scala 如何摆脱：需要类类型但找到了 T
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时间：2020-10-22 06:39:08 来源:igfitidea点击:
How to get rid of : class type required but T found
提问by Yann Moisan
How to solve this compilation error :
如何解决这个编译错误：
trait Container {
def getInts() : Seq[Int]
def getStrings() : Seq[String]
def put[T](t: T)
def get[T] : Seq[T]
}
class MutableContainer extends Container {
val entities = new mutable.HashMap[Class[_], mutable.Set[Any]]() with mutable.MultiMap[Class[_], Any]
override def getStrings(): Seq[String] = entities.get(classOf[String]).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[String]] //strings
override def getInts(): Seq[Int] = entities.get(classOf[Int]).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[Int]]
override def get[T]: Seq[T] = entities.get(classOf[T]).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[T]]
override def put[T](t: T): Unit = entities.addBinding(t.getClass, t)
}
Here is the error :
这是错误：
[error] Container.scala:23: class type required but T found
[error] override def get[T]: Seq[T] = entities.get(classOf[T]).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[T]]
回答by rightfold
Tis not a class type, but a type parameter. Request a ClassTag:
T不是类类型，而是类型参数。请求ClassTag：
import scala.reflect._
override def get[T](implicit ct: ClassTag[T]): Seq[T] =
entities.get(ct.runtimeClass)
.map(_.toSeq)
.getOrElse(Seq.empty)
.asInstanceOf[Seq[T]]
But this brings another problem; this is not an override!
但这又带来了另一个问题；这不是覆盖！
So you have to modify the base class to declare getas follows:
所以你必须修改基类来声明get如下：
def get[T: ClassTag]: Seq[T]
回答by mohit
A solution without reflection
没有反射的解决方案
trait Container {
def getInts() : Seq[Int]
def getStrings() : Seq[String]
def put[T](clazz: Class[T],t :T)
def get[T](clazz: Class[T]) : Seq[T]
}
class MutableContainer extends Container {
val entities = new mutable.HashMap[Class[_], mutable.Set[Any]]() with mutable.MultiMap[Class[_],Any]
override def getStrings(): Seq[String] = entities.get(classOf[String]).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[String]] //strings
override def getInts(): Seq[Int] = entities.get(classOf[Int]).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[Int]]
override def get[T](clazz: Class[T]): Seq[T] = entities.get(clazz).map(_.toSeq).getOrElse(Seq.empty).asInstanceOf[Seq[T]]
override def put[T](clazz: Class[T],t :T): Unit = entities.addBinding(clazz, t)
}
