将为用于将值绑定到变量 x 和 y 。

此后，此模式匹配包含：Tuple2 [Int，Int] ，因此它尝试执行 isInstanceOf [Tuple2 [Int，Int] ] 动态检查对象是否另外具有类型 Tuple2 [Int，Int] 。但是，泛型类型信息在运行时会被删除，因此编译器警告说，它实际上无法生成验证类型参数 [Int，Int] 实例化该对象的代码。 / p>

以相同的方式，按照以下模式匹配：

  val a：AnyRef =（1、2）
a比赛{
情况t2：Tuple [Int，Int] => 
} 
  

您也会收到类似的警告。

Have a look at this Scala class:

class Example {
  val (x, y): (Int, Int) = (1, 2)
}

Compiling this results in a warning:

Example.scala:2: warning: non variable type-argument Int in type pattern
               (Int, Int) is unchecked since it is eliminated by erasure
    val (x, y): (Int, Int) = (1, 2)
                ^

Removing the explicit type annotation gets rid of this warning:

class Example {
  val (x, y) = (1, 2)
}

Why do I get the warning and why does removing the explicit type annotation get rid of it? As far as I can see nothing really changes, x and y are still of type Int without the type annotation.

解决方案

You could rewrite your example to:

class Example {
  val Tuple2(x, y): Tuple2[Int, Int] = Tuple2(1, 2)
}

This pattern match actually consists of 2 matches - it now says: take the right hand side object of type Tuple2[Int, Int] and call the method unapply[Int, Int] on the Tuple2 companion object. The unapply[Int, Int] will then verify that the object really has the type Tuple2, and its result value will be used to bind values to variables x and y.

After that, this pattern match contains : Tuple2[Int, Int], so it tries to do an isInstanceOf[Tuple2[Int, Int]] check dynamically to see if the object additionally has the type Tuple2[Int, Int]. However, generic type information is erased at runtime, so the compiler warns that it cannot actually produce code which verifies that the object is instantiated for type parameters [Int, Int].

In the same way, in the following pattern match:

val a: AnyRef = (1, 2)
a match {
  case t2: Tuple[Int, Int] => 
}

you would get a similar warning.

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