What does [B >: A] do in Scala?

Question

What does [B >: A] mean in Scala? And what are the effects?

Example reference: http://www.scala-lang.org/node/129

class Stack[+A] {
             


        

Answer 1

[B >: A] is a lower type bound. It means that B is constrained to be a supertype of A.

Similarly [B <: A] is an upper type bound, meaning that B is constrained to be a subtype of A.

In the example you've shown, you're allowed to push an element of type B onto a stack containing A elements, but the result is a stack of B elements.

The page where you saw this actually has a link to another page about lower type bounds, which includes an example showing the effect.