Question

It is my understanding that parametric polymorphism is a technique which allows uniform actions over a variety of data(types). Is my knowledge correct?

Is this example parametric polymorphism?
I believe it is since Animal.talk allows talk to be called despite the specific animal type (Cat or Dog).

public interface Animal
{
  public String talk();
}

public class Cat implements Animal
{
  public String talk()
  {
    return "Cat says Meow!";
  }
}

public class Dog implements Animal
{
  public String talk()
  {
    return "Dog says Woof! Woof!";
  }
}

import java.util.*;

public class PolymorphismExample
{
  public static void main(String[] args)
  {
    Collection<Animal> animals = new ArrayList<Animal>();
    animals.add(new Cat());
    animals.add(new Dog());
    for (Animal a : animals)
    {
      System.out.println(a.talk());
    }
  }
}

Regards.

edit: if my example is not specifically exhibiting parametric polymorphism please would you provide one? thank you.

Answer 1

“Parametric Polymorphism” is just another term for “Generics” in Java. The idea is simple: you state what types will be used by a particular class, a clear example of this is present in all the collections of the java.util package.

For learning all the nuances of generics in Java, I highly recommend Angelika Langer’s FAQ, it explores every corner of the specification.

In your code, this line is an example of using generics:

Collection<Animal> animals = new ArrayList<Animal>();

A collection is specified to hold any object that is an animal.