When to use isInstanceOf and when to use a match-c

Question

sealed class A
class B1 extends A    
class B2 extends A

Assuming we have a List of objects of class A : val l: List[A] = List(new B1, new B2, new B1, new B1)

And we want to filter out the elements of the type B1. Then we need a predicate and could use the following two alternatives:

l.filter(_.isInstanceOf[B1])

Or

l.filter(_ match {case b: B1 => true; case _ => false})

Personally, I like the first approach more, but I often read, one should use the match-case statement more often (for reasons I do not know).

Therefore, the question is: Are there drawbacks of using isInstanceOf instead of the match-case statement ? When should one use which approach (and which approach should be used here and why) ?

Answer 1

There are no difference

cat t.scala:

class A {
  def x(o: AnyRef) = o.isInstanceOf[A]
  def y(o: AnyRef) = o match {
    case s: A => true
    case _ => false
  }
}

$ scalac -print t.scala

[[syntax trees at end of cleanup]]// Scala source: t.scala
package <empty> {
  class A extends java.lang.Object with ScalaObject {
    def x(o: java.lang.Object): Boolean = o.$isInstanceOf[A]();
    def y(o: java.lang.Object): Boolean = {
      <synthetic> val temp1: java.lang.Object = o;
      temp1.$isInstanceOf[A]()
    };
    def this(): A = {
      A.super.this();
      ()
    }
  }
}

Answer 2

The advantage of match-case is that you don't have to cast the object in case you want to perform operations on it that depend on its narrower type.

In the following snippet, using isInstanceOf seems to be fine since you don't perform such an operation:

if (obj.isInstanceOf[A]) println(obj)

However, if you do the following:

if (obj.isInstanceOf[A]) {
  val a = obj.asInstanceOf[A]
  println(a.someField) // someField is declared by A
}

then I'd be in favour of using match-case:

obj match {
  case a: A => println(a.someField)
  case _ =>
}

It is slightly annoying that you have to include the "otherwise"-case, but using collect (as hinted at by om-nom-nom) could help, at least if you work with collections inherit from Seq:

collectionOfObj.collect{ case a: A => a}.foreach(println(_.someField))

Answer 3

You can filter like that:

l.collect{ case x: B1 => x }

That is much more readable, IMO.

Answer 4

There's no problem using isInstanceOf, as long as you don't use asInstanceOf.

Code that uses both is brittle, because checking and casting are separate actions, whereas using matching you have a single action doing both.