Polymorphism , Dynamic Binding, Widening and I'm totally confused

Question

class Fruit
     private String name;
     public String getName()
          return this.name;
class Apple extends Fruit
     private String typeOfApple;
     public void setTypeOfApple(String type)
          this.typeOfApple=type;
     public String getTypeOfApple()
          return this.typeOfApple;
class Banana extends Fruit
     private String typeOfBanana;
     public void setTypeOfBanana(String type)
          this.typeOfBanana=type;
     public String getTypeOfBanana()
          return this.typeOfBanana;
public class ExceptionTesting
     public static void main(String args[])
          Fruit obj=new Apple();
          obj.setTypeOfApple();  // ??? Why can't I do this ???
}how can I achieve something like underneath by modifying above code ???
Fruit whateverFruit=new Apple();
whateverFruit.setTypeOfApple();
whateverFruit=new Banana();
whateverFruit.setTypeOfBanana();
whateverFruit=new Orange();
whateverFruit.setTypeOfOrange();
...

Answer 1

Harry_lynn_17 wrote:
I do apologize if it's become sound like stupid or something . I know somehow it's not very suitable to think in that way but I was just thinking of the object reference variable as a pointer becauseA reference is like a "filtered" pointer to an object: it will only show that part of the object that corresponds with the type of the reference. So after "Fruit whatever = new Banana();", the "whatever" reference only shows the "Fruit" part of the "Banana" object. This means you can only call methods that are defined in Fruit, not those defined in Banana.
Dynamic binding means the JVM will decide at runtime which method implementation to invoke based on the class of the object. So, I thought
Fruit whateverFruit=new Banana(); and then if i tried to invoke whateverFruit.yellowBanana(); then the compiler will try to find the method out and executed automatically.That's not what it means. It means when you have "Fruit whatever = new Banana()" and you call "whatever.getName()", it will call the getName() method from Banana, if Banana overrides the implementation of Fruit. You can still only use methods that are defined in Fruit through a reference of type Fruit. In this case, it doesn't seem that the behaviour of setTypeOfApple() is different from setTypeOfBanana(), so ejp's recommendation to just create a setType() method in Fruit is correct.
Polymorphism means a variable of a superclass type to hold a reference to an object whose class is the superclass or any of its subclasses. So, I thought Fruit whateverFruit is somehow capable to hold the reference of its subclasses and can be used to reference of its subclasses. That's basically correct. However, the variable will only expose the superclass part of the subclass object.
I got confused with all those stuff and kinda lost. There's one more thing. What's the point of doing something like this Fruit apple=new Apple() if I can't reference to an instance of apple object ?? I mean I could just simply write like this Fruit aFruit=new Fruit(). Sorry, if i've become sound like a fool but i'm really confused. Please help me out of this and thanks in advance.The point of having any superclass/subclass combo is that your problem domain demands that you make the distinction: sometimes you need to be able to treat an object as Fruit, sometimes you need to be specific and treat it like an Apple. For example, if you sell fruit and need to calculate the total price of a fruit basket, all you need to know is that each piece of Fruit has a getPrice(), you don't need to know that one piece is an Apple and another piece is a Banana. However, if a customer demands a kilogram of apples, you need to make sure you give him only Apples and not Banana's.List<Fruit> fruitBasket = new ArrayList<Fruit>();
fruitBasket.add(new Apple("Granny Smith"));
fruitBasket.add(new Banana("Chiquita"));
fruitBasket.add(new Apple("Golden Delicious"));
int priceOfBasket = 0;
for(Fruit pieceOfFruit:fruitBasket) {
  priceOfBasket += pieceOfFruit.getPrice();
Set<Apple> appleScale = new HashSet<Apple>();
appleScale.add(new Apple("Granny Smith"));
appleScale.add(new Apple("Golden Delicious"));
appleScale.add(new Banana("Chiquita")); // ERROR