Scala：使用泛型类型参数声明方法

Scala: Declaring method with generic type parameter

提问by Neil

What is the equivalent of the following Java method declaration in Scala:

Scala 中以下 Java 方法声明的等效项是什么：

public <T>  T readValue(java.lang.String s, java.lang.Class<T> tClass)

In other words, I'd like to declare a method that takes a class of type T, and returns an instance of that type.

换句话说，我想声明一个方法，它接受一个类型为 T 的类，并返回该类型的一个实例。

回答by Arseniy Zhizhelev

• I. Very close to what you want:

  def readValue[T:ClassTag](s:String):T = {
    val tClass = implicitly[ClassTag[T]].runtimeClass
    //implementation for different classes.
  }
  

  usage is a bit clearer than in Java:

  val myDouble = readValue[Double]("1.0")
  

• I. 非常接近你想要的：

  def readValue[T:ClassTag](s:String):T = {
    val tClass = implicitly[ClassTag[T]].runtimeClass
    //implementation for different classes.
  }
  

  用法比Java更清楚一点：

  val myDouble = readValue[Double]("1.0")
  

回答by Arseniy Zhizhelev

• II. Another, more involved way, is to externalize implementation of readValue to some user-provided object (type class):

  trait ValueReader[T] {
    def readValue(s: String): T
  }
  
  def readValue[T: ValueReader](s: String): T = {
    val reader = implicitly[ValueReader[T]]
    reader.readValue(s)
  }
  
  implicit val doubleReader = new ValueReader[Double] {
    def readValue(s:String) = // implementation for Double
  }
  

• 二、另一种更复杂的方法是将 readValue 的实现外部化为一些用户提供的对象（类型类）：

  trait ValueReader[T] {
    def readValue(s: String): T
  }
  
  def readValue[T: ValueReader](s: String): T = {
    val reader = implicitly[ValueReader[T]]
    reader.readValue(s)
  }
  
  implicit val doubleReader = new ValueReader[Double] {
    def readValue(s:String) = // implementation for Double
  }