How can Scala receive multiple parameters in a method definition?

Question

Java has:

public void someMethod(int ... intArray) { // question: what is the equivalent to "..." 
    // do something with intArray
}

how can I achieve the same functionality in Scala? That is, passing an undefined number of parameters to a method?

Answer 1

Both Java and Scala have varargs, and both only support it for the last parameters.

 def varargTest(ints:Int*) { ints.foreach(println) }  

From this article, the difference is in the type used for the varargs arguments:

• array for Java
• Seq (Sequence) for Scala: it can be iterated and many methods such as collections foreach, map, filter, find, ... are available

The '*' stands for 0 or more arguments.

Note: if the parameter values are already "packaged" as a sequence, such as a list, it fails:

# varargTest(List(1,2,3,4,5))  
# //--> error: type mismatch;  
# //-->  found   : List[Int]  
# //-->  required: Int  
# //-->        varargTest(List(1,2,3,4,5))  
# //--> 

But this will pass:

  varargTest(List(1,2,3):_*)  
  //--> 1  
  //--> 2  
  //--> 3  

'_' is a placeholder shortcut for type inference. '_*' is here applyied to a 'repeated type.
Section 4.6.2 of Scala Specification mentions:

The last value parameter of a parameter section may be suffixed by “*”, e.g. (..., x:T *).
The type of such a repeated parameter inside the method is then the sequence type scala.Seq[T].
Methods with repeated parameters T* take a variable number of arguments of type T.

 (T1, . . . , Tn,S*)U => (T1, . . . , Tn,S, . . . , S)U, 

The only exception to this rule is if the last argument is marked to be a sequence argument via a _* type annotation.

 (e1, . . . , en,e0: _*) => (T1, . . . , Tn, scala.Seq[S]).

Note bis: beware of the underlying type erasure of Java:

//--> error: double definition:
//--> method varargTest:(ints: Seq[Int])Unit and
//--> method varargTest:(ints: Int*)Unit at line 10
//--> have same type after erasure: (ints: Sequence)Unit
//-->   def varargTest(ints:Seq[Int]) { varargTest(ints: _*) }