New a scala class with another class as param meet “type mismatch” error?

Question

class base[T] () {
    def tell(t: T) { }
}

class test[T](a: T, b: base[T]){
    b.tell("sssssssssss")
}

val b = new base[String]()
val s:String = "12354555"
val t = new test[String](s, b)

I have two classes, one is class base , the other is test .
Class test has two params.

After run the code, I get such a type mismatch error.

error: type mismatch;
 found   : String("sssssssssss")
 required: T
    b.tell("sssssssssss")
               ^
one error found

In my code, already defined a type String b, but in class test the type of b is T .

Answer 1

At compilation moment type variable T in class test is not String , but you try to call it with String

Two possible solutions:

a) Set Lower bounds for paramter T

class test[T >: String](a: T, b: base[T]){
    b.tell("sssssssssss")
}

b) Or change type of prameter b :

class test[T](a: T, b: base[String]){
    b.tell("sssssssssss")
}

Answer 2

In the example you gave, T is a String, but it doesn't have to be. It could be anything.

You pass a String to b.tell() no matter what, but b.tell()'s argument type is not guaranteed to be a String.