Reference and Instance variables

[Jyoti Kaushal]

Hi,

I want to discuss about an example from Overriding topic. Following is the example:-

public class TestAnimals {
      public static void main(String[] arr) {
             Animal a = new Animal();
             Animal b = new Horse();
             a.eat();
             b.eat();
      }
}
class Animal {
     public void eat() {
          System.out.println("Generic Animal Eating....");
     }
}
class Horse extends Animal {
    public void eat() {
          System.out.println("Horse eating hay.....");
    }
     public void buck() { }
}

In this, b is referencing Animal but has instance of Horse. This means it will call eat() of Horse. Similarly, why can't it access b.buck(), as b has an instance of Horse class and Horse class has buck method?
Please correct me if I am wrong in giving the description and explain me how it works when we use instance of one class and refer some other.

Also , Is this also legal?

Horse b = new Animal();

Thanks in advance

[Ganish Patil]

Please always write code in code tag. For code convention read this Java Programming Style

[Jyoti Kaushal]

Thanks for the link Ganish. But I did all the indenting and formatting of my code but it all vanished when it was posted. :-(

[Paul Clapham]

I've added the code tags this time.

(When you post code, you'll see a "Code" button above the box you're typing in. Highlight the code and click that button to surround it with the "code" tags.)

[Sergej Smoljanov]

you want use method from child but use type of parent, becouse parent have not this method you get compiler error. some variant of this situation (use parent variable) you may get when pass your child to method with parent parametr.
you can't assign type of parent to variable of type child. you can do this with cast, but if you have actual object that not IS child (or sub child) you will get exception

[Jyoti Kaushal]

So, as per my understanding, method of only that class will be invoked whose object is instantiated. But only if the method is present in the referenced class. Is this right?

[Ganish Patil]

class Animal { 
    public void eat() { 
    System.out.println("Animal: Generic Animal Eating...."); 
    } 
} 

class Horse extends Animal { 
    public void eat() { 
    System.out.println("Horse: Horse eating hay....."); 
    } 
    public void buck(){
    System.out.println("Horse: buck eating hay....."); 
    } 
}

public class TestAnimals { 
    public static void main(String[] arr) { 
        // a is reference variable of Animal class and store an instance of class Animal        
        Animal a = new Animal();   
        
       //  b is reference variable of Animal class and store an instance of class Horse
        Animal b = new Horse();    
       
        // a as reference variable of Animal stores an instance of Animal class, it will call its instance variable's method eat defined in Animal class   
        a.eat();  
       
        //  b as reference variable of Animal stores an instance of Horse class, it will call its instance variable's method eat defined in Horse
        b.eat();

        //b.buck();  error: can'y find symbol b. buck, location: variable b of type Animal
        
    } 
} 
 

Here relation between Animal and Horse is IS-A relation. Animal is super type and Horse is sub type. so All Horse can be animals but all animals can't be Horse Ex. It can be cat, dog etc.
So it is an assignment conversion where widening conversion is safe but narrowing conversions is not recommended as it's not safe. When you say Animal a, b ; These reference variables points to a type of Animal which has it's instance variable method called eat() but it doesn't have buck method which Horse has so when you try to call method buck, using Animal reference variable which has no buck method as part of Animal type, it gives above error.

[Ganish Patil]

non-static methods belong to object and static members belong to class. When you want to invoke non-static methods then an object of that type(class) must be instantiated. When you try to invoke non-static method of a sub type using reference variable of super type by assigning an instance of sub type to reference variable of super type then the no-static method of sub type which you want to invoke must have to be method of super type.

[Jyoti Kaushal]

Nice explanation Ganish. Thanks:-)

[Ganish Patil]

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