Re: Visitor pattern vs if-ladder

From:
Tom Anderson <twic@urchin.earth.li>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 25 Apr 2009 11:54:33 +0100
Message-ID:
<alpine.DEB.1.10.0904251141310.27031@urchin.earth.li>
On Fri, 24 Apr 2009, Daniel Pitts wrote:

Philipp wrote:

I think about replacing an if-ladder (with instanceof calls) in my
code with a visitor pattern,


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

where:
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".


I suspect my discomfort with type tokens is the same as yours, and stems
from the incomplete static typing: there's nothing stopping Car.getType()
returning VehicleType.BIKE.

But is there any clever generic trickery we could do to fix that?

[goes off and has a play]

Well, you can't make enums generic, so no, apparently not. Hmm.

It also would reduce the size of your count method/visitor
dramatically:

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) {
     counts.get(vehical.getType()).increment();
  }
  return counts;
}


Good call! I was thinking you'd still have to write a switch on the enum,
but the EnumMap makes things very compact, but still fast.

tom

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