In Scala, type parameters and abstract types may be constrained by a type bound. Such type bounds limit the concrete values of the type variables and possibly reveal more information about the members of such types. An upper type bound T <: Adeclares that type variable T refers to a subtype of type A.

Here is an example which relies on an upper type bound for the implementation of the polymorphic method findSimilar:

trait Similar {def isSimilar(x: Any): Boolean}case class MyInt(x: Int)extends Similar {def isSimilar(m: Any): Boolean =    m.isInstanceOf[MyInt] &&    m.asInstanceOf[MyInt].x == x}object UpperBoundTestextends Application {def findSimilar[T <: Similar](e: T, xs: List[T]): Boolean =if (xs.isEmpty)falseelse if (e.isSimilar(xs.head))trueelse findSimilar[T](e, xs.tail)val list: List[MyInt] = List(MyInt(1), MyInt(2), MyInt(3))  println(findSimilar[MyInt](MyInt(4), list))  println(findSimilar[MyInt](MyInt(2), list))}

Without the upper type bound annotation it would not be possible to call method isSimilar in method findSimilar.

The usage of lower type bounds is discussed here. 

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