Polimorfismul

1. Balenele și vacile

Iată un fapt zoologic interesant: o vaca este mult mai aproape de o balenă decât, de exemplu, de un hipopotam. Se dovedește că vacile și balenele sunt rude relativ apropiate.

Uite aici. Să vă spunem despre polimorfism — un alt instrument foarte puternic al OOP . Are patru caracteristici.

2. Moștenirea nu este un panaceu

Imaginează-ți că ai scris o Cowclasă pentru un joc. Are multe domenii și metode. Obiectele acestei clase pot face diverse lucruri: să meargă, să mănânce și să doarmă. Vacile poartă și un clopoțel care sună când merg. Să presupunem că ați implementat totul în clasă până la cel mai mic detaliu.

And then your client comes and says she wants to release a new game level, in which all the action takes place at sea, and the main character is a whale.

You start designing a Whale class and realize that it is only slightly different from the Cow class. The logic of both classes is very similar and you decide to use inheritance.

The Cow class is ideal for taking on the role of the parent class: it has all the necessary variables and methods. All we need to do is give the whale the ability to swim. But there's a problem: your whale has legs, horns, and a bell. After all, this functionality is implemented inside the Cow class. What can be done here?

3. Method overriding

Method overriding comes to our rescue. If we inherited a method that doesn't quite do what we want in our new class, we can replace that method with another one.

How is this done? In our descendant class, we declare the same method as the method of the parent class that we want to override. We write our new code in it. And that's it — it's as if the old method in the parent class simply doesn't exist.

This is how it works:

class Cow
{
   public void printColor ()
   {
      System.out.println("I'm a white whale");
   }

   public void printName()
   {
      System.out.println("I'm a cow");
   }
}

class Whale extends Cow
{
   public void printName()
   {
      System.out.println("I'm a whale");
   }
}

public static void main(String[] args)
{
   Cow cow = new Cow();
   cow.printName();
}

I'm a cow

public static void main(String[] args)
{
   Whale whale = new Whale();
   whale.printName();
}

I'm a whale

After inheriting the Cow class and overriding the printName method, the Whale class actually contains the following data and methods:

class Whale
{
   public void printColor()
   {
      System.out.println("I'm a white whale");
   }

   public void printName()
   {
      System.out.println("I'm a whale");
   }
}