scala中的协变,逆变

trait Queue[+T] { ... }

Prefixing a formal type parameter with a + indicates that subtyping is covariant (flexible) in that parameter. By adding this single character, you are telling Scala that you want Queue[String], for example, to be considered a subtype of Queue[AnyRef]. The compiler will check that Queue is defined in a way that this subtyping is sound.Besides +, there is also a prefix -, which indicates contravariant subtyping.
If Queue were defined like this:

trait Queue[-T] { ... }

then if T is a subtype of type S, this would imply that Queue[S] is a subtype of Queue[T] (which in the case of queues would be rather surprising!).Whether a type parameter is covariant, contravariant, or nonvariant is called the parameter’s variance . The + and - symbols you can place next to type parameters are called variance annotations.