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Zalek Bloom
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 Posted: Fri Dec 26, 2003 2:19 pm Post subject: Newbie: Inheritage, overloading and casting |
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I am learning about heritage and have some questions about it.
Here is a simple example:
class A {
void m1(A a) {
System.out.print("A");
}
}
class B extends A {
void m1(B b) {
System.out.print("B");
}
}
class C extends B {
void m1(C c) {
System.out.print("C");
}
}
class D {
public static void main(String[] args) {
A a = new A();
C c = new C();
a.m1(c);
}
}
As I understand the statement a.m1(c) takes a method m1, defined in a
class A. Now the method m1 defined in the class A works on variable
type A only. I know that class C extends the class A, so type C is
also a class A, but how Java knows it? For example - Container class
extends Component class, that means I can apply all methods that work
on Component class on the Container without any casting?
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Ryan Stewart
Guest
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| Posted: Fri Dec 26, 2003 2:57 pm Post subject: Re: Newbie: Inheritage, overloading and casting |
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"Zalek Bloom" <zalekbloom (AT) hotmail (DOT) com> wrote
| Quote: | I am learning about heritage and have some questions about it.
Here is a simple example:
class A {
void m1(A a) {
System.out.print("A");
}
}
class B extends A {
void m1(B b) {
System.out.print("B");
}
}
class C extends B {
void m1(C c) {
System.out.print("C");
}
}
class D {
public static void main(String[] args) {
A a = new A();
C c = new C();
a.m1(c);
}
}
As I understand the statement a.m1(c) takes a method m1, defined in a
class A. Now the method m1 defined in the class A works on variable
type A only. I know that class C extends the class A, so type C is
also a class A, but how Java knows it? For example - Container class
extends Component class, that means I can apply all methods that work
on Component class on the Container without any casting?
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First, it's inheritance, not inheritage. Second, yes classes will upcast
automatically. Take java.util.Vector for instance. It's a collection that
has an add(Object o) method. You can use that to add any object to the
Vector like so:
String blah = "blah";
someVector.add(blah);
Graphics g = someImage.getGraphics();
someVector.add(g);
No casting is required because every object is a descendant of Object.
However, when getting objects back out of a Vector, you have to cast them
appropriately because the get and remove methods of Vector return Objects,
and Java doesn't automatically downcast. Consider casting primitives, like
int and long. A long is bigger than an int, so casting from int to long will
never cause a problem, so it is done automatically if needed. However,
casting from long to int could lose data, therefore it is never done
automatically. It works basically the same way for objects. Any cast to a
more general object will be done automatically. Any cast to a more specific
object must be done manually.
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