Polymorphic Methods

Methods in Scala can be parameterized by type as well as value. The syntax is similar to that of generic classes. Type parameters are declared within a pair of brackets while value parameters are enclosed in a pair of parentheses.

Here is an example:

def listOfDuplicates[A](x: A, length: Int): List[A] = {
    if (length < 1)
        x :: listOfDuplicates(x, length - 1)
println(listOfDuplicates[Int](3, 4))  // List(3, 3, 3, 3)
println(listOfDuplicates("La", 8))  // List(La, La, La, La, La, La, La, La)

The method listOfDuplicates takes a type parameter A and values parameters x and n. In this case, value x is of type A. If length < 1 we return an empty list. Otherwise we prepend x to the the list of duplicates returned by the recursive call to listOfDuplicates. (note: :: means prepend an element on the left to a sequence on the right).

When we call listOfDuplicates with [Int] as the type parameter, the first argument must be an int and the return type will be List[Int]. However, you don’t always need to explicitly provide the the type parameter because the compiler can often figure it out based on the type of value argument ("La" is a String). In fact, if calling this method from Java it is impossible to provide the type parameter.