class DefaultListMap[A, B <: List[B]] extends HashMap[A, B] {
override def default(key: A) = List[B]()
}
I wan't to create map A -> List[B]. In my case it is Long -> List[String] but when I get key from map that doesn't have value I would like to create empty List instead of Exception being thrown. I tried different combinations but I don't know how to make code above pass the compiler.
Thanks in advance.
Why not to use withDefaultValue(value)?
scala> val m = Map[Int, List[String]]().withDefaultValue(List())
m: scala.collection.immutable.Map[Int,List[String]] = Map()
scala> m(123)
res1: List[String] = List()
Maps. m(123) will work, but m.get(123) will give you None.Rather than using apply to access the map, you could always use get, which returns Option[V] and then getOrElse:
map.get(k) getOrElse Nil
One great feature of the scalaz functional-programming library is the unary operator ~, which means "or zero",as long as the value type has a "zero" defined (which List does, the zero being Nil of course). So the code then becomes:
~map.get(k)
This is doubly useful because the same syntax works where (for example) your values are Int, Double etc (anything for which there is a Zero typeclass).
There has been a great deal of debate on the scala mailing list about using Map.withDefault because of how this then behaves as regards the isDefinedAt method, among others. I tend to steer clear of it for this reason.
There's a method withDefaultValue on Map:
scala> val myMap = Map(1 -> List(10), 2 -> List(20, 200)).withDefaultValue(Nil)
myMap: scala.collection.immutable.Map[Int,List[Int]] = Map((1,List(10)), (2,List(20, 200)))
scala> myMap(2)
res0: List[Int] = List(20, 200)
scala> myMap(3)
res1: List[Int] = List()
withDefault, which takes a function.Maps. myMap(3) will work, but myMap.get(3) will give you None.Why do you want to manipulate a map when it has already a method for this?
val m = Map(1L->List("a","b"), 3L->List("x","y","z"))
println(m.getOrElse(1L, List("c"))) //--> List(a, b)
println(m.getOrElse(2L, List("y"))) //--> List(y)
withDefault can also be used.
/** The same map with a given default function.
* Note: `get`, `contains`, `iterator`, `keys`, etc are not affected
* by `withDefault`.
*
* Invoking transformer methods (e.g. `map`) will not preserve the default value.
*
* @param d the function mapping keys to values, used for non-present keys
* @return a wrapper of the map with a default value
*/
def withDefault[B1 >: B](d: A => B1): immutable.Map[A, B1]
Example:
scala> def intToString(i: Int) = s"Integer $i"
intToString: (i: Int)String
scala> val x = Map[Int, String]().withDefault(intToString)
x: scala.collection.immutable.Map[Int,String] = Map()
scala> x(1)
res5: String = Integer 1
scala> x(2)
res6: String = Integer 2
Hope this helps.
code val map = new HashMap[Long, List[String]]() { override def default(key: Long) = List[String]() }