Scala compiler says: value & is not a member of java.lang.object

Question

I am very new to Scala, I try some tutorials but I didn't get the point in this problem here:

val reverse = new mutable.HashMap[String, String]() with mutable.SynchronizedMap[String, String]  

def search(query: String) = Future.value {
  val tokens = query.split(" ")
  val hits = tokens map { token => reverse.getOrElse(token, Set()) }
  if (hits.isEmpty)
    Nil
  else
    hits reduceLeft {_ & _} toList // value & is not a member of java.lang.Object
}

The compiler says value & is not a member of java.lang.Object. Can somebody explain me why I am getting a compiler error ? I have this from this tutorial here: https://twitter.github.io/scala_school/searchbird.html

The future object has no method value, you have to move it after the curly bracket. — Ende Neu
– Ende Neu, Commented Oct 15, 2014 at 15:59
I edited the source, I forgot the "reverse" and the "toList". The problem is at the last line with the binary operation "&" — okarahan
– okarahan, Commented Oct 15, 2014 at 16:07

mohit · Accepted Answer · 2014-10-15 18:01:20Z

4

"tokens" is of type Array[String]. Now, when you iterate over the array, there are two possibilities. Either reverse will have a value for the token or not. If it has, then the Array element get a string value otherwise an empty set.

For example: Lets say reverse has two values - ("a" -> "a1", "b" ->"b1") a maps to a1 and b maps to b1.

Suppose, The query string is "a c". tokens will be ["a","c"] after splitting. After mapping you will get in array ["a1", Set()] (a got mapped to a1 and there is no value for "c" in the map hence, you got an empty Set())

Now, the overall type of the hits array is Array[Object]. So, now you are getting an error as the last line will be "&" operator on 2 Objects according to the compiler.

edited Oct 15, 2014 at 18:01

answered Oct 15, 2014 at 16:19

mohit

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Comments

Kadreal · Accepted Answer · 2014-10-16 18:58:03Z

2

Mohit Has The right answer, you end up with an Array of objects. This is because your HashMap for reverse has a value type of String, so it'll return a String for a given key. Your getOrElse however, will return a Set type if the key is not found in your HashMap reverse. These need to return the same type so you don't end up with an Array[Objects]

If you notice a few lines above in the tutorial you linked, reverse is defined as follows:

val reverse = new mutable.HashMap[String, Set[String]] with mutable.SynchronizedMap[String, Set[String]]

edited Oct 16, 2014 at 18:58

answered Oct 15, 2014 at 17:09

Kadreal

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1 Comment

okarahan Over a year ago

Thank you. Maybe I overlooked this detail.

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