Varargs

Both Java and Scala have varargs. In Scala the syntax for a varargs argument is def method (param:String*). In the off-chance you are not aware of what varargs are; they allow an arbitrary number of values to be passed to the method. Some restrictions on when varargs can be used are:

• The vararg parameter must be the last parameter in the list
• There can not be default values for any parameters in the method containing the varargs (Scala 2.8+)

From the caller's point of view; varargs can be called as follows: method("p1", "p2", "p3") where the number of values is not limited. However, the () must be used.

In Java (if I remember right) only arrays can be passed in place of varargs:

1. class C {
2.   public static void main(String... args) {
3.     System.out.println(args.length)
4.   }
5. }
6. String[] args = new String[]{"arg1", "arg2"}
7. C.main (args)

However Scala is more general and allows any sequence to be passed to varargs, with a caveat. When a sequence is passed to varargs a hint must be provided to indicate that you are intending to have the sequence be expanded to be the varargs.

1. def method (args:Int*) = println(args)
2. val list = List(1,2,3)
3. method (list:_*)  // note the use of _*

Examples:

1. scala> def method(varargs:Int*)(more:String*) = println(varargs,more)
2. method: (Int*)(String*)Unit
4. scala> method(1,2,3,4)("one")
5. (Array(1, 2, 3, 4),Array(one))
7. scala> method(1,2,3,4)       
8. < console>:6: error: missing arguments for method method in object $iw;
9. follow this method with '_' if you want to treat it as a partially applied function
10.        method(1,2,3,4)
11.        ^
13. scala> method(1,2,3,4)()
14. (Array(1, 2, 3, 4),Array())
16. scala> method()("one")  
17. (Array(),Array(one))
19. scala> method("one")  
20. < console>:6: error: type mismatch;
21.  found   : java.lang.String("one")
22.  required: Int
23.        method("one")
24.               ^
26. scala> method()()     
27. (Array(),Array())
29. scala> val method2 = method(1,2,3)_  
30. method2: (String*) => Unit = < function>
32. scala> val paramList = List("hi","ho")
33. paramList: List[java.lang.String] = List(hi, ho)
35. scala> method2(paramList)
36. < console>:8: error: type mismatch;
37.  found   : List[java.lang.String]
38.  required: String
39.        method2(paramList)
40.                ^
42. scala> method2(paramList:_*)
43. (Array(1, 2, 3),List(hi, ho))
45. scala> val range = (1 to 5) map {_.toString} 
46. range: RandomAccessSeq.Projection[java.lang.String] = RangeM(1, 2, 3, 4, 5)
48. scala> method2(range:_*)                     
49. (Array(1, 2, 3),RangeM(1, 2, 3, 4, 5))