Question

I have a question about Parametric Polymorphism. How do I determine the actual type and avoid casting If I have a collection with a mix of child types. For example

    class Animal{

    }

    class Bird extends Animal{
          void fly(){}
    }

    class Dog extends Animal{
          void bark(){}
    }

    ArrayList<Animal> list = new ArrayList<Animal>();

The problem is how do I know which one is which when I iterating through the Animal collection. Do I need to use instanceof to check the actual type every time?

    for(Animal animal : list){
          if(animal instanceof Dog){
                ((Dog) animal).bark();
          } 
          else ((Bird) animal).fly();             
    }

Answer 1

If you need to do this, that means that’s a common action. You usually would have this :

abstract class Animal {
     abstract void act();
}

class Bird extends Animal{
      void fly(){}
      void act(){
         fly();
      }
}

class Dog extends Animal{
      void bark(){}
      void act(){
         bark();
      }
}

and in your loop you would simply call the act method :

for(Animal animal : list){
    animal.act();
}