Scala: Erroneous type mismatch error using ArrayList

Question

Can anyone explain why the following error occurs (Scala 2.10.3)?

scala> new java.util.ArrayList[Integer]()
res0: java.util.ArrayList[Integer] = []

scala> res0.add(0)
res1: Boolean = true

scala> java.util.Collections.binarySearch(res0, 0)
<console>:9: error: type mismatch;
 found   : java.util.ArrayList[Integer]
 required: java.util.List[_ <: Comparable[_ >: Any]]
              java.util.Collections.binarySearch(res0, 0)
                                                 ^

The following does work:

scala> java.util.Collections.binarySearch[Integer](res0, 0)
res4: Int = 0

It seems odd that the compiler would complain about a particular type until I was more explicit about that incorrect type and then it will accept it.

EDIT:

Also note the if you change the first step to:

scala> new java.util.ArrayList[Int]()

there is also a very similar error.

Answer 1

Try this:

java.util.Collections.binarySearch(res0, 0: Integer)

As you can see, it compiles fine. The problem was that 0 has type Int, not Integer. So you have to tell scala somehow that you want to convert 0 into a Integer value.

As it stands, your code triggers the compiler to look for a method binarySearch which takes and ArrayList[Integer] as its first parameter, and an Int as its second parameter.

Answer 2

Definition of binarySearch is as follows:

Searches the specified list for the specified object using the binary search algorithm.

static <T> int
binarySearch(List<? extends Comparable<? super T>> list, T key)

As you can see, list and key are parameterized on a type T. Now Scala compiler tries to infer the type of list in following invocation:

scala> java.util.Collections.binarySearch(l, 0)
<console>:9: error: type mismatch;
 found   : java.util.ArrayList[Integer]
 required: java.util.List[_ <: Comparable[_ >: Any]]
              java.util.Collections.binarySearch(l, 0)

And the common type between the parameters list and key is Any. Because former is of type List<Integer> and later is of type int as per Java. So this converts in Scala to Integer and int respectively, as explained below:

scala> classOf[Integer]
res9: Class[Integer] = class java.lang.Integer

scala> classOf[Int]
res10: Class[Int] = int