What makes a "doubt" a "valid doubt"? I think you need to read about single inheritance versus mulitple inheritance and the basic notion of a class hierarchy.
Java has single class inheritance. When you define a class you can write
class B extends X {...}
but not
class B extends X, Y {...} //as one can do in C++, for example.
Because you can continue this process and define a class to extend B:
class C extends B {...}
a hierachy may result. Consider javax.swing.JButton:
JButton extends AbstractButton
AbstractButton extends JComponent
JComponent extends Container
Container extends Component
Component extends Object
Because at each step the newly defined class can only *directly* extend
one class, this is not multiple inheritance.
[ May 19, 2006: Message edited by: Jeff Albertson ]