What is parametric polymorphism in Java (with example)?

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.

Looks correct. Basically, you treat Cat and Dog without regard for their actual type based on the Animal type. — dvallejo
As a Haskeller, I wouldn't call that parametric polymorphism. A parametric polymorphic function would be something like LinkedList<T> reverse(LinkedList<T>) that works the same for all parameter types T. A parametric polymorphic function can't call interface methods. — Daniel Fischer

Óscar López Óscar López · Accepted Answer · 2012-04-16T18:24:44

"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.

What is parametric polymorphism in Java (with example)?

3 Answers