Java Inheritance...

[Justin Fox]

Hello,

I was looking through the SCJP from the beginning, and I was just wondering if
when Class A extends Class B, does Class A have access to Class B's Public methods
as well as any Protected?

This is the beginning of what may seem to be noobish questions..

Thanks a lot,

Justin Fox

[Campbell Ritchie]

No.

A superclass does not have access to a subclass. A subclass automatically includes all accessible methods from the superclass.

Animal fido = new Dog();

fido has all the public methods of Animal.

But if you want to use the public methods of Dog in Animal, you would have to create a Dog instance. To Animal, Dog is any old class, and it treats it as it would Food or LampPost objects.

[Vikas Kapoor]

If this is just for curiosity than it's ok otherwise in real life application parent class should not access/dependent on child class.

[Justin Fox]

Ha, I asked the question backwards lol.

I mean to ask if B has access to the public/protected methods of A.

Thanks for the info. though.

Justin

[Don Solomon]

Yes, when a method in class B overrides a method in class A. When class A envokes a method in class A that has been overridden by class B, the class B method will execute. In reference to your original question.

[Justin Fox]

Yes, when a method in class B overrides a method in class A. When class A envokes a method in class A that has been overridden by class B, the class B method will execute. In reference to your original question.

Ok, now I'm getting kind of confused...

If B is the super class and A is the subclass:
no method in B can override a method in class A.

If A is the super class and B is the subclass:
if B overrid a method that it inherited from A,
and A calls its method, A's version will execute. If
B calls the same method, the overrid version in class B will Execute.

Example:

//CLASS A

public class A {

public void message()
{ System.out.println("A");}

}

//CLASS B
public class B extends A{

public void message()
{ System.out.println("B");}

public void specialmessage()
{System.out.println("Special Message");}

}

//MAIN METHOD
public class classtest{

public class classtest{

public static void main (String[] args)
{
	A a = new A();
        B b = new B();
        A c = new B();

	a.message();
        b.message();
        c.message();

	c.specialmessage(); // fails at compile time.

	if ( c instanceof B)
        {
	((B)c).specialmessage();}
}

}

Output:
A
B
B
Special Message

What I'm trying to say is the quote of above (to me) doesn't make much sense
unless there is backwards inheritance or something like that....

Thanks,

Justin Fox

[Don Solomon]

Try this...

 

public class A {

   public void print() {
      method();
   }

   public void method() {
      System.out.println("A.method()");
   }


   public static void main(String[] args) {
      A a=new B();
      a.print();
   }
}

class B extends A {

   public void method() {
      System.out.println("B.method()");
   }

}

[Justin Fox]

lol that's just twisting it around....

you might as well say:

B b = new B();
b.method();

when would this atcually be usefull?

Thanks,

Justin

[Justin Fox]

because technically, A isn't 'overriding' B's method() method, you're just
using the fact that at runtime the JVM will know that the instance of a is actually
and object of B and call its method instead..

Justin

[Sujai Kaarthik]

Consider Father as super class and son as sub class.

A son can aquire all the properties from the father whereas the father cannot. This is the basic concept of inheritance.

`
On overriding,

If a son posses some habits, you may tell those habits are aquired from his father, but you cannot say the father aquired the habits from his son. hope you got it

[Don Solomon]

I recommend that you study the core fundamental property of Polymorphism. Very important concept/behavior.

This concept allows one to change behavior without changing interface. Thanks, Don

[Justin Fox]

I understand polymorphism and inheritance fully,

What I'm trying to say is, A super class CANNOT override a sub classes method.

Justin

lol nice analogy

[Campbell Ritchie]

Justin Fox wrote:I understand polymorphism and inheritance fully,

What I'm trying to say is, A super class CANNOT override a sub classes method.

Justin

lol nice analogy

Correct. A superclass doesn't "know" about its subclasses at all.