如何在 Scala 中使用默认操作实现 Map

Question

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.

Answer 1

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()

Answer 2

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.

Answer 3

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()

Answer 4

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)

Answer 5

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.