get objects of the certain type from Collection

[Kuba Zygmunt]

Hi,

Below you can see a simple hierarchy of Bike's family.


So, I have a Collection<Bike> and I would like to get all bikes of the certain type. What I've done is :

 
public Collection<Bike> getBikesType(Bike bikeType){
  ArrayList<Bike> newList = new ArrayList<Bike>();
  for(Bike bike : allBikes) {
    if (bike().getClass() == bikeType.getClass()) 
newList.add(bike);
  }
  return newList;
}

so I compare classes ( I dont want to use instanceof because I'm interested in a concrete class ( not superclass ).

Another way to do it is to create a BikeType class which stores and defines all used type of bikes, and then a super class Bike will have an additional field
BikeType bikeType;

and then I could compare bikes in collection using that property .
I dont know if the 2nd option is better, because when you add new class you will have to change bikeType.

What's your opinion ?

[Kuba Zygmunt]

there is an error in the code

should be:

if (bike.getClass() == bikeType.getClass())
newList.add(booking);
}

[Rob Spoor]

I'd go with a slightly modified version of your original code:

public Collection<Bike> getBikesType(Class<?> cls)
{
    // your code here
}

Of course this allows also non-bike classes to be passed.

So another modification, to allow only the Bike class and its subclasses:

public <T extends Bike> Collection<Bike> getBikesType(Class<T> cls)
{
    // your code here
}

There is another improvement to be made now: you know that everything inside the collection will be of the given type. So:

public <T extends Bike> Collection<T> getBikesType(Class<T> cls)
{
    ArrayList<T> newList = new ArrayList<T>();
    for (Bike bike: allBikes)
    {
        if (bike.getClass() == cls)
        {
            // now you have to cast to allow the adding
            // the cast is safe because cls.isInstance(bike) yields true
            T tBike = cls.cast(bike);
            newList.add(tBike);
        }
    }
    return newList;
}

You can now do stuff like this:

Collection<MountainBike> = getBikesType(MountainBike.class);
Collection<RacingBike> = getBikesType(RacingBike.class);

[ November 22, 2007: Message edited by: Rob Prime ]

[Kuba Zygmunt]

Great !

Thank you for showing me this approach