Re: Visitor pattern vs if-ladder

Daniel Pitts <>
Fri, 24 Apr 2009 13:05:06 -0700
Philipp wrote:

I think about replacing an if-ladder (with instanceof calls) in my
code with a visitor pattern, but the advantages are not really obvious
to me.
- cleaner code (although I think this is debatable)
- Needs modification of the original interface and classes (to add the
accept(Visitor) method)
- If new subclasses of the acceptor interface are added, both the
Visitor interface and it's implementation needs change (adding a new
visit(MyNewClass c); to the interface). In the ladder implementation,
you only need to change at one point and add an if(...)
- A bit slower than the ladder implementation (see below)

What advantages am I missing? Why is an if ladder seen as code smell
and not the visitor pattern?

About speed:
I compared the speed of execution of an if-ladder with the visitor
pattern for counting different types of Vehicles.
If ladder:
public void count(Vehicle vehicle){
  if (vehicle instanceof Car) {
  } else if (vehicle instanceof Motorbike) {
  } else if (vehicle instanceof Bike) {

Visitor pattern:
public void visit(Car car) {
public void visit(Motorbike motorbike) {
public void visit(Bike bike) {

I ran the counting with 8 types of vehicle on 10^8 randomly produced
vehicles 10 times alternatively (each run is ~16 sec, all 20 runs are
in one JVM execution, so warm up should be accounted for). The ladder
implementation is about 10% faster (mean time per run: ladder 16800ms,
visitor 18380).
I'm not saying that this is a big difference or should influence the
decision very much. It's just FYI.


Maybe consider adding an enum type-token, and using an
EnumMap<Vehical.Type, CountAccumulator>

public class CountAccumulator {
   private int count;
   public void increment() { ++count; }
   public int get() { return count; }

I usually avoid enum for type-tokens, but this seems like an appropriate
place to apply them, since you are looking for a very specific set of
"types". It also would reduce the size of your count method/visitor

Map<Vehical.Type, CountAccumulator> count(Collection<Vehical> vehicals){
   Map<Vehical.Type, CountAccumulator> counts =
       new EnumMap<Vehical.Type, CountAccumulator>(CarType.class);
    for (Vehical.Type type: Vehical.Type.values()) {
       counts.put(type, new CountAccumulator());
    for (Vehical vehical: vehicals) {
    return counts;

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