I don't know what to name this topic

[purusingh]

class X
{
public:
double A(double x)
{
return x*x;
}
double B(double x)
{
return A(x)/2;
}
};

class Y ublic X
{
public:
double A(double x)
{
return x*x*x;
}
};
void main()
{
Y y;
cout<<y.B(3);
}

this gives 4.5

but in java


public class X
{
public:
double A(double x)
{
return x*x;
}
double B(double x)
{
return A(x)/2;
}

}
public class Y extends X
{
public:
double A(double x)
{
return x*x*x;
}
}
public class App
{
public static void main(String s[])
{
Y y=new Y();
System.out.println(y.B(3));
}
}

this gives 13.5

Why

Then what is the use of interface in java?

[Zaei]

Horribly wrong forum.

Z.

[CornedBee]

Try this in the C++ part:

class X
{
public:
virtual double A(double x)
{
return x*x;
}
double B(double x)
{
return A(x)/2;
}
};

class Y : public X
{
public:
virtual double A(double x)
{
return x*x*x;
}
};
void main()
{
Y y;
cout<<y.B(3);
}

It now should output 13.5 too.
Methods in Java are virtual by default. In C++ they are not because it would often produce unnecessary overhead. In Java this isn't important as in interpreted (byte-)code virtual functions don't create overhead.

If you don't know what virtual means look up polymorphism in your favourite C++ tutorial.


Z: It is the right forum. He's more likely to get an answer here than in the Java forums (why the hell should any Java programmer know about virtual functions or that there is something else unless he/she comes from a C++ background).

[MoMad]

Hehe, I was about to say "virtual" was missing but I guess you beat me at it

[EDIT]
Doesnt it just overload the function if there is no virtual and override it when there is? So, unless you say something like Y::A(x)/2 in the base function (and this wont work inline because Y is unknonwn at the time), it will always resolve to using X::A(x)/2!!

[Zaei]

Originally posted by CornedBee
Z: It is the right forum. He's more likely to get an answer here than in the Java forums (why the hell should any Java programmer know about virtual functions or that there is something else unless he/she comes from a C++ background). [/B]

True, forgive, it was too early in the morning =).

Z.

[CornedBee]

Mo: Y:: won't ever work in the base class because it's not guaranteed that it is really a Y (might be a X or even a Z derived from X too). It will resolve to X:: except if it's virtual, then it will resolve to ActualType::

[MoMad]

isnt it something like typeof(*this):: or yea something to do with the object and not necesarily the class itself. And the Y:: was just for example. I meant it has to be able to do that kind of forward declaration or something.

[purusingh]

I know it works with virtual function.
But why it works with java.I think only using interface it should work.

[CornedBee]

No, extends also uses only virtual functions.

But why exactly would better be asked in the Java forum.