如何在 Scala 中查找值类型的实例？
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How to find instance of a value type in Scala?
提问by Bala
I know isInstanceOfcan be used to find value type, but how do I find the type of 'str'?
我知道isInstanceOf可用于查找值类型，但是如何查找“str”的类型？
What type is it?
它是什么类型？
scala> val str = ("Scala", "Elixir","Spark")
str: (String, String, String) = (Scala, Elixir, Spark)
The following throws an error (exclude Any/AnyRef etc for now):
以下会引发错误（暂时排除 Any/AnyRef 等）：
scala> str.isInstanceOf[List]
<console>:13: error: type List takes type parameters
str.isInstanceOf[List]
scala> str.isInstanceOf[String]
<console>:13: warning: fruitless type test: a value of type (String, String, String) cannot also be a String (the underlying of String)
str.isInstanceOf[String]
^
res9: Boolean = false
I can check it this way but is there a name for this?
我可以通过这种方式检查它，但是有名称吗？
scala> str.isInstanceOf[(String, String, String)]
res12: Boolean = true
回答by pamu
Use :typein scala repl to find type
:type在 scala repl 中使用以查找类型
Actual type is Tuple3[String, String, String]
实际类型是 Tuple3[String, String, String]
str.isInstanceOf[Tuple3[String, String, String]]
Scala REPL
Scala REPL
scala> val str = ("Scala", "Elixir","Spark")
str: (String, String, String) = (Scala,Elixir,Spark)
scala> :type str
(String, String, String)
scala> str.isInstanceOf[Tuple3[String, String, String]]
res2: Boolean = true
scala> str.getClass
res3: Class[_ <: (String, String, String)] = class scala.Tuple3
回答by fabfas
How to determine val type programmatically?
如何以编程方式确定 val 类型？
For instance, you want to know the type of the value is some specific type?
例如，您想知道值的类型是某种特定类型吗？
// assign x value
val x: Any = "this is string"
// custom function evaluate type
def f[T](v: T) = v match {
case _: Int => println("Int")
case _: String => println("String")
case _ => println("Unknown")
}
// determine val type
f(x)
回答by Alexey Svyatkovskiy
Yet another way to determine a type programmatically is using Manifest:
另一种以编程方式确定类型的方法是使用 Manifest：
scala> def getType[T: Manifest](t: T): Manifest[T] = manifest[T]
getType: [T](t: T)(implicit evidence: Manifest[T])Manifest[T]
scala> val str = ("Scala", "Elixir","Spark")
str: (String, String, String) = (Scala,Elixir,Spark)
scala> getType(str)
res0: Manifest[(String, String, String)] = scala.Tuple3[java.lang.String, java.lang.String, java.lang.String]
回答by Joseph
This method can help you get the type of any val/var at runtime, it's also works in compiled code.
此方法可以帮助您在运行时获取任何 val/var 的类型，它也适用于已编译的代码。
import scala.reflect.runtime.universe._
def printType[T](x: T)(implicit tag: TypeTag[T]): Unit = println(tag.tpe.toString)
printType(List[Int](1,2,3)) // output: List[Int]
printType(("xxx", 123, 0.1)) // output: (String, Int, Double)
printType(2) // output: Int
回答by Jasper-M
The modern (meaning Scala 2.10 or later), Scala way of programmatically getting the compile time type of something is using a TypeTag.
现代（意味着 Scala 2.10 或更高版本），Scala 以编程方式获取某事物的编译时类型的方法是使用TypeTag.
scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._
scala> def getType[T: TypeTag](value: T) = typeOf[T]
getType: [T](value: T)(implicit evidence: reflect.runtime.universe.TypeTag[T])reflect.runtime.universe.Type
scala> val str = ("Scala", "Elixir","Spark")
str: (String, String, String) = (Scala,Elixir,Spark)
scala> println(getType(str))
(java.lang.String, java.lang.String, java.lang.String)
scala> getType(str) <:< typeOf[(String,String,String)]
res1: Boolean = true
scala> getType((1,2,3)) <:< typeOf[(String,String,String)]
res2: Boolean = false
getClasswill give your the erased runtime class. isInstanceOf[T]will test whether the erased runtime class is the same as or a subclass of the erased runtime class of T.
getClass会给你擦除的运行时类。isInstanceOf[T]将测试擦除的运行时类是否与 的擦除运行时类相同或子类T。
And "erased" means that the following evaluates to true.
“擦除”意味着以下评估为true.
(1,2,3).isInstanceOf[(String,String,String)]
"runtime" and "compile time" mean that this is true:
“运行时”和“编译时”的意思是true：
val a: Any = (1,2,3)
a.isInstanceOf[(_,_,_)]
while this is false:
虽然这是false：
val a: Any = (1,2,3)
getType(a) <:< typeOf[(Int,Int,Int)]
