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Scala - ClassTag and TypeTag Performance Penalty

If you program in Java, you probably know that the compiler performs type erasure for performance and backwards compatibility reasons. Thus, parameterized types of generic collections are normally not accessible (although you can work around the limitation using reflection or by passing the class  explicitly).

Scala comes with the concept of ClassTag and TypeTag which enable you to enrich the accessible type information. Using type bounds, it makes it easy to access type information at runtime.

However, using ClassTags and TypeTags requires importing from scala.reflect. Does this mean that it uses reflection, a scary thought when considering performance? Here is a quick unscientific benchmark to figure things out with Scala 2.12.4:


As you can see, using ClassTags incurs no performance penalty compared to case class pattern matching. On the other hand, TypeTags is significantly slower due to it bringing along richer type information. Only use the latest if you really need it!

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