Parametric polymorphism in java with simple example

Question

I have researched the topic further and i realise that for an example to purely detail parametric polymorphism, it must not utilise "implements" (which would detail subtype polymorphism) but instead should utilise generics .

This is my previous question: What is parametric polymorphism in Java (with example)?

This is the reworked code to use generics with a single issue.

class Pet<T>
{
  private T pet;

  public Pet(T pet)
  {
    this.pet = pet;
  }

  public String talks()
  {
    // what do i write here?
    // pet.talks(); is undefined for type T
  }
}

class Cat
{
  public String talks()
  {
    return "Meow";
  }
}

class Dog
{
  public String talks()
  {
    return "Woof";
  }
}

public class TestPet
{

  public static void main(String[] args)
  {
    Pet<Cat> cat = new Pet<Cat>(new Cat());
    System.out.println("Cat says " + cat.talks());

    Pet<Dog> dog = new Pet<Dog>(new Dog());
    System.out.println("Dog says " + dog.talks());
  }

}

There is a single issue with my code detailed in comments. I'm unsure how to write it without using the implements command. Do you know what to write?

Answer 1

You need to add an upper bound to T such that it implements an interface which guarantees that there is a talks() method in the type T .

class Pet<T extends Talker>
{
    private T pet;

    public Pet(T pet)
    {
        this.pet = pet;
    }

    public String talks()
    {
        return pet.talks(); // is now defined for type T
    }
}

interface Talker
{
    String talks();
}

Demo, thanks to Ray Toal : http://ideone.com/yiAqM