Found Option[CSVWriter] but required CSVWriter? `var` caused the issue? - scala

Question

The following code
var writers = new HashMap[String, CSVWriter]()
var writer = writers.get(pool)
if (writer == null) {
//writer = new CSVWriter(new FileWriter(s"..."))
writers.put(pool, writer) // Errr
}
Got the error of
[error] found : Option[au.com.bytecode.opencsv.CSVWriter]
[error] required: au.com.bytecode.opencsv.CSVWriter
[error] writers.put(pool, writer)
[error] ^
[error] one error found
Does var automatically add an Option wrapper? What I should do to put CSVWriter in the hashmap?

Answer 1

When a value is retrieved from a HashMap in scala using .get(key), the default returned value is of Option instance.
scala> val writers = collection.mutable.HashMap("abc" -> "def")
writers: scala.collection.mutable.HashMap[String,String] = Map(abc -> def)
scala> writers.get("abc")
res10: Option[String] = Some(def)
Option will return Some instance if the key exists in the HashMap and returns None if the key is not present.
scala> writers.get("a")
res13: Option[String] = None
This helps to avoid the nullpointer exceptions.
We need to get the real value using another get
scala> writers.get("abc").get
res11: String = def
But to be on the safe-side we can use getOrElse
scala> writers.getOrElse("abc", "no value")
res1: String = def
scala> writers.getOrElse("a", "no value")
res2: String = no value
I hope the explanation is clear